ESLintRequire === 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:
[] == false[] == ![]3 == "03"
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.
Examples of incorrect code for this rule:
/*eslint eqeqeq: "error"*/
if (x == 42) { }
if ("" == text) { }
if (obj.getStuff() != undefined) { }
Options
always
The "always" option (default) enforces the use of === and !== in every situation.
Examples of incorrect code for the "always" option:
/*eslint eqeqeq: ["error", "always"]*/
a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
Examples of correct code for the "always" option:
/*eslint eqeqeq: ["error", "always"]*/
a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null
smart
The "smart" option enforces the use of === and !== except for these cases:
- Comparing two literal values
- Evaluating the value of
typeof - Comparing against
null
Examples of incorrect code for the "smart" option:
/*eslint eqeqeq: ["error", "smart"]*/
// comparing two variables requires ===
a == b
// only one side is a literal
foo == true
bananas != 1
// comparing to undefined requires ===
value == undefined
Examples of correct code for the "smart" option:
/*eslint eqeqeq: ["error", "smart"]*/
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
allow-null
The "allow-null" 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.
Examples of incorrect code for the "allow-null" option:
/*eslint eqeqeq: ["error", "allow-null"]*/
bananas != 1
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
foo == undefined
Examples of correct code for the "allow-null" option:
/*eslint eqeqeq: ["error", "allow-null"]*/
foo == null
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.