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NullDev JavaScript Styleguide

NullDev JavaScript StyleGuide

Now with the new ESLint Flat Config Format!

A mostly reasonable approach to JavaScript

Or... Mainly NodeJS...


Setup ⚙️

To integrate into your own project:

  • Make sure ESLint is installed. For VSCode, download the extension here
  • Navigate inside your project
  • Install Babel-ESLint*:
    $ npm install @babel/eslint-parser @babel/core eslint --save-dev
  • Get the config:
    $ wget https://raw.githubusercontent.com/NullDev/JavaScript-Styleguide/master/eslint.config.js
  • Done! :)

*) The rationale for using Babel-ESLint is that it supports the newest Stage-3 ECMAScript features.

... with already preset config files and project structure

DOWNLOAD

$ npm i - and you're ready to go!


Table of Contents 📑

  1. 📋 Types

    View contents
  2. 🔗 References

    View contents
    • 2.1 Prefer const
    • 2.2 Disallow var
    • 2.3 Block scope
  3. 📦 Objects

    View contents
    • 3.1 Literals
    • 3.2 Computed properties
    • 3.3 Object shorthand
    • 3.4 Object concise
    • 3.5 Grouped shorthand
    • 3.6 Quoted properties
    • 3.7 Prototype Builtins
    • 3.8 Rest spread
  4. 📑 Arrays

    View contents
    • 4.1 Literals
    • 4.2 Push
    • 4.3 Array spreads
    • 4.4 spread for iterable
    • 4.5 Array-like objects
    • 4.6 Mapping
    • 4.7 Callback return
    • 4.8 Linebreaks
  5. 🔨 Destructuring

    View contents
    • 5.1 Object destructuring
    • 5.2 Array destructuring
    • 5.3 Object over array
  6. 📄 Strings

    View contents
    • 6.1 Quotes
    • 6.2 Length
    • 6.3 Template literals
    • 6.4 Eval
    • 6.5 Escaping
    • 6.6 Regular Expressions (RegEx)
  7. 📟 Functions

    View contents
    • 7.1 Strict mode
    • 7.2 Declarations
    • 7.3 IIFE's
    • 7.4 Non-function blocks
    • 7.5 Blocks (Info)
    • 7.6 Arguments shadow
    • 7.7 Rest
    • 7.8 Default parameters
    • 7.9 Side effects
    • 7.10 Defaults last
    • 7.11 Function constructor
    • 7.12 Signature spacing
    • 7.13 Mutating parameters
    • 7.14 Reassigning parameters
    • 7.15 Spread
    • 7.16 Signature invocation indentation
  8. ➡️ Arrow Functions

    View contents
    • 8.1 Usage
    • 8.2 Implicit return
    • 8.3 Wrap
    • 8.4 Omitting parentheses
    • 8.5 Confusing functions
    • 8.6 Linebreaks
  9. 📐 Classes & Constructors

    View contents
    • 9.1 Use class
    • 9.2 Inheritance
    • 9.3 Chaining
    • 9.4 toString methods
    • 9.5 No empty constructors
    • 9.6 No duplicate members
    • 9.7 this in Class-Methods
  10. 📮 Modules

    View contents
    • 10.1 CommonJS
    • 10.2 Duplicate imports
    • 10.3 Mutable exports
    • 10.4 Imports first
    • 10.5 Imports extensions
    • 10.6 Multiline imports
  11. 🔃 Iterators and Generators

    View contents
  12. 📑 Properties

    View contents
    • 12.1 Dot notation
    • 12.2 Bracket notation
    • 12.3 Exponentiation operator
  13. 💾 Variables

    View contents
    • 13.1 Global namespace
    • 13.2 Declaration per variable
    • 13.3 Grouping
    • 13.4 Placing
    • 13.5 Chain assignment
    • 13.6 Increment & decrement
    • 13.7 Linebreaks
    • 13.8 Unused variables
    • 13.9 Bitwise floor
  14. ⤴️ Hoisting

    View contents
    • 14.1 About
    • 14.2 Anonymous function expressions
    • 14.3 Named function expressions
    • 14.4 Function declarations
  15. ↔️ Comparison Operators & Equality

    View contents
    • 15.1 Strict comparison
    • 15.2 Conditional statements
    • 15.3 Shortcuts
    • 15.4 Truth Equality
    • 15.5 Switch blocks
    • 15.6 Nested ternaries
    • 15.7 Unneeded-ternaries
    • 15.8 Mixed operators
  16. 🔲 Blocks

    View contents
  17. 🔧 Control Statements

    View contents
  18. 📝 Comments

    View contents
  19. Whitespace

    View contents
  20. 🔻 Commas

    View contents
    • 20.1 Leading commas
    • 20.2 Comma Dangling
  21. Semicolons

    View contents
    • 21.1 Use semicolons
  22. 🔀 Type Casting & Coercion

    View contents
  23. 📜 Naming Conventions

    View contents
  24. ⬇️ Accessors

    View contents
    • 24.1 Not required accessors
    • 24.2 Getters & Setters
    • 24.3 Boolean prefix
    • 24.4 Consistency
  25. 🔆 Events

    View contents
  26. 📖 Standard Library

    View contents
  27. 🔩 jQuery

    View contents
  28. 🔄 ECMAScript 5 Compatibility

    View contents
    • 28.1 Compatibility table
  29. 6️⃣ ECMAScript 6+ (ES 2015+) Styles

    View contents
  30. 🔌 Testing

    View contents
  31. 📈 Performance

    View contents
  32. 📚 Resources

    View contents
  33. ©️ Copyright

    View contents
  34. ♻️ Amendments

    View contents

Types

  • 1.1 Primitives: When you access a primitive type you work directly on its value.

    • string
    • number
    • boolean
    • null
    • undefined
    • symbol
    • bigint
    const foo = 1;
    let bar = foo;
    
    bar = 9;
    
    console.log(foo, bar); // => 1, 9
    • Symbols and BigInts cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don't support them natively.

  • 1.2 Complex: When you access a complex type you work on a reference to its value.

    • object
    • array
    • function
    const foo = [1, 2];
    const bar = foo;
    
    bar[0] = 9;
    
    console.log(foo[0], bar[0]); // => 9, 9

⬆ back to top

References

  • 2.1 Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign

    Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.

    // bad
    var a = 1;
    var b = 2;
    
    // good
    const a = 1;
    const b = 2;

  • 2.2 If you must reassign references, use let instead of var. eslint: no-var

    Why? let is block-scoped rather than function-scoped like var.

    // bad
    var count = 1;
    if (true){
        count += 1;
    }
    
    // good
    let count = 1;
    if (true){
        count += 1;
    }

  • 2.3 Note that both let and const are block-scoped.

    // const and let only exist in the blocks they are defined in.
    {
        let a = 1;
        const b = 1;
        var c = 1;
    }
    console.log(a); // ReferenceError: a is not defined
    console.log(b); // ReferenceError: b is not defined
    console.log(c); // 1

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Objects

  • 3.1 Use the literal syntax for object creation. eslint: no-new-object

    // bad
    const item = new Object();
    
    // good
    const item = {};

  • 3.2 Use computed property names when creating objects with dynamic property names.

    Why? They allow you to define all the properties of an object in one place.

    function getKey(k){
        return `a key named ${k}`;
    }
    
    // bad
    const obj = {
        id: 5,
        name: "foo",
    };
    obj[getKey("enabled")] = true;
    
    // good
    const obj = {
        id: 5,
        name: "foo",
        [getKey("enabled")]: true,
    };

  • 3.3 Use object method shorthand. eslint: object-shorthand

    // bad
    const foo = {
        value: 1,
    
        addValue: function (value) {
            return foo.value + value;
        }
    };
    
    // good
    const foo = {
        value: 1,
    
        addValue(value) {
            return foo.value + value;
        }
    };

  • 3.4 Use property value shorthand. eslint: object-shorthand

    Why? It is shorter and descriptive.

    const foo = "bar";
    
    // bad
    const obj = {
        foo: foo,
    };
    
    // good
    const obj = {
        foo,
    };

  • 3.5 Group your shorthand properties at the beginning of your object declaration.

    Why? It’s easier to tell which properties are using the shorthand.

    const foo = "bar";
    const bar = "foo";
    
    // bad
    const obj = {
        test: 1,
        key: 2,
        foo,
        abc: 3,
        xyz: 4,
        bar,
    };
    
    // good
    const obj = {
        foo,
        bar,
        test: 1,
        key: 2,
        abc: 3,
        xyz: 4,
    };

  • 3.6 Only quote properties that are invalid identifiers. eslint: quote-props

    Why? In general it is subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.

    // bad
    const bad = {
        "foo": 3,
        "bar": 4,
        "data-foo": 5,
    };
    
    // good
    const good = {
        foo: 3,
        bar: 4,
        "data-foo": 5,
    };

  • 3.7 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. eslint: no-prototype-builtins

    Why? These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).

    // bad
    object.hasOwnProperty(key);
    
    // good
    Object.prototype.hasOwnProperty.call(object, key);
    
    // best - Stage 3 Proposal:
    // https://github.com/tc39/proposal-accessible-object-hasownproperty
    Object.hasOwn(object, key);

  • 3.8 Prefer the object spread operator over Object.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

    // very bad
    const original = { a: 1, b: 2 };
    const copy = Object.assign(original, { c: 3 }); // this mutates `original`
    delete copy.a; // so does this
    
    // bad
    const original = { a: 1, b: 2 };
    const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }
    
    // good
    const original = { a: 1, b: 2 };
    const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }
    
    const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

⬆ back to top

Arrays

  • 4.1 Use the literal syntax for array creation. eslint: no-array-constructor

    // bad
    const items = new Array();
    
    // good
    const items = [];

  • 4.2 Use Array#push instead of direct assignment to add items to an array.

    const someStack = [];
    
    // bad
    someStack[someStack.length] = "foobar";
    
    // good
    someStack.push("foobar");

  • 4.3 Use array spreads ... to copy arrays. (Read more...)

    // bad
    const len = items.length;
    const itemsCopy = [];
    let i;
    
    for (i = 0; i < len; i += 1){
        itemsCopy[i] = items[i];
    }
    
    // good
    const itemsCopy = [...items];

  • 4.4 To convert an iterable object to an array, use spreads ... instead of Array.from.

    const foo = document.querySelectorAll(".foo");
    
    // good
    const nodes = Array.from(foo);
    
    // best
    const nodes = [...foo];

  • 4.5 Use Array.from for converting an array-like object to an array.

    const arrLike = { 0: "foo", 1: "bar", 2: "baz", length: 3 };
    
    // bad
    const arr = Array.prototype.slice.call(arrLike);
    
    // good
    const arr = Array.from(arrLike);

  • 4.6 Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.

    // bad
    const baz = [...foo].map(bar);
    
    // good
    const baz = Array.from(foo, bar);

  • 4.7 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint: array-callback-return

    // good
    [1, 2, 3].map((x) => {
        const y = x + 1;
        return x * y;
    });
    
    // good
    [1, 2, 3].map(x => x + 1);
    
    // bad - no returned value means `acc` becomes undefined after the first iteration
    [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
        const flatten = acc.concat(item);
    });
    
    // good
    [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
        const flatten = acc.concat(item);
        return flatten;
    });
    
    // bad
    inbox.filter((msg) => {
        const { subject, response } = msg;
        if (subject === "foo") return response === "bar";
        else return false;
    });
    
    // good
    inbox.filter((msg) => {
        const { subject, response } = msg;
        if (subject === "foo") return response === "bar";
        return false;
    });

  • 4.8 Use line breaks after open and before close array brackets if an array has multiple lines

    // bad
    const arr = [[0, 1], [2, 3], [4, 5]];
    
    const objectInArray = [{
        id: 1,
    }, {
        id: 2,
    }];
    
    const numberInArray = [
        1, 2, ...
    ];
    
    // good
    const arr = [
        [0, 1], 
        [2, 3], 
        [4, 5],
        ...
    ];
    
    const objectInArray = [
        {
            id: 1,
        },
        {
            id: 2,
        },
    ];
    
    const numberInArray = [
        1,
        2,
        ...
    ];

⬆ back to top

Destructuring

  • 5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring

    Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.

    // bad
    function getFullName(user){
        const firstName = user.firstName;
        const lastName = user.lastName;
    
        return `${firstName} ${lastName}`;
    }
    
    // good
    function getFullName(user){
        const { firstName, lastName } = user;
        return `${firstName} ${lastName}`;
    }
    
    // best
    function getFullName({ firstName, lastName }){
        return `${firstName} ${lastName}`;
    }

  • 5.2 Use array destructuring. eslint: prefer-destructuring

    const arr = [1, 2, 3, 4];
    
    // bad
    const first = arr[0];
    const second = arr[1];
    
    // good
    const [first, second] = arr;

  • 5.3 Use object destructuring for multiple return values, not array destructuring.

    Why? You can add new properties over time or change the order of things without breaking call sites.

    // bad
    function processInput(input){
        // then a miracle occurs
        return [left, right, top, bottom];
    }
    
    // the caller needs to think about the order of return data
    const [left, __, top] = processInput(input);
    
    // good
    function processInput(input){
        // then a miracle occurs
        return { left, right, top, bottom };
    }
    
    // the caller selects only the data they need
    const { left, top } = processInput(input);

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Strings

  • 6.1 Use double quotes "" for strings instead of single quotes ''. eslint: quotes

    Why? While other Styleguides may enforce single quotes, they mostly do it because of consistency (in favor of older projects). Here are some reasons for using double quotes:

    • Double quotes eliminate the need to escape apostrophes: "I'm" vs 'I\'m'.
    • From a linguistic point of view, double quotes identify a passage of quoted text while single quotes are commonly used as a contraction.
    • Double quotes are used to define strings in many other languages. Single quotes are used to define char's in some.
    • JSON Strings are only valid with double quotes.
    // bad
    const name = 'foo bar';
    
    // bad - template literals should contain interpolation or newlines
    const name = `foo bar`;
    
    // good
    const name = "foo bar";

  • 6.2 Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.

    Why? Broken strings are painful to work with and make code less searchable.

    // bad
    const errorMessage = "This is a super long error - lorem ipsum dolor \
    sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget \
    dolor. Aenean massa. Cum sociis natoque \
    penatibus et.";
    
    // bad
    const errorMessage = "This is a super long error - lorem ipsum dolor " +
        "sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget " +
        "dolor. Aenean massa. Cum sociis natoque " +
        "penatibus et.";
    
    // good
    const errorMessage = "This is a super long error - lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor. Aenean massa. Cum sociis natoque penatibus et.";

  • 6.3 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing

    Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

    // bad
    function sayHi(name){
        return "How are you, " + name + "?";
    }
    
    // very bad
    function sayHi(name){
        return ["How are you, ", name, "?"].join();
    }
    
    // bad as well
    function sayHi(name){
        return `How are you, ${ name }?`;
    }
    
    // good
    function sayHi(name){
        return `How are you, ${name}?`;
    }

  • 6.4 Never use eval() on a string, it opens too many vulnerabilities. eslint: no-eval

  • 6.5 Do not unnecessarily escape characters in strings. eslint: no-useless-escape

    Why? Backslashes harm readability, thus they should only be present when necessary.

    // bad
    const foo = '\'this\' \i\s \"quoted\"';
    
    // good
    const foo = "\"this\" is 'quoted'";
    const foo = `my name is "${name}"`;

  • 6.6 Do not split regular expressions, even if some parts are used multiple times. The only exception are computed RegEx'es.

    Why? It has a great impact on readability and can lead to extremely confusing code

    // bad
    const baseSite = "http(?:s?):\/\/website\.com\/";
    const topic = "(?:top|new|user\/\w+\/(?:uploads|likes))(?:(?:\/\w+)?)\/(\d+)";
    const comment = "(?:(?::)comment(\d+))";
    
    const uploadsRegex = new RegExp(baseSite + topic, "gi");
    const commentRegex = new RegExp(baseSite + topic + comment, "gi");
    const profileRegex = new RegExp(baseSite + "user\/(\w+)", "gi");
    
    // good
    const uploadsRegex = /http(?:s?):\/\/website\.com\/(?:top|new|user\/\w+\/(?:uploads|likes))(?:(?:\/\w+)?)\/(\d+)/gi;
    const commentRegex = /http(?:s?):\/\/website\.com\/(?:top|new|user\/\w+\/(?:uploads|likes))(?:(?:\/\w+)?)\/(\d+)(?:(?::)comment(\d+))/gi;
    const profileRegex = /http(?:s?):\/\/website\.com\/user\/(\w+)/gi;

⬆ back to top

Functions

  • 7.1 Write the "use strict"; directive in each and every script you code and put it on the first line to scope it globally. Also, make an empty line below it. eslint: strict

    Why? It is a good way to make your code safer. This is because strict mode doesn't allow the usage of dangerous features which could work in a way you wouldn't expect. Some things it does:

    • It disallows non-explicit global variables.
    • Silent failing assignments will throw errors instead.
    • It requires all property names in an object literal to be unique.
    • Function parameter names must be unique as well.

    Also you do not need to worry about browser compartibility. It is not a statement, but a literal expression, ignored by earlier versions of JavaScript. Read more about strict mode and why you should use it.

    Note: Modules are exempt from this rule because they always run in strict-mode.

    0    // bad
    1    console.log("Foo");
    2    let x = function(){
    3        console.log("Bar");
    4    };
    5
    
    ---
    
    0    // bad
    1    console.log("Foo");
    2    let x = function(){
    3        "use strict";
    4        // strict mode is enabled in the scope of this function only!
    5        console.log("Bar");
    6    };
    7
    
    ---
    
    0    "use strict";
    1
    2    // ^ Good
    3    console.log("Foo");
    4    let x = function(){
    5        console.log("Bar");
    6    };
    7

  • 7.2 Use named function expressions instead of function declarations. eslint: func-style

    Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to explicitly name the expression, regardless of whether or not the name is inferred from the containing variable (which is often the case in modern browsers or when using compilers such as Babel). This eliminates any assumptions made about the Error's call stack. (Discussion)

    // bad
    function foo(){
        // ...
    }
    
    // okay
    let foo = function(){
        // ...
    };
    
    // good
    // lexical name distinguished from the variable-referenced invocation(s)
    let short = function longUniqueMoreDescriptiveLexicalFoo(){
        // ...
    };

  • 7.3 Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife

    Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.

    // immediately-invoked function expression (IIFE)
    
    // bad
    !function(){ /* ... */ }();
    ~function(){ /* ... */ }();
    -function(){ /* ... */ }();
    +function(){ /* ... */ }();
    void function(){ /* ... */ }();
    
    // good
    (function(){
        console.log("I'm an IIFE");
    }());

  • 7.4 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func

  • 7.5 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.

    // bad
    if (currentUser){
        function test(){
            console.log("Foo");
        }
    }
    
    // good
    let test;
    if (currentUser){
        test = () => {
            console.log("Foo");
        };
    }

  • 7.6 Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

    // bad
    function foo(name, options, arguments){
        // ...
    }
    
    // good
    function foo(name, options, args){
        // ...
    }

  • 7.7 Never use arguments, opt to use rest syntax ... instead. eslint: prefer-rest-params

    Why? ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments.

    // bad
    function concatenateAll(){
        const args = Array.prototype.slice.call(arguments);
        return args.join("");
    }
    
    // good
    function concatenateAll(...args){
        return args.join("");
    }

  • 7.8 Use default parameter syntax rather than mutating function arguments.

    // really bad
    function handleThings(opts){
        // No! We shouldn't mutate function arguments.
        // Double bad: if opts is falsy it'll be set to an object which may
        // be what you want but it can introduce subtle bugs.
        opts = opts || {};
        // ...
    }
    
    // still bad
    function handleThings(opts){
        if (opts === void 0){
            opts = {};
        }
        // ...
    }
    
    // good
    function handleThings(opts = {}){
        // ...
    }

  • 7.9 Avoid side effects with default parameters.

    Why? They are confusing to reason about.

    let b = 1;
    // bad
    function count(a = b++){
        console.log(a);
    }
    count();  // 1
    count();  // 2
    count(3); // 3
    count();  // 3

  • 7.10 Always put default parameters last.

    // bad
    function handleThings(opts = {}, name){
        // ...
    }
    
    // good
    function handleThings(name, opts = {}){
        // ...
    }

  • 7.11 Never use the Function constructor to create a new function. eslint: no-new-func

    Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.

    // bad
    let add = new Function("a", "b", "return a + b");
    
    // still bad
    let subtract = Function("a", "b", "return a - b");

  • 7.12 Spacing in a function signature. eslint: space-before-function-paren space-before-blocks

    Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

    // bad
    const f = function () {};
    const g = function (){};
    const h = function() {};
    
    // good
    const x = function(){};
    const y = function a(){};

  • 7.13 Never mutate parameters. eslint: no-param-reassign

    Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

    // bad
    function f1(obj){
        obj.key = 1;
    }
    
    // good
    function f2(obj){
        const key = Object.prototype.hasOwnProperty.call(obj, "key") ? obj.key : 1;
    }

  • 7.14 Never reassign parameters. eslint: no-param-reassign

    Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8 (and therefore NodeJS as well).

    // bad
    function f1(a){
        a = 1;
        // ...
    }
    
    function f2(a){
        if (!a) a = 1;
        // ...
    }
    
    // good
    function f3(a){
        const b = a || 1;
        // ...
    }
    
    function f4(a = 1){
        // ...
    }

  • 7.15 Prefer the use of the spread operator ... to call variadic functions. eslint: prefer-spread

    Why? It’s cleaner, you don’t need to supply a context, and you can not easily compose new with apply.

    // bad
    const x = [1, 2, 3, 4, 5];
    console.log.apply(console, x);
    
    // good
    const x = [1, 2, 3, 4, 5];
    console.log(...x);
    
    // bad
    new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));
    
    // good
    new Date(...[2016, 8, 5]);

  • 7.16 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself and with a trailing comma on the last item. eslint: function-paren-newline

    // bad
    function foo(bar,
                 baz,
                 quux){
      // ...
    }
    
    // good
    function foo(
        bar,
        baz,
        quux,
        ...
    ){
      // ...
    }
    
    // bad
    console.log(foo,
        bar,
        baz);
    
    // good
    console.log(
        foo,
        bar,
        baz,
        ...
    );

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Arrow Functions

  • 8.1 When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing

    Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.

    Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression.

    // bad
    [1, 2, 3].map(function (x){
        const y = x + 1;
        return x * y;
    });
    
    // good
    [1, 2, 3].map((x) => {
        const y = x + 1;
        return x * y;
    });

  • 8.2 If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style

    Why? Syntactic sugar. It reads well when multiple functions are chained together.

    // bad
    [1, 2, 3].map(number => {
        const nextNumber = number + 1;
        `A string containing the ${nextNumber}.`;
    });
    
    // good
    [1, 2, 3].map(number => `A string containing the ${number + 1}.`);
    
    // good
    [1, 2, 3].map((number) => {
        const nextNumber = number + 1;
        return `A string containing the ${nextNumber}.`;
    });
    
    // good
    [1, 2, 3].map((number, index) => ({
        [index]: number,
    }));
    
    // No implicit return with side effects
    function foo(callback){
        const val = callback();
        if (val === true){
            // Do something if callback returns true
        }
    }
    
    let bool = false;
    
    // bad
    foo(() => bool = true);
    
    // good
    foo(() => {
        bool = true;
    });

  • 8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

    Why? It shows clearly where the function starts and ends.

    // bad
    ["get", "post", "put"].map(httpMethod => Object.prototype.hasOwnProperty.call(
            httpMagicObjectWithAVeryLongName,
            httpMethod,
        )
    );
    
    // good
    ["get", "post", "put"].map(httpMethod => (
        Object.prototype.hasOwnProperty.call(
            httpMagicObjectWithAVeryLongName,
            httpMethod,
        )
    ));

  • 8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments for clarity and consistency. Note: it is also acceptable to always use parentheses, in which case use the “always” option for eslint. eslint: arrow-parens

    Why? Less visual clutter.

    // bad
    [1, 2, 3].map((x) => x * x);
    
    // good
    [1, 2, 3].map(x => x * x);
    
    // good
    [1, 2, 3].map(number => (
        `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
    ));
    
    // bad
    [1, 2, 3].map(x => {
        const y = x + 1;
        return x * y;
    });
    
    // good
    [1, 2, 3].map((x) => {
        const y = x + 1;
        return x * y;
    });

  • 8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow

    // bad
    const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize;
    
    // bad
    const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize;
    
    // good
    const itemHeight = item => (item.height > 256 ? item.largeSize : item.smallSize);
    
    // good
    const itemHeight = (item) => {
        const { height, largeSize, smallSize } = item;
        return height > 256 ? largeSize : smallSize;
    };

  • 8.6 Enforce the location of arrow function bodies with implicit returns. eslint: implicit-arrow-linebreak

    // bad
    (foo) =>
        bar;
    
    (foo) =>
        (bar);
    
    // good
    (foo) => bar;
    (foo) => (bar);
    (foo) => (
         bar
    )

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Classes & Constructors

  • 9.1 Always use class. Avoid manipulating prototype directly.

    Why? class syntax is more concise and easier to reason about.

    // bad
    function Queue(contents = []){
        this.queue = [...contents];
    }
    
    Queue.prototype.pop = function(){
        const value = this.queue[0];
        this.queue.splice(0, 1);
        return value;
    };
    
    // good
    class Queue {
        constructor(contents = []){
            this.queue = [...contents];
        }
        pop(){
            const value = this.queue[0];
            this.queue.splice(0, 1);
            return value;
        }
    }

  • 9.2 Use extends for inheritance.

    Why? It is a built-in way to inherit prototype functionality without breaking instanceof.

    // bad
    const inherits = require("inherits");
    function PeekableQueue(contents){
        Queue.apply(this, contents);
    }
    
    inherits(PeekableQueue, Queue);
    PeekableQueue.prototype.peek = function(){
        return this.queue[0];
    };
    
    // good
    class PeekableQueue extends Queue {
        peek(){
            return this.queue[0];
        }
    }

  • 9.3 Methods can return this to help with method chaining.

    // bad
    Player.prototype.jump = function(){
        this.jumping = true;
        return true;
    };
    
    Player.prototype.setHeight = function(height){
        this.height = height;
    };
    
    const foo = new Player();
    foo.jump(); // => true
    foo.setHeight(20); // => undefined
    
    // good
    class Player {
        jump(){
            this.jumping = true;
            return this;
        }
    
        setHeight(height){
            this.height = height;
            return this;
        }
    }
    
    const foo = new Player();
    
    foo.jump().setHeight(20);

  • 9.4 It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

    class Player {
        constructor(options = {}) {
            this.name = options.name || "no name";
        }
    
        getName(){
            return this.name;
        }
    
        toString(){
            return `Player - ${this.getName()}`;
        }
    }

  • 9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint: no-useless-constructor

    // bad
    class Player {
        constructor() {}
    
        getName(){
            return this.name;
        }
    }
    
    // bad
    class Foo extends Player {
        constructor(...args){
            super(...args);
        }
    }
    
    // good
    class Foo extends Player {
        constructor(...args){
            super(...args);
            this.name = "Foo";
        }
    }

  • 9.6 Avoid duplicate class members. eslint: no-dupe-class-members

    Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

    // bad
    class Foo {
        bar(){ return 1; }
        bar(){ return 2; }
    }
    
    // good
    class Foo {
        bar(){ return 1; }
    }
    
    // good
    class Foo {
        bar(){ return 2; }
    }

  • 9.7 Class methods should use this or be made into a static method unless an external library or framework requires to use specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint: class-methods-use-this

    // bad
    class Foo {
        bar(){
            console.log("bar");
        }
    }
    
    // good - this is used
    class Foo {
        bar(){
            console.log(this.bar);
        }
    }
    
    // good - constructor is exempt
    class Foo {
        constructor(){
            // ...
        }
    }
    
    // good - static methods aren't expected to use this
    class Foo {
        static bar(){
            console.log("bar");
        }
    }

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Modules

  • 10.1 Stick to CommonJS Imports (RequireJS / module.exports).

    Why? Not many of the current browser engines implements import/export from the ES6 standard. In NodeJS, require() is still the standard way of importing modules. Plus, in NodeJS you can make use of dynamic module loading and Require's API's in general, where you can control caching as well.

    // bad
    import foo from "bar";
    export default bar;
    
    // bad
    import { foo } from "bar";
    export default bar;
    
    // good
    let foo = require("bar");
    module.exports = bar;

  • 10.2 Only import from a path in one place. eslint: no-duplicate-imports

    Why? Having multiple lines that import from the same path can make code harder to maintain.

    // bad
    let foo = require("bar");
    // … some other imports … 
    let { foo2, foo3 } = require("bar");
    
    // good
    let { foo, foo1, foo2 } = require("bar");
    
    // good
    let {
        foo,
        foo1,
        foo2,
    } = require("bar");

  • 10.3 Do not export mutable bindings. eslint: import/no-mutable-exports

    Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.

    // bad
    let foo = 3;
    module.exports = foo;
    
    // good
    const foo = 3;
    module.exports = foo;

  • 10.4 Put all imports above non-import statements. eslint: import/first

    Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

    // bad
    let foo = require("foo");
    foo.init();
    
    let bar = require("bar");
    
    // good
    let foo = require("foo");
    let bar = require("bar");
    
    foo.init();

  • 10.5 Do not include JavaScript filename extensions eslint: import/extensions

    Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.

    // bad
    import foo from "./foo.js";
    import bar from "./bar.jsx";
    import baz from "./baz/index.jsx";
    
    // good
    import foo from "./foo";
    import bar from "./bar";
    import baz from "./baz";

  • 10.6 Multiline imports should be indented just like multiline array and object literals. eslint: object-curly-newline

    Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide.

    // bad
    let { longNameA, longNameB, longNameC, longNameD, longNameE } = require("path");
    
    // good
    let {
        longNameA,
        longNameB,
        longNameC,
        longNameD,
        longNameE,
    } = require("path");

Iterators and Generators

  • 11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint: no-iterator no-restricted-syntax

    Why? Dealing with pure functions that return values is easier to reason about than side effects.

    Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.

    Note: for-in and for-of are mostly OK as well. Especially for small operations. Higher-order functions however are best practice.

    const numbers = [1, 2, 3, 4, 5];
    
    // ok...
    let sum = 0;
    for (let num of numbers){
        sum += num;
    }
    sum === 15;
    
    // good
    let sum = 0;
    numbers.forEach((num) => {
        sum += num;
    });
    sum === 15;
    
    // best (use the functional force)
    const sum = numbers.reduce((total, num) => total + num, 0);
    sum === 15;
    
    // bad
    const increasedByOne = [];
    for (let i = 0; i < numbers.length; i++){
        increasedByOne.push(numbers[i] + 1);
    }
    
    // good
    const increasedByOne = [];
    numbers.forEach((num) => {
        increasedByOne.push(num + 1);
    });
    
    // best (keeping it functional)
    const increasedByOne = numbers.map(num => num + 1);

  • 11.2 Use generators only in NodeJS for now.

    Why? They don’t transpile well to ES5.

  • 11.3 If you must use generators, or if you disagree with 11.2, make sure their function signature is spaced properly. eslint: generator-star-spacing

    Why? function and * are part of the same conceptual keyword - * is not a modifier for function, function* is a unique construct, different from function.

    // bad
    function * foo(){
        // ...
    }
    
    // bad
    let bar = function * (){
        // ...
    };
    
    // bad
    let baz = function *(){
        // ...
    };
    
    // bad
    let quux = function* (){
        // ...
    };
    
    // bad
    function*foo(){
        // ...
    }
    
    // bad
    function *foo(){
        // ...
    }
    
    // very bad
    function
    *
    foo(){
        // ...
    }
    
    // very bad
    let foo = function
    *
    (){
        // ...
    };
    
    // good
    function* foo(){
        // ...
    }
    
    // good
    let foo = function*(){
        // ...
    };

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Properties

  • 12.1 Use dot notation when accessing properties. eslint: dot-notation

    const foo = {
        bar: true,
        baz: 5,
    };
    
    // bad
    const isTrue = foo["bar"];
    
    // good
    const isTrue = foo.bar;

  • 12.2 Use bracket notation [] when accessing properties with a variable or if the key includes illegal characters.

    const foo = {
        bar: true,
        baz: 5,
        "test-1": "foo"
    };
    
    function getProp(prop) {
        return foo[prop];
    }
    
    const isTrue = getProp("bar");
    const bar = foo["test-1"];

  • 12.3 Use exponentiation operator ** when calculating exponentiations. eslint: no-restricted-properties.

    // bad
    const binary = Math.pow(2, 10);
    
    // good
    const binary = 2 ** 10;

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Variables

  • 13.1 Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Variables declared without const, let or var are disallowed in strict mode as well. eslint: no-undef prefer-const

    // bad
    foo = new Foo();
    
    // good
    const foo = new Foo();

  • 13.2 Use one const or let declaration per variable or assignment. eslint: one-var

    Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

    // bad
    const items = getItems(),
        foo = true,
        bar = "baz";
    
    // bad
    // (compare to above, and try to spot the mistake)
    const items = getItems(),
        foo = true;
        bar = "baz";
    
    // good
    const items = getItems();
    const foo = true;
    const bar = "baz";

  • 13.3 Group all your consts and then group all your lets.

    Why? This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.

    // bad
    let i, len, foo,
        items = getItems(),
        bar = true;
    
    // bad
    let i;
    const items = getItems();
    let foo;
    const bar = true;
    let len;
    
    // good
    const bar = true;
    const items = getItems();
    let foo;
    let i;
    let length;

  • 13.4 Assign variables where you need them, but place them in a reasonable place.

    Why? let and const are block scoped and not function scoped.

    // bad - unnecessary function call
    function checkName(hasName){
        const name = getName();
    
        if (hasName === "test") return false;
    
        if (name === "test"){
            this.setName("");
            return false;
        }
    
        return name;
    }
    
    // good
    function checkName(hasName){
        if (hasName === "test") return false;
    
        const name = getName();
    
        if (name === "test"){
            this.setName("");
            return false;
        }
    
        return name;
    }

  • 13.5 Don’t chain variable assignments. eslint: no-multi-assign

    Why? Chaining variable assignments creates implicit global variables.

    // bad
    (function example(){
        /**
         * JavaScript interprets this as
         * let a = ( b = ( c = 1 ) );
         * The let keyword only applies to variable a; variables b and c become
         * global variables.
         */
        let a = b = c = 1;
    }());
    
    console.log(a); // throws ReferenceError
    console.log(b); // 1
    console.log(c); // 1
    
    // good
    (function example(){
        let a = 1;
        let b = a;
        let c = a;
    }());
    
    console.log(a); // throws ReferenceError
    console.log(b); // throws ReferenceError
    console.log(c); // throws ReferenceError
    
    // the same applies for `const`

  • 13.6 Avoid using unary increments and decrements (++, --). eslint no-plusplus

    Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.

    // bad
    
    const array = [1, 2, 3];
    let num = 1;
    num++;
    --num;
    
    let sum = 0;
    let truthyCount = 0;
    for (let i = 0; i < array.length; i++){
        let value = array[i];
        sum += value;
        if (value) truthyCount++;
    }
    
    // good
    
    const array = [1, 2, 3];
    let num = 1;
    num += 1;
    num -= 1;
    
    const sum = array.reduce((a, b) => a + b, 0);
    const truthyCount = array.filter(Boolean).length;

  • 13.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

    Why? Linebreaks surrounding = can obfuscate the value of an assignment.

    // bad
    const foo =
      superLongLongLongLongLongLongLongLongFunctionName();
    
    // bad
    const foo
      = "superLongLongLongLongLongLongLongLongString";
    
    // good
    const foo = (
        superLongLongLongLongLongLongLongLongFunctionName()
    );
    
    // good
    const foo = "superLongLongLongLongLongLongLongLongString";

  • 13.8 Disallow unused variables. eslint: no-unused-vars

    Why? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

    // bad
    
    let some_unused_var = 42;
    
    // Write-only variables are not considered as used.
    let y = 10;
    y = 5;
    
    // A read for a modification of itself is not considered as used.
    let z = 0;
    z = z + 1;
    
    // Unused function arguments.
    function getX(x, y){
        return x;
    }
    
    // good
    
    function getXPlusY(x, y){
        return x + y;
    }
    
    let x = 1;
    let y = a + 2;
    
    alert(getXPlusY(x, y));
    
    // 'type' is ignored even if unused because it has a rest property sibling.
    // This is a form of extracting an object that omits the specified keys.
    let { type, ...coords } = data;
    // 'coords' is now the 'data' object without its 'type' property.

  • 13.9 Do not floor variables with the bitwise or (x | 0). Use Math.floor() or if you must Math.trunc().

    Why? First off, it does not floor the number. It truncates it (rounding towards 0). It causes odd Comparative behavior as well: Math.floor(NaN) === NaN, while (NaN | 0) === 0. Also, it works with 32-bit signed integers only. As mentioned above, use Math.trunc() if you have to. It is the ES5 equivalent of | 0 and it is able to work with numbers higher or equal to 2^31.

    let x = 42.835
    
    // Bad
    let y = x | 0;
    
    // Okay
    let y = Math.trunc(x);
    
    // Good 
    let y = Math.floor(x);

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Hoisting

  • 14.1 var declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It’s important to know why typeof is no longer safe.

    // this wouldn’t work (assuming there
    // is no notDefined global variable)
    function example(){
        console.log(notDefined); // => throws a ReferenceError
    }
    
    // creating a variable declaration after you
    // reference the variable will work due to
    // variable hoisting. Note: the assignment
    // value of `true` is not hoisted.
    function example(){
        console.log(declaredButNotAssigned); // => undefined
        var declaredButNotAssigned = true;
    }
    
    // the interpreter is hoisting the variable
    // declaration to the top of the scope,
    // which means our example could be rewritten as:
    function example(){
        let declaredButNotAssigned;
        console.log(declaredButNotAssigned); // => undefined
        declaredButNotAssigned = true;
    }
    
    // using const and let
    function example(){
        console.log(declaredButNotAssigned); // => throws a ReferenceError
        console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
        const declaredButNotAssigned = true;
    }

  • 14.2 Anonymous function expressions hoist their variable name, but not the function assignment.

    function example(){
        console.log(anonymous); // => undefined
    
        anonymous(); // => TypeError anonymous is not a function
    
        var anonymous = function(){
            console.log("anonymous function expression");
        };
    }

  • 14.3 Named function expressions hoist the variable name, not the function name or the function body.

    function example(){
        console.log(named); // => undefined
    
        named(); // => TypeError named is not a function
    
        foo(); // => ReferenceError foo is not defined
    
        var named = function foo(){
            console.log("bar");
        };
    }
    
    // the same is true when the function name
    // is the same as the variable name.
    function example(){
        console.log(named); // => undefined
    
        named(); // => TypeError named is not a function
    
        var named = function named(){
            console.log("named");
        };
    }

  • 14.4 Function declarations hoist their name and the function body.

    function example(){
        foo(); // => bar
    
        function foo(){
            console.log("bar");
        }
    }
  • For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

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Comparison Operators & Equality

  • 15.1 Use === and !== over == and != (Strict comparison). eslint: eqeqeq

  • 15.2 Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:

    • Objects evaluate to true
    • Undefined evaluates to false
    • Null evaluates to false
    • Booleans evaluate to the value of the boolean
    • Numbers evaluate to false if +0, -0, or NaN, otherwise true
    • Strings evaluate to false if an empty string '', otherwise true
    if ([0] && []){
        // true
        // an array (even an empty one) is an object, objects will evaluate to true
    }

  • 15.3 Use shortcuts for booleans, but explicit comparisons for strings and numbers.

    // bad
    if (isValid === true){
        // ...
    }
    
    // good
    if (isValid){
        // ...
    }
    
    // bad
    if (name){
        // ...
    }
    
    // good
    if (name !== ""){
        // ...
    }
    
    // bad
    if (collection.length){
        // ...
    }
    
    // good
    if (collection.length > 0){
        // ...
    }

  • 15.5 Use braces to create blocks in case and default clauses. This is especially useful for those, which contain lexical declarations (e.g. let, const, function, and class). eslint: no-case-declarations

    Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

    // bad
    switch (foo){
        case 1:
            let x = 1;
            break;
        case 2:
            const y = 2;
            break;
        case 3:
            function f(){
                // ...
            }
            break;
        default:
            class C {}
    }
    
    // good
    switch (foo) {
        case 1: {
            let x = 1;
            break;
        }
        case 2: {
            const y = 2;
            break;
        }
        case 3: {
            function f(){
                // ...
            }
            break;
        }
        case 4: {
            bar();
            break;
        }
        default: {
            class C {}
        }
    }

  • 15.6 Ternaries should not be nested and generally be single line expressions. eslint: no-nested-ternary

    // bad
    const foo = maybe1 > maybe2
      ? "bar"
      : value1 > value2 ? "baz" : null;
    
    // split into 2 separated ternary expressions
    const maybeNull = value1 > value2 ? "baz" : null;
    
    // better
    const foo = maybe1 > maybe2
      ? "bar"
      : maybeNull;
    
    // best
    const foo = maybe1 > maybe2 ? "bar" : maybeNull;

  • 15.7 Avoid unneeded ternary statements. eslint: no-unneeded-ternary

    // bad
    const foo = a ? a : b;
    const bar = c ? true : false;
    const baz = c ? false : true;
    
    // good
    const foo = a || b;
    const bar = !!c;
    const baz = !c;

  • 15.8 When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators (+, -, *, & /) since their precedence is broadly understood. eslint: no-mixed-operators

    Why? This improves readability and clarifies the developer’s intention.

    // bad
    const foo = a && b < 0 || c > 0 || d + 1 === 0;
    
    // bad
    const bar = a ** b - 5 % d;
    
    // bad
    // one may be confused into thinking (a || b) && c
    if (a || b && c){
        return d;
    }
    
    // good
    const foo = (a && b < 0) || c > 0 || (d + 1 === 0);
    
    // good
    const bar = (a ** b) - (5 % d);
    
    // good
    if (a || (b && c)){
        return d;
    }
    
    // good
    const bar = a + b / c * d;

⬆ back to top

Blocks

  • 16.1 Use braces with all multi-line blocks. eslint: nonblock-statement-body-position

    Note: Use one-line statements only with short commands. Like if (x) y();
    They look nicer but can be hard to debug

    // bad
    if (test)
      return false;
    
    // good
    if (test) return false;
    
    // bad - it is too bulky and doesn't fit nicely in one row
    if (test1) if (test2) stuff();
    
    // good
    if (test1){
        if (test2) stuff();
    }
    
    // good (multiline)
    if (test){
        doStuff();
        // ...
        return false;
    }
    
    // bad
    function foo(){ return false; }
    
    // good
    function bar(){
        return false;
    }

  • 16.2 If you're using multi-line blocks with if and else, do not put else on the same line as your if block’s closing brace. eslint: brace-style

    // bad
    if (test){
        thing1();
        thing2();
    } else {
        thing3();
        thing4();
    }
    
    // good
    if (test){
        thing1();
        thing2();
    }
    else {
        thing3();
        thing4();
    }
    
    // bad 
    if (test){
        thing1();
        thing2();
    } else thing3();
    
    // good
    if (test){
        thing1();
        thing2();
    } 
    else thing3();

  • 16.3 If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

    // bad
    function foo(){
        if (x) return x;
        else return y;
    }
    
    // bad
    function bar(){
        if (x) return x;
        else if (y) return y;
    }
    
    // bad
    function baz(){
        if (x) return x;
        else {
            if (y) return y;
        }
    }
    
    // good
    function foo(){
        if (x) return x;
        return y;
    }
    
    // good
    function bar(){
        if (x) return x;
        if (y) return y;
    }
    
    // good
    function baz(x){
        if (x){
            if (z) return y;
        } 
        else return z;
    }

⬆ back to top

Control Statements

  • 17.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.

    Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.

    // bad
    if ((foo === 123 || bar === "abc") && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()){
        thing1();
    }
    
    // bad
    if (foo === 123 &&
        bar === "abc"){
        thing1();
    }
    
    // bad
    if (foo === 123
        && bar === "abc"){
        thing1();
    }
    
    // bad
    if (
        foo === 123 &&
        bar === "abc"
    ){
        thing1();
    }
    
    // good
    if (
        foo === 123
        && bar === "abc"
    ){
        thing1();
    }
    
    // good
    if (
        (foo === 123 || bar === "abc")
        && doesItLookGoodWhenItBecomesThatLong()
        && isThisReallyHappening()
    ){
        thing1();
    }
    
    // good
    if (foo === 123 && bar === "abc"){
        thing1();
    }

  • 17.2 Don't use selection operators in place of control statements.

    // bad
    !isRunning && startRunning();
    
    // good
    if (!isRunning) startRunning();

⬆ back to top

Comments

  • 18.1 Stick to the english language. Always write variable names, function names, comments and co in english.

    Why? Some reasons:

    • Consistency.
    • English is a global language. What if you're part of a german developer team, write code in german and then want to hire someone from another country?
    • JavaScript's keywords are english.
    • Some languages use symbols from different charsets (ö, ä, ü, ß, Ѱ, Ω, etc. pp.). Some of them are illegal as variable/function names and others could break your encoding.

  • 18.2 Use /** ... */ for multi-line comments.

    // bad
    // make() returns a new element
    // based on the passed in tag name
    //
    // @param {String} tag
    // @return {Element} element
    function make(tag){
    
        // ...
    
        return element;
    }
    
    // good
    /**
     * make() returns a new element
     * based on the passed-in tag name
     */
    function make(tag){
    
        // ...
    
        return element;
    }

  • 18.3 Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block.

    // bad
    const active = true;  // is current tab
    
    // good
    // is current tab
    const active = true;
    
    // bad
    function getType(){
        console.log("fetching type...");
        // set the default type to 'no type'
        const type = this.type || "no type";
    
        return type;
    }
    
    // good
    function getType(){
        console.log("fetching type...");
    
        // set the default type to 'no type'
        const type = this.type || "no type";
    
        return type;
    }
    
    // also good
    function getType(){
        // set the default type to 'no type'
        const type = this.type || "no type";
    
        return type;
    }

  • 18.4 Start all comments with a space to make it easier to read. eslint: spaced-comment

    // bad
    //is current tab
    const active = true;
    
    // good
    // is current tab
    const active = true;
    
    // bad
    /**
     *make() returns a new element
     *based on the passed-in tag name
     */
    function make(tag){
    
        // ...
    
        return element;
    }
    
    // good
    /**
     * make() returns a new element
     * based on the passed-in tag name
     */
    function make(tag){
    
        // ...
    
        return element;
    }

  • 18.5 Prefixing your comments with FIXME or TODO (action-items) helps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: -- need to figure this out or TODO: -- need to implement.

  • 18.6 Use // FIXME: to annotate problems.

    class Calculator extends Abacus {
        constructor(){
            super();
    
            // FIXME: shouldn’t use a global here
            total = 0;
        }
    }

  • 18.7 Use // TODO: to annotate solutions to problems.

    class Calculator extends Abacus {
        constructor(){
            super();
    
            // TODO: total should be configurable by an options param
            this.total = 0;
        }
    }

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Whitespace

  • 19.1 Use soft tabs (space character instead of tabulator) set to 4 spaces. eslint: indent

    // bad
    function foo(){
    ∙∙let name;
      let foo;
    }
    
    // bad
    function bar(){
    ∙let name;
     let foo;
    }
    
    // good
    function baz(){
    ∙∙∙∙let name;
        let foo;
    }

  • 19.2 Place 1 space before the leading brace if the character before it is not a parenthesis. eslint: space-before-blocks

    Why? Technically speaking, the block is part of the expression. A space doesn't really make sense unless it is simpler to read.

    // bad
    function test() {
        console.log("test");
    }
    
    // good
    function test(){
        console.log("test");
    }
    
    // bad
    foo.set("attr",{
        bar: "baz"
    });
    
    // good
    foo.set("attr", {
        bar: "baz"
    });
    
    // bad
    class Foo{
        // ...
    }
    
    // good 
    class Foo {
        // ...
    }
    
    // bad
    constructor() {
        // ...
    }
    
    // good
    constructor(){
        // ...
    }
    
    // bad
    if (foo) {
        bar();
    }
    else{
        baz();
    }
    
    // good
    if (foo){
        bar();
    }
    else {
        baz();
    }
    
    // bad
    let x ={
        foo: "bar",
    };
    
    // good
    let x = {
        foo: "bar",
    };

  • 19.3 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: keyword-spacing

    // bad
    if(foo){
        bar ();
    }
    
    // good
    if (foo){
        bar();
    }
    
    // bad
    function foo (){
        console.log ("bar");
    }
    
    // good
    function foo(){
        console.log("bar");
    }

  • 19.4 Set off operators with spaces. eslint: space-infix-ops

    // bad
    const x=y+5;
    
    // good
    const x = y + 5;

  • 19.5 Use Unix/Linux-Style Linebreaks - LF (\n) instead of CR + LF (\r\n). eslint: linebreak-style

  • 19.6 End files with a single newline character. eslint: eol-last

    Why? Because that's how the POSIX standard defines a line. Read more...

    // bad
    let foo = require("foo");
    // ...
    module.exports = foo;
    // bad
    let foo = require("foo");
    // ...
    module.exports = foo;
    
    // good
    let foo = require("foo");
    // ...
    module.exports = foo;

  • 19.7 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which emphasizes that the line is a method call, not a new statement. eslint: newline-per-chained-call no-whitespace-before-property

    // bad
    $("#items").find(".selected").highlight().end().find(".open").updateCount();
    
    // bad
    $("#items").
        find(".selected").
            highlight().
            end().
        find(".open").
            updateCount();
    
    // good
    $("#items")
        .find(".selected")
            .highlight()
            .end()
        .find(".open")
            .updateCount();
    
    // bad
    const leds = stage.selectAll(".led").data(data).enter().append("svg:svg").classed("led", true)
        .attr("width", (radius + margin) * 2).append("svg:g")
        .attr("transform", `translate(${radius + margin}, ${radius + margin})`)
        .call(tron.led);
    
    // good
    const leds = stage.selectAll(".led")
            .data(data)
        .enter().append("svg:svg")
            .classed("led", true)
            .attr("width", (radius + margin) * 2)
        .append("svg:g")
            .attr("transform", `translate(${radius + margin}, ${radius + margin})`)
            .call(tron.led);
    
    // good
    const leds = stage.selectAll(".led").data(data);

  • 19.8 Leave a blank line after blocks and before the next statement.

    // bad
    if (foo){
        return bar;
    }
    return baz;
    
    // good
    if (foo){
        return bar;
    }
    
    return baz;
    
    // bad
    const obj = {
        foo(){
        },
        bar(){
        }
    };
    return obj;
    
    // good
    const obj = {
        foo(){
        },
    
        bar(){
        }
    };
    
    return obj;
    
    // bad
    const arr = [
        function foo(){
        },
        function bar(){
        }
    ];
    return arr;
    
    // good
    const arr = [
        function foo(){
        },
    
        function bar(){
        }
    ];
    
    return arr;

  • 19.9 Do not pad your blocks with blank lines. eslint: padded-blocks

    // bad
    function bar(){
    
        console.log(foo);
    
    }
    
    // bad
    if (baz){
    
        console.log(qux);
    } 
    else {
        console.log(foo);
    
    }
    
    // bad
    class Foo {
    
        constructor(bar){
            this.bar = bar;
        }
    }
    
    // good
    function bar(){
        console.log(foo);
    }
    
    // good
    if (baz){
        console.log(qux);
    } 
    else {
        console.log(foo);
    }

  • 19.10 Do not add spaces inside parentheses. eslint: space-in-parens

    // bad
    function bar( foo ){
        return foo;
    }
    
    // good
    function bar(foo){
        return foo;
    }
    
    // bad
    if ( foo ){
        console.log(foo);
    }
    
    // good
    if (foo){
        console.log(foo);
    }

  • 19.11 Do not add spaces inside brackets. eslint: array-bracket-spacing

    // bad
    const foo = [ 1, 2, 3 ];
    console.log(foo[ 0 ]);
    
    // good
    const foo = [1, 2, 3];
    console.log(foo[0]);

  • 19.12 Add spaces inside curly braces. eslint: object-curly-spacing

    // bad
    const foo = {foo: "bar"};
    
    // good
    const foo = { foo: "bar" };

  • 19.13 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint: max-len

    Why? This ensures readability and maintainability.

    // bad
    const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;
    
    // bad
    $.ajax({ method: "POST", url: "https://nulldev.org", data: { foo: "bar" } }).done(() => console.log("Done")).fail(() => console.log("Error"));
    
    // good
    const foo = jsonData
        && jsonData.foo
        && jsonData.foo.bar
        && jsonData.foo.bar.baz
        && jsonData.foo.bar.baz.quux
        && jsonData.foo.bar.baz.quux.xyzzy;
    
    // good
    $.ajax({
        method: "POST", 
        url: "https://nulldev.org", 
        data: { 
            foo: "bar" 
        },
    }).done(() => {
        console.log("Done");
    }).fail(() => {
        console.log("Error");
    });

  • 19.14 Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: block-spacing

    // bad
    function foo(){return true;}
    if (foo){ bar = 0;}
    
    // good
    function foo(){ return true; }
    if (foo){ bar = 0; }

  • 19.15 Avoid spaces before commas and require a space after commas. eslint: comma-spacing

    // bad
    let foo = 1,bar = 2;
    let arr = [1 , 2];
    
    // good
    let foo = 1, bar = 2;
    let arr = [1, 2];

  • 19.16 Enforce spacing inside of computed property brackets. eslint: computed-property-spacing

    // bad
    obj[foo ]
    obj[ "foo"]
    let x = {[ b ]: a}
    obj[foo[ bar ]]
    
    // good
    obj[foo]
    obj["foo"]
    let x = { [b]: a }
    obj[foo[bar]]

  • 19.17 Avoid spacing between functions and their invocations. eslint: func-call-spacing

    // bad
    func ();
    
    func
    ();
    
    // good
    func();

  • 19.18 Enforce spacing between keys and values in object literal properties. eslint: key-spacing

    // bad
    let obj = { "foo" : 42 };
    let obj2 = { "foo":42 };
    
    // good
    let obj = { "foo": 42 };

  • 19.20 Avoid multiple empty lines and only allow one newline at the end of files. eslint: no-multiple-empty-lines

    // bad
    let x = 1;
    
    
    
    let y = 2;
    
    // good
    let x = 1;
    
    let y = 2;

⬆ back to top

Commas

  • 20.1 Do not write leading commas. eslint: comma-style

    // bad
    const x = [
          foo
        , bar
        , baz
    ];
    
    // good
    const x = [
        foo,
        bar,
        baz,
    ];
    
    // bad
    const x = {
          foo: "foo1"
        , bar: "bar1"
        , baz: "baz1"
        , abc: "abc1"
    };
    
    // good
    const x = {
        foo: "foo1",
        bar: "bar1",
        baz: "baz1",
        abc: "abc1",
    };

  • 20.2 Write additional trailing commas. eslint: comma-dangle

    Why? It leads to cleaner git diffs and allows easier copy-pasting. Careful: A comma must not appear after a rest element. Read more...

    // git diff without trailing comma (bad)
    const hero = {
         bar: "bar",
    -    baz: "baz"
    +    baz: "baz",
    +    abc: [1, 2, 3]
    };
    
    // git diff with trailing comma (good)
    const foo = {
         bar: "bar",
         baz: "baz",
    +    abc: [1, 2, 3],
    };
    // bad
    const foo = {
        bar: true,
        baz: false
    };
    
    const foo = [
        "bar",
        "baz"
    ];
    
    // good
    const foo = {
        bar: true,
        baz: false,
    };
    
    const foo = [
        "bar",
        "baz",
    ];
    
    // bad
    function foo(
        arg1,
        arg2,
        agr3
    ){
        // ..
    }
    
    // good
    function foo(
        arg1,
        arg2,
        agr3,
    ){
        // ..
    }
    
    // bad
    createUser(
        firstName,
        lastName,
        birthday
    );
    
    // good
    createUser(
        firstName,
        lastName,
        birthday,
    );

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Semicolons

  • 21.1 Use semicolons. eslint: semi

    Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether or not it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues. In other words: You could say ASI is a feature to help out, when you forget a semicolon. This doesn't mean you shouldn't use them. Not using semicolons may also slow down the execution because of the additional parsing.

    // bad - raises exception
    const foo = {}
    const bar = {}
    [foo, bar].forEach(baz => baz.x = "test")
    
    // bad - raises exception
    const abc = "Another test"
    (async function tempFoo(){
        // ...
    }())
    
    // bad - returns `undefined` instead of the value on the next line
    // this always happens when `return` is on a line by itself because of ASI!
    function tempBar(){
      return
          "Some string..."
    }
    
    // good
    const foo = {};
    const bar = {};
    [foo, bar].forEach((baz) => {
        baz.x = "test";
    });
    
    // good
    const abc = "Another test";
    (async function tempFoo(){
        // ...
    }());
    
    // good
    function tempBar(){
        return "Some string...";
    }

    Read more.

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Type Casting & Coercion

  • 22.1 Perform type coercion at the beginning of the statement.

  • 22.2 Strings: Prefer String() over .toString() eslint: no-new-wrappers

    Why? .toString() is a prototype of Number. String() on the other hand, is globally available and thus allows casting of any type. Also, .toString() can be overridden as seen in section 9.4

    // => this.reviewScore = 9;
    
    // bad
    const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"
    
    // bad
    const totalScore = this.reviewScore + ""; // invokes this.reviewScore.valueOf()
    
    // bad
    const totalScore = this.reviewScore.toString(); // isn't guaranteed to return a string
    
    // good
    const totalScore = String(this.reviewScore);

  • 22.3 Numbers: Use Number() for type casting and parseInt() only with a radix for parsing strings. Do prefer Number() over parseInt() though. eslint: radix no-new-wrappers

    Why? Mostly because of the same reasons listed in the section above. Also, since parseInt() always expects a string, it does show odd behaviour when parsing very small numbers (source)

    const inputValue = "4";
    
    // bad
    const val = new Number(inputValue);
    
    // bad
    const val = +inputValue;
    
    // bad
    const val = inputValue >> 0;
    
    // bad
    const val = parseInt(inputValue);
    
    // best
    const val = Number(inputValue);
    
    // good
    const val = parseInt(inputValue, 10);

  • 22.4 If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you're doing.

    // good
    /**
     * parseInt was the reason my code was slow.
     * Bitshifting the String to coerce it to a
     * Number made it a lot faster.
     */
    const val = inputValue >> 0;

  • 22.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. More info. Largest signed 32-bit Int is 2,147,483,647:

    2147483647 >> 0; // => 2147483647
    2147483648 >> 0; // => -2147483648
    2147483649 >> 0; // => -2147483647

  • 22.6 Booleans: eslint: no-new-wrappers

    const age = 0;
    
    // bad
    const hasAge = new Boolean(age);
    
    // good
    const hasAge = Boolean(age);
    
    // best
    const hasAge = !!age;

  • 22.6 Only compare returned strings by typeof to valid strings: eslint: valid-typeof

    // bad - will be prevented by linter
    typeof foo === "strnig";
    
    // good
    typeof foo === "string";
    typeof bar === typeof foo;

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Naming Conventions

  • 23.0 Stick to the english language. Always write variable names, function names, comments and co in english.

    Why? Some reasons:

    • Consistency.
    • English is a global language. What if you're part of a german developer team, write code in german and then want to hire someone from another country?
    • JavaScript's keywords are english.
    • Some languages use symbols from different charsets (ö, ä, ü, ß, Ѱ, Ω, etc. pp.). Some of them are illegal as variable/function names and others could break your encoding.

  • 23.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length

    // bad
    function q(){
        // ...
    }
    
    // good
    function query(){
        // ...
    }

  • 23.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase

    // bad
    const OBJEcttsssss = {};
    const this_is_my_object = {};
    function c(){}
    
    // good
    const thisIsMyObject = {};
    function thisIsMyFunction(){}

  • 23.3 Use PascalCase only when naming constructors or classes. eslint: new-cap

    // bad
    function user(options){
        this.name = options.name;
    }
    
    const bad = new user({
        name: "...",
    });
    
    // good
    class User {
        constructor(options){
            this.name = options.name;
        }
    }
    
    const good = new User({
        name: "...",
    });

  • 23.4 Do not use trailing or leading underscores. eslint: no-underscore-dangle

    Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.

    // bad
    this.__firstName__ = "Foo";
    this.firstName_ = "Foo";
    this._firstName = "Foo";
    
    // good
    this.firstName = "Foo";
    
    // good, in environments where WeakMaps are available
    // see https://kangax.github.io/compat-table/es6/#test-WeakMap
    const firstNames = new WeakMap();
    firstNames.set(this, "Foo");

  • 23.5 Don’t save references to this. Use arrow functions or Function#bind.

    // bad
    function foo(){
        const self = this;
        return function(){
            console.log(self);
        };
    }
    
    // bad
    function foo(){
        const that = this;
        return function(){
            console.log(that);
        };
    }
    
    // good
    function foo(){
        return () => {
            console.log(this);
        };
    }

  • 23.6 A base filename should exactly match the name of its default export.

    // file 1 contents
    class CheckBox {
        // ...
    }
    module.exports = CheckBox;
    
    // file 2 contents
    module.exports = function fortyTwo(){ return 42; }
    
    // file 3 contents
    module.exports = function insideDirectory(){}
    
    // in some other file
    // bad
    let CheckBox = require("./checkBox"); // PascalCase import/export, camelCase filename
    let FortyTwo = require("./FortyTwo"); // PascalCase import/filename, camelCase export
    let InsideDirectory = require("./InsideDirectory"); // PascalCase import/filename, camelCase export
    
    // bad
    let CheckBox = require("./check_box"); // PascalCase import/export, snake_case filename
    let forty_two = require("./forty_two"); // snake_case import/filename, camelCase export
    let inside_directory = require("./inside_directory"); // snake_case import, camelCase export
    let index = require("./inside_directory/index"); // requiring the index file explicitly
    let insideDirectory = require("./insideDirectory/index"); // requiring the index file explicitly
    
    // good
    let CheckBox = require("./CheckBox"); // PascalCase export/import/filename
    let fortyTwo = require("./fortyTwo"); // camelCase export/import/filename
    let insideDirectory = require("./insideDirectory"); // camelCase export/import/directory name/implicit "index"
    // ^ supports both insideDirectory.js and insideDirectory/index.js

  • 23.7 Use camelCase when you export-default a function. Your filename should be identical to your function’s name.

    function makeStyleGuide(){
        // ...
    }
    
    module.exports = makeStyleGuide;

  • 23.8 Use PascalCase when you export a constructor / class / singleton / function library / bare object.

    const Foo = {
        bar: {
        }
    };
    
    module.exports = Foo;

  • 23.9 Acronyms and initialisms should always be all capitalized, or all lowercased.

    Why? Names are for readability, not to appease a computer algorithm.

    // bad
    let SmsContainer = require("./containers/SmsContainer");
    
    // bad
    const HttpRequests = [
        // ...
    ];
    
    // good
    let SMSContainer = require("./containers/SMSContainer");
    
    // good
    const HTTPRequests = [
        // ...
    ];
    
    // also good (sticks to camelCase)
    const httpRequests = [
        // ...
    ];
    
    // best
    let TextMessageContainer = require("./containers/TextMessageContainer");
    
    // best
    const requests = [
        // ...
    ];

  • 23.10 You may optionally uppercase a constant only if it (1) is exported, (2) is a const (it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change.
    Note: This is about ES6 import/export, not CommonJS require()

    Why? This is an additional tool to assist in situations where the programmer would be unsure if a variable might ever change. UPPERCASE_VARIABLES are letting the programmer know that they can trust the variable (and its properties) not to change.

    • What about all const variables? This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however.
    • What about exported objects? Uppercase at the top level of export (e.g. EXPORTED_OBJECT.key) and maintain that all nested properties do not change.
    // bad
    const PRIVATE_VARIABLE = "should not be unnecessarily uppercased within a file";
    
    // bad
    export const THING_TO_BE_CHANGED = "should obviously not be uppercased";
    
    // bad
    export let REASSIGNABLE_VARIABLE = "do not use let with uppercase variables";
    
    // ---
    
    // allowed but does not supply semantic value
    export const apiKey = "SOMEKEY";
    
    // better in most cases
    export const API_KEY = "SOMEKEY";
    
    // ---
    
    // bad - unnecessarily uppercases key while adding no semantic value
    export const MAPPING = {
        KEY: "value",
    };
    
    // good
    export const MAPPING = {
        key: "value",
    };

  • 23.11 Write isCondition instead of conditionState for boolean state checks.

    Why? It makes the intentions clear.

    // bad - this is very vague
    if (activeState) doSomething();
    
    // bad - the "=== true" check makes the intentions clear, but is also unnecessary
    if (activeState === true) doSomething();
    
    // good - short and clear
    if (isActive) doSomething();

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Accessors

  • 24.1 Accessor functions for properties are not required.

  • 24.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal("foo").

    // bad
    class Foo {
        get bar(){
            // ...
        }
    
        set bar(value){
            // ...
        }
    }
    
    // good
    class Foo {
        getBar(){
            // ...
        }
    
        setBar(value){
            // ...
        }
    }

  • 24.3 If the property/method is a boolean, use isVal() or hasVal().

    // bad
    if (!foo.bar()){
        return false;
    }
    
    // good
    if (!foo.hasBar()){
        return false;
    }

  • 24.4 It’s okay to create get() and set() functions, but be consistent.

    class Foo {
        constructor(options = {}){
            const bar = options.bar || "Default bar";
            this.set("bar", bar);
        }
    
        set(key, val){
            this[key] = val;
        }
    
        get(key){
            return this[key];
        }
    }

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Events

  • 25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as a "hash") instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:

    // bad
    $(this).trigger("listingUpdated", listing.id);
    
    // ...
    
    $(this).on("listingUpdated", (e, listingID) => {
        // do something with listingID
    });

    prefer:

    // good
    $(this).trigger("listingUpdated", { 
        listingID: listing.id 
    });
    
    // ...
    
    $(this).on("listingUpdated", (e, data) => {
        // do something with data.listingID
    });

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Standard Library

The Standard Library contains utilities that are functionally broken but remain for legacy reasons.

  • 26.1 Use Number.isNaN instead of global isNaN. eslint: no-restricted-globals

    Why? The global isNaN coerces non-numbers to numbers, returning true for anything that coerces to NaN. If this behavior is desired, make it explicit.

    // bad
    isNaN("1.2"); // false
    isNaN("1.2.3"); // true
    
    // good
    Number.isNaN("1.2.3"); // false
    Number.isNaN(Number("1.2.3")); // true

  • 26.2 Use Number.isFinite instead of global isFinite. eslint: no-restricted-globals

    Why? The global isFinite coerces non-numbers to numbers, returning true for anything that coerces to a finite number. If this behavior is desired, make it explicit.

    // bad
    isFinite("2e3"); // true
    
    // good
    Number.isFinite("2e3"); // false
    Number.isFinite(parseInt("2e3", 10)); // true

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jQuery

  • 27.1 Prefix jQuery object variables with a $.

    // bad
    const sidebar = $(".sidebar");
    
    // good
    const $sidebar = $(".sidebar");
    
    // good
    const $sidebarBtn = $(".sidebar-btn");

  • 27.2 Cache jQuery lookups.

    // bad
    function setSidebar(){
        $(".sidebar").hide();
    
        // ...
    
        $(".sidebar").css({
            "background-color": "green",
        });
    }
    
    // good
    function setSidebar(){
        const $sidebar = $(".sidebar");
        $sidebar.hide();
    
        // ...
    
        $sidebar.css({
            "background-color": "green",
        });
    }

  • 27.3 For DOM queries use Cascading $(".sidebar ul") or parent > child $(".sidebar > ul"). jsPerf

  • 27.4 Use find with scoped jQuery object queries.

    // bad
    $("ul", ".sidebar").hide();
    
    // bad
    $(".sidebar").find("ul").hide();
    
    // good
    $(".sidebar ul").hide();
    
    // good
    $(".sidebar > ul").hide();
    
    // good
    $sidebar.find("ul").hide();

  • 27.5 Use .on instead of the event name (shorthands) when doing bindings.

    Why? .on() works on dynamically added elements and is better in performance. Read more...

    // bad
    $("a.test").click(function(){
        // ...
    });
    
    $("div.test").mouseover(function(){
        // ...
    });
    
    // good
    $("a.test").on("click", function(){
        // ...
    });
    
    $("div.test").on("mouseover", function(){
        // ...
    });

  • 27.6 Don't bloat the $(document).ready() function

    Why? It actively harms readability and generally the structure of the code.

    // Bad
    $(document).ready(function(){
        $(foo).on("click", function(){
            doStuff();
            doMoreStuff();
            doEvenMoreStuff(function(stuff){
                // ...
            });
        });
        
        $(bar).on("click", function(){
            doMoreStuff();
            doStuff();
        });
        
        $(baz).on("click", function(){
            doMoreStuff();
            doEvenMoreStuff(function(stuff){
                // ...
            });
        });
    });
    
    // Good
    function stuffHandler(){
        // ...
    }
    
    $(document).ready(function(){
        $(foo).on("click", stuffHandler);
        $(bar).on("click", stuffHandler);
        $(baz).on("click", stuffHandler);
    });

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ECMAScript 5 Compatibility

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ECMAScript 6+ (ES 2015+) Styles

  • 29.1 This is a collection of links to the various ES6+ features.
  1. Arrow Functions
  2. Classes
  3. Object Shorthand
  4. Object Concise
  5. Object Computed Properties
  6. Template Strings
  7. Destructuring
  8. Default Parameters
  9. Rest
  10. Array Spreads
  11. Let and Const
  12. Exponentiation Operator
  13. Iterators and Generators
  14. Modules

  • 29.2 Do not use TC39 proposals that have not reached stage 3.

    Why? They are not finalized, and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.

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Testing

  • 30.1 Tests
    • Whichever testing framework you use, you should be writing tests!
    • Strive to write many small pure functions, and minimize where mutations occur.
    • Be cautious about stubs and mocks - they can make your tests more brittle.
    • Recommendations: mocha and jest or tape for small, separate modules.
    • 100% test coverage is a good goal to strive for, even if it’s not always practical to reach it.
    • Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

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Performance

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Resources

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Copyright

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Amendments

  • 34.1 We encourage you to fork this guide and change the rules to fit your team’s style guide. 😸

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NullDev JavaScript Styleguide