eslint/docs/rules/no-eq-null.md

   1 # Disallow Null Comparisons (no-eq-null)
   2
   3 Comparing to `null` without a type-checking operator (`==` or `!=`), can have unintended results as the comparison will evaluate to true when comparing to not just a `null`, but also an `undefined` value.
   4
   5 ```js
   6 if (foo == null) {
   7   bar();
   8 }
   9 ```
  10
  11 ## Rule Details
  12
  13 The `no-eq-null` rule aims reduce potential bug and unwanted behavior by ensuring that comparisons to `null` only match `null`, and not also `undefined`. As such it will flag comparisons to null when using `==` and `!=`.
  14
  15 Examples of **incorrect** code for this rule:
  16
  17 ```js
  18 /*eslint no-eq-null: "error"*/
  19
  20 if (foo == null) {
  21   bar();
  22 }
  23
  24 while (qux != null) {
  25   baz();
  26 }
  27 ```
  28
  29 Examples of **correct** code for this rule:
  30
  31 ```js
  32 /*eslint no-eq-null: "error"*/
  33
  34 if (foo === null) {
  35   bar();
  36 }
  37
  38 while (qux !== null) {
  39   baz();
  40 }
  41 ```
  42
  43 ## Compatibility
  44
  45 * **JSHint**: This rule corresponds to `eqnull` rule of JSHint.
  46
  47 ## When Not To Use It
  48
  49 If you want to enforce type-checking operations in general, use the more powerful [eqeqeq](./eqeqeq) instead.