ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidently overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
The following patterns are considered problems:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
The following patterns are not considered problems:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.