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New: Add no-dupe-else-if rule (fixes #12469) #12504

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merged 4 commits into from Nov 18, 2019
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New: Add no-dupe-else-if rule (fixes #12469) #12504

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New: Add no-dupe-else-if rule (fixes #12469)
mdjermanovic Oct 29, 2019
f07ea75
Add || and && logic
mdjermanovic Nov 5, 2019
d9f0d66
Rename variables
mdjermanovic Nov 13, 2019
ec49da1
Update lib/rules/no-dupe-else-if.js
mdjermanovic Nov 18, 2019
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130 changes: 130 additions & 0 deletions docs/rules/no-dupe-else-if.md
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# Disallow duplicate conditions in `if-else-if` chains (no-dupe-else-if)

`if-else-if` chains are commonly used when there is a need to execute only one branch or at most one branch out of several possible branches, based on certain conditions.

```js
if (a) {
foo();
} else if (b) {
bar();
} else if (c) {
baz();
}
```

Two identical test conditions in the same chain are almost always a mistake in the code. Unless there are side effects in the expressions, a duplicate will evaluate to the same `true` or `false` value as the identical expression earlier in the chain, meaning that its branch can never execute.

```js
if (a) {
foo();
} else if (b) {
bar();
} else if (b) {
baz();
}
```

In the above example, `baz()` can never execute. Obviously, `baz()` could be executed only when `b` evaluates to `true`, but in that case `bar()` would be executed instead, since it's earlier in the chain.

## Rule Details

This rule disallows duplicate conditions in the same `if-else-if` chain.

Examples of **incorrect** code for this rule:

```js
/*eslint no-dupe-else-if: "error"*/

if (isSomething(x)) {
foo();
} else if (isSomething(x)) {
bar();
}

if (a) {
foo();
} else if (b) {
bar();
} else if (c && d) {
baz();
} else if (c && d) {
quux();
} else {
quuux();
}

if (n === 1) {
foo();
} else if (n === 2) {
bar();
} else if (n === 3) {
baz();
} else if (n === 2) {
quux();
} else if (n === 5) {
quuux();
}
```

Examples of **correct** code for this rule:

```js
/*eslint no-dupe-else-if: "error"*/

if (isSomething(x)) {
foo();
} else if (isSomethingElse(x)) {
bar();
}

if (a) {
foo();
} else if (b) {
bar();
} else if (c && d) {
baz();
} else if (c && e) {
quux();
} else {
quuux();
}

if (n === 1) {
foo();
} else if (n === 2) {
bar();
} else if (n === 3) {
baz();
} else if (n === 4) {
quux();
} else if (n === 5) {
quuux();
}
```

Please note that this rule does not compare conditions from the chain with conditions inside statements, and will not warn in the cases such as follows:

```js
if (a) {
if (a) {
foo();
}
}

if (a) {
foo();
} else {
if (a) {
bar();
}
}
```

## When Not To Use It

In rare cases where you really need identical test conditions in the same chain, which necessarily means that the expressions in the chain are causing and relying on side effects, you will have to turn this rule off.

## Related Rules

* [no-duplicate-case](no-duplicate-case.md)
* [no-lonely-if](no-lonely-if.md)
1 change: 1 addition & 0 deletions lib/rules/index.js
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Expand Up @@ -108,6 +108,7 @@ module.exports = new LazyLoadingRuleMap(Object.entries({
"no-div-regex": () => require("./no-div-regex"),
"no-dupe-args": () => require("./no-dupe-args"),
"no-dupe-class-members": () => require("./no-dupe-class-members"),
"no-dupe-else-if": () => require("./no-dupe-else-if"),
"no-dupe-keys": () => require("./no-dupe-keys"),
"no-duplicate-case": () => require("./no-duplicate-case"),
"no-duplicate-imports": () => require("./no-duplicate-imports"),
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54 changes: 54 additions & 0 deletions lib/rules/no-dupe-else-if.js
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/**
* @fileoverview Rule to disallow duplicate conditions in if-else-if chains
* @author Milos Djermanovic
*/

"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const astUtils = require("./utils/ast-utils");

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

module.exports = {
meta: {
type: "problem",

docs: {
description: "disallow duplicate conditions in if-else-if chains",
category: "Possible Errors",
recommended: false,
url: "https://eslint.org/docs/rules/no-dupe-else-if"
},

schema: [],

messages: {
unexpected: "Duplicate condition in if-else-if chain."
}
},

create(context) {
const sourceCode = context.getSourceCode();

return {
IfStatement(node) {
let current = node;

while (current.parent && current.parent.type === "IfStatement" && current.parent.alternate === current) {
current = current.parent;

if (astUtils.equalTokens(node.test, current.test, sourceCode)) {
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I'm guessing that to check each condition of the || expression individually is useful if the test nodes are LogicalExpression with || operator. Thoughts?

For example:

if (a || b) {
    // ...
} else if (a) { // duplicated condition
    // ...
}

I'm imaging code like:

                    const nodeConds = getOrConditions(node.test)
                    const currentConds = getOrConditions(current.test)
                    if (nodeConds.every(a => currentConds.some(b => astUtils.equalTokens(a, b, sourceCode)))) {

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I think it's a very good idea.

We could even compare with the union of || conditions from all parents in the chain?

For example:

if (a) {
    // ...
} else if (b) {
    // ...
} else if (a || b) { // duplicated condition: { a, b } is a subset of { a, b }
    // ...
}

That would also catch things like this:

if (a || b) {
    // ...
} else if (c || d) {
    // ...
} else if (a || d) { // duplicated condition: { a, d } is a subset of { a, b, c, d }
    // ...
}

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Nice!

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Is this really necessary? This seems like it would add some complexity and go beyond the original proposal.

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Looks like there are 3 options at the moment:

  1. To check for equal ancestors only, as originally proposed in New rule proposal: no-dupe-else-if #12469
  2. To check for a subset of an ancestor as in this comment
  3. To check for a subset of the union of all ancestors as in this comment

I'm fine with all three 👍

Options 2. and 3. look very nice, but only 1. is accepted, so I don't how to proceed 😕

It doesn't look too complicated to do 2. or 3. Though, compared to the simple 1. it would be certainly much more code. Not sure how to weight how useful these enhancements would be.

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I don't have any complexity concerns anymore based on the implementation shared earlier.

I don't understand why this only applies to || operators only, though. Shouldn't we look at && as well?

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I like this addition! Agreed with @platinumazure. It does seem like it could be useful to also check for and warn on:

if (a && b) {} 
else if (b && a) {}

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👍 to account for && as well.

The logic for && in a chain is a bit different, so option 3 wouldn't apply well. For instance, the following is valid code:

if (a && b) {
    // ...
} else if (a) {
    // ...
} else if (b) {
    // ... 
}

if (a && b) {
    // ...
} else if (b && c) {
    // ...
} else if (a && c) {
    // ...
}

On the other hand, 'inverted' option 2 should work quite well for && and is easy to implement. If the condition split by && is a superset of one of the previous conditions, then it's a 'duplicate'. For example, all these are invalid:

if (a) {
    // ...
} else if (a && b) {
    // ...
}

if (a && b) {
    // ...
} else if (b && a) {
    // ...
}

if (a && b) {
    // ...
} else if (a && b && c) {
    // ...
}

In addition, it also looks easy to split the original condition by && and do the || check with each element of the resulting array, which would catch cases that might not even look like a bug at first glance:

if (a || b) {
    // ...
} else if (b && c) { // perhaps a not-so-obvious bug
    // ...  
}

if (a) {
    // ...
} else if (b) {
    // ...
} else if ((a || b) && c) {
   // ...
}

That shouldn't even add new lines, just .map and .some to the existing lines in this draft

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This is done now, the rule should be able to catch some relatively complex errors ('more complex errors with || and &&' section in invalid tests).

Examples from the documentation:

/*eslint no-dupe-else-if: "error"*/

if (a || b) {
    foo();
} else if (a) {
    bar();
}

if (a) {
    foo();
} else if (b) {
    bar();
} else if (a || b) {
    baz();
}

if (a) {
    foo();
} else if (a && b) {
    bar();
}

if (a && b) {
    foo();
} else if (a && b && c) {
    bar();
}

if (a || b) {
    foo();
} else if (b && c) {
    bar();
}

if (a) {
    foo();
} else if (b && c) {
    bar();
} else if (d && (c && e && b || a)) {
    baz();
}

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Tried the rule on ESLint codebase, reports only 1 bug at indent-legacy.js#L779 and it's indeed a bug because of !parentVarNode which is used in the previous condition with ||.

context.report({ node: node.test, messageId: "unexpected" });
break;
}
}
}
};
}
};
172 changes: 172 additions & 0 deletions tests/lib/rules/no-dupe-else-if.js
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/**
* @fileoverview Tests for the no-dupe-else-if rule
* @author Milos Djermanovic
*/

"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const rule = require("../../../lib/rules/no-dupe-else-if");
const { RuleTester } = require("../../../lib/rule-tester");

//------------------------------------------------------------------------------
// Tests
//------------------------------------------------------------------------------

const ruleTester = new RuleTester();

ruleTester.run("no-dupe-else-if", rule, {
valid: [

// different test conditions
"if (a) {} else if (b) {}",
"if (a); else if (b); else if (c);",
"if (true) {} else if (false) {} else {}",
"if (1) {} else if (2) {}",
"if (f) {} else if (f()) {}",
"if (f(a)) {} else if (g(a)) {}",
"if (f(a)) {} else if (f(b)) {}",
"if (a === 1) {} else if (a === 2) {}",
"if (a === 1) {} else if (b === 1) {}",

// not an if-else-if chain
"if (a) {}",
"if (a);",
"if (a) {} else {}",
"if (a) if (a) {}",
"if (a) if (a);",
"if (a) { if (a) {} }",
"if (a) {} else { if (a) {} }",
"if (a) {} if (a) {}",
"if (a); if (a);",
"while (a) if (a);",
"if (a); else a ? a : a;",

// not same conditions in the chain
"if (a) { if (b) {} } else if (b) {}",
"if (a) if (b); else if (a);",

// not equal tokens
"if (a) {} else if (!!a) {}",
"if (a === 1) {} else if (a === (1)) {}"
],

invalid: [

// basic tests
{
code: "if (a) {} else if (a) {}",
errors: [{ messageId: "unexpected", type: "Identifier", column: 20 }]
},
{
code: "if (a); else if (a);",
errors: [{ messageId: "unexpected", type: "Identifier", column: 18 }]
},
{
code: "if (a) {} else if (a) {} else {}",
errors: [{ messageId: "unexpected", type: "Identifier", column: 20 }]
},
{
code: "if (a) {} else if (b) {} else if (a) {} else if (c) {}",
errors: [{ messageId: "unexpected", type: "Identifier", column: 35 }]
},
{
code: "if (a) {} else if (b) {} else if (a) {}",
errors: [{ messageId: "unexpected", type: "Identifier", column: 35 }]
},
{
code: "if (a) {} else if (b) {} else if (c) {} else if (a) {}",
errors: [{ messageId: "unexpected", type: "Identifier", column: 50 }]
},
{
code: "if (a) {} else if (b) {} else if (b) {}",
errors: [{ messageId: "unexpected", type: "Identifier", column: 35 }]
},
{
code: "if (a) {} else if (b) {} else if (b) {} else {}",
errors: [{ messageId: "unexpected", type: "Identifier", column: 35 }]
},
{
code: "if (a) {} else if (b) {} else if (c) {} else if (b) {}",
errors: [{ messageId: "unexpected", type: "Identifier", column: 50 }]
},
{
code: "if (a); else if (b); else if (c); else if (b); else if (d); else;",
errors: [{ messageId: "unexpected", type: "Identifier", column: 44 }]
},
{
code: "if (a); else if (b); else if (c); else if (d); else if (b); else if (e);",
errors: [{ messageId: "unexpected", type: "Identifier", column: 57 }]
},

// multiple duplicates of the same condition
{
code: "if (a) {} else if (a) {} else if (a) {}",
errors: [
{ messageId: "unexpected", type: "Identifier", column: 20 },
{ messageId: "unexpected", type: "Identifier", column: 35 }
]
},

// multiple duplicates of different conditions
{
code: "if (a) {} else if (b) {} else if (a) {} else if (b) {} else if (a) {}",
errors: [
{ messageId: "unexpected", type: "Identifier", column: 35 },
{ messageId: "unexpected", type: "Identifier", column: 50 },
{ messageId: "unexpected", type: "Identifier", column: 65 }
]
},

// inner if statements do not affect chain
{
code: "if (a) { if (b) {} } else if (a) {}",
errors: [{ messageId: "unexpected", type: "Identifier", column: 31 }]
},

// various kinds of test conditions
{
code: "if (a === 1) {} else if (a === 1) {}",
errors: [{ messageId: "unexpected", type: "BinaryExpression", column: 26 }]
},
{
code: "if (1 < a) {} else if (1 < a) {}",
errors: [{ messageId: "unexpected", type: "BinaryExpression", column: 24 }]
},
{
code: "if (true) {} else if (true) {}",
errors: [{ messageId: "unexpected", type: "Literal", column: 23 }]
},
{
code: "if (a && b) {} else if (a && b) {}",
errors: [{ messageId: "unexpected", type: "LogicalExpression", column: 25 }]
},
{
code: "if (a && b || c) {} else if (a && b || c) {}",
errors: [{ messageId: "unexpected", type: "LogicalExpression", column: 31 }]
},
{
code: "if (f(a)) {} else if (f(a)) {}",
errors: [{ messageId: "unexpected", type: "CallExpression", column: 23 }]
},

// spaces and comments do not affect comparison
{
code: "if (a === 1) {} else if (a===1) {}",
errors: [{ messageId: "unexpected", type: "BinaryExpression", column: 26 }]
},
{
code: "if (a === 1) {} else if (a === /* comment */ 1) {}",
errors: [{ messageId: "unexpected", type: "BinaryExpression", column: 26 }]
},

// extra parens around the whole test condition do not affect comparison
{
code: "if (a === 1) {} else if ((a === 1)) {}",
errors: [{ messageId: "unexpected", type: "BinaryExpression", column: 27 }]
}
]
});
1 change: 1 addition & 0 deletions tools/rule-types.json
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Expand Up @@ -95,6 +95,7 @@
"no-div-regex": "suggestion",
"no-dupe-args": "problem",
"no-dupe-class-members": "problem",
"no-dupe-else-if": "problem",
"no-dupe-keys": "problem",
"no-duplicate-case": "problem",
"no-duplicate-imports": "problem",
Expand Down