eslint/docs/src/rules/prefer-spread.md

   1 ---
   2 title: prefer-spread
   3 layout: doc
   4 rule_type: suggestion
   5 related_rules:
   6 - no-useless-call
   7 ---
   8
   9
  10 Before ES2015, one must use `Function.prototype.apply()` to call variadic functions.
  11
  12 ```js
  13 var args = [1, 2, 3, 4];
  14 Math.max.apply(Math, args);
  15 ```
  16
  17 In ES2015, one can use spread syntax to call variadic functions.
  18
  19 ```js
  20 /*eslint-env es6*/
  21
  22 var args = [1, 2, 3, 4];
  23 Math.max(...args);
  24 ```
  25
  26 ## Rule Details
  27
  28 This rule is aimed to flag usage of `Function.prototype.apply()` in situations where spread syntax could be used instead.
  29
  30 ## Examples
  31
  32 Examples of **incorrect** code for this rule:
  33
  34 ::: incorrect
  35
  36 ```js
  37 /*eslint prefer-spread: "error"*/
  38
  39 foo.apply(undefined, args);
  40 foo.apply(null, args);
  41 obj.foo.apply(obj, args);
  42 ```
  43
  44 :::
  45
  46 Examples of **correct** code for this rule:
  47
  48 ::: correct
  49
  50 ```js
  51 /*eslint prefer-spread: "error"*/
  52
  53 // Using spread syntax
  54 foo(...args);
  55 obj.foo(...args);
  56
  57 // The `this` binding is different.
  58 foo.apply(obj, args);
  59 obj.foo.apply(null, args);
  60 obj.foo.apply(otherObj, args);
  61
  62 // The argument list is not variadic.
  63 // Those are warned by the `no-useless-call` rule.
  64 foo.apply(undefined, [1, 2, 3]);
  65 foo.apply(null, [1, 2, 3]);
  66 obj.foo.apply(obj, [1, 2, 3]);
  67 ```
  68
  69 :::
  70
  71 Known limitations:
  72
  73 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.
  74
  75 ```js
  76 /*eslint prefer-spread: "error"*/
  77
  78 // This warns.
  79 a[i++].foo.apply(a[i++], args);
  80
  81 // This does not warn.
  82 a[++i].foo.apply(a[i], args);
  83 ```
  84
  85 ## When Not To Use It
  86
  87 This rule should not be used in ES3/5 environments.
  88
  89 In ES2015 (ES6) or later, if you don't want to be notified about `Function.prototype.apply()` callings, you can safely disable this rule.