no-var - ESLint - Pluggable JavaScript Linter (original) (raw)

Require let or const instead of var

🔧 Fixable

Some problems reported by this rule are automatically fixable by the --fix command line option

Table of Contents

  1. Rule Details
  2. Examples
  3. When Not To Use It
  4. Version
  5. Resources

ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the letand 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; // accidentally 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!");

Rule Details

This rule is aimed at discouraging the use of var and encouraging the use of const or let instead.

Examples

Examples of incorrect code for this rule:

Examples of correct code for this rule:

This rule additionally supports TypeScript type syntax. There are multiple ways to declare global variables in TypeScript. Only using var works for all cases. See this TypeScript playground for reference.

Examples of incorrect TypeScript code for this rule:

/*eslint no-var: "error"*/

declare var x: number

declare namespace ns {
    var x: number
}

declare module 'module' {
    var x: number
}

Examples of correct TypeScript code for this rule:

/*eslint no-var: "error"*/

declare global {
    declare var x: number
}

When Not To Use It

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.

Version

This rule was introduced in ESLint v0.12.0.

Resources