eslint/docs/rules/prefer-spread.md

   1 # Suggest using spread syntax instead of `.apply()`. (prefer-spread)
   2
   3 Before ES2015, one must use `Function.prototype.apply()` to call variadic functions.
   4
   5 ```js
   6 var args = [1, 2, 3, 4];
   7 Math.max.apply(Math, args);
   8 ```
   9
  10 In ES2015, one can use spread syntax to call variadic functions.
  11
  12 ```js
  13 /*eslint-env es6*/
  14
  15 var args = [1, 2, 3, 4];
  16 Math.max(...args);
  17 ```
  18
  19 ## Rule Details
  20
  21 This rule is aimed to flag usage of `Function.prototype.apply()` in situations where spread syntax could be used instead.
  22
  23 ## Examples
  24
  25 Examples of **incorrect** code for this rule:
  26
  27 ```js
  28 /*eslint prefer-spread: "error"*/
  29
  30 foo.apply(undefined, args);
  31 foo.apply(null, args);
  32 obj.foo.apply(obj, args);
  33 ```
  34
  35 Examples of **correct** code for this rule:
  36
  37 ```js
  38 /*eslint prefer-spread: "error"*/
  39
  40 // Using spread syntax
  41 foo(...args);
  42 obj.foo(...args);
  43
  44 // The `this` binding is different.
  45 foo.apply(obj, args);
  46 obj.foo.apply(null, args);
  47 obj.foo.apply(otherObj, args);
  48
  49 // The argument list is not variadic.
  50 // Those are warned by the `no-useless-call` rule.
  51 foo.apply(undefined, [1, 2, 3]);
  52 foo.apply(null, [1, 2, 3]);
  53 obj.foo.apply(obj, [1, 2, 3]);
  54 ```
  55
  56 Known limitations:
  57
  58 This rule analyzes code statically to check whether or not the `this` argument is changed. So, if the `this` argument is computed in a dynamic expression, this rule cannot detect a violation.
  59
  60 ```js
  61 /*eslint prefer-spread: "error"*/
  62
  63 // This warns.
  64 a[i++].foo.apply(a[i++], args);
  65
  66 // This does not warn.
  67 a[++i].foo.apply(a[i], args);
  68 ```
  69
  70 ## When Not To Use It
  71
  72 This rule should not be used in ES3/5 environments.
  73
  74 In ES2015 (ES6) or later, if you don't want to be notified about `Function.prototype.apply()` callings, you can safely disable this rule.
  75
  76 ## Related Rules
  77
  78 * [no-useless-call](no-useless-call.md)