Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

The following patterns are considered problems:

/* eslint eqeqeq: 2 */

if (x == 42) { }                     /*error Expected '===' and instead saw '=='.*/

if ("" == text) { }                  /*error Expected '===' and instead saw '=='.*/

if (obj.getStuff() != undefined) { } /*error Expected '!==' and instead saw '!='.*/

Options

“smart”

This option enforces the use of === and !== except for these cases:

You can specify this option using the following configuration:

"eqeqeq": [2, "smart"]

The following patterns are not considered problems:

/* eslint eqeqeq: [2, "smart"] */

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

The following patterns are considered problems with “smart”:

/* eslint eqeqeq: [2, "smart"] */

// comparing two variables requires ===
a == b              /*error Expected '===' and instead saw '=='.*/

// only one side is a literal
foo == true         /*error Expected '===' and instead saw '=='.*/
bananas != 1        /*error Expected '!==' and instead saw '!='.*/

// comparing to undefined requires ===
value == undefined  /*error Expected '===' and instead saw '=='.*/

“allow-null”

This option will enforce === and !== in your code with one exception - it permits comparing to null to check for null or undefined in a single expression.

You can specify this option using the following configuration:

"eqeqeq": [2, "allow-null"]

The following patterns are not considered problems:

/* eslint eqeqeq: [2, "allow-null"] */

foo == null

The following patterns are considered problems with “allow-null”:

/* eslint eqeqeq: [2, "allow-null"] */

bananas != 1              /*error Expected '!==' and instead saw '!='.*/
typeof foo == 'undefined' /*error Expected '===' and instead saw '=='.*/
'hello' != 'world'        /*error Expected '!==' and instead saw '!='.*/
0 == 0                    /*error Expected '===' and instead saw '=='.*/
foo == undefined          /*error Expected '===' and instead saw '=='.*/

When Not To Use It

If you don’t want to enforce a style for using equality operators, then it’s safe to disable this rule.

Version

This rule was introduced in ESLint 0.0.2.

Resources