eslint/docs/rules/func-style.md

   1 # enforce the consistent use of either `function` declarations or expressions (func-style)
   2
   3 There are two ways of defining functions in JavaScript: `function` declarations and `function` expressions. Declarations contain the `function` keyword first, followed by a name and then its arguments and the function body, for example:
   4
   5 ```js
   6 function doSomething() {
   7     // ...
   8 }
   9 ```
  10
  11 Equivalent function expressions begin with the `var` keyword, followed by a name and then the function itself, such as:
  12
  13 ```js
  14 var doSomething = function() {
  15     // ...
  16 };
  17 ```
  18
  19 The primary difference between `function` declarations and `function expressions` is that declarations are *hoisted* to the top of the scope in which they are defined, which allows you to write code that uses the function before its declaration. For example:
  20
  21 ```js
  22 doSomething();
  23
  24 function doSomething() {
  25     // ...
  26 }
  27 ```
  28
  29 Although this code might seem like an error, it actually works fine because JavaScript engines hoist the `function` declarations to the top of the scope. That means this code is treated as if the declaration came before the invocation.
  30
  31 For `function` expressions, you must define the function before it is used, otherwise it causes an error. Example:
  32
  33 ```js
  34 doSomething();  // error!
  35
  36 var doSomething = function() {
  37     // ...
  38 };
  39 ```
  40
  41 In this case, `doSomething()` is undefined at the time of invocation and so causes a runtime error.
  42
  43 Due to these different behaviors, it is common to have guidelines as to which style of function should be used. There is really no correct or incorrect choice here, it is just a preference.
  44
  45 ## Rule Details
  46
  47 This rule enforces a particular type of `function` style throughout a JavaScript file, either declarations or expressions. You can specify which you prefer in the configuration.
  48
  49 ## Options
  50
  51 This rule has a string option:
  52
  53 * `"expression"` (default) requires the use of function expressions instead of function declarations
  54 * `"declaration"` requires the use of function declarations instead of function expressions
  55
  56 This rule has an object option for an exception:
  57
  58 * `"allowArrowFunctions"`: `true` (default `false`) allows the use of arrow functions. This option applies only when the string option is set to `"declaration"` (arrow functions are always allowed when the string option is set to `"expression"`, regardless of this option)
  59
  60 ### expression
  61
  62 Examples of **incorrect** code for this rule with the default `"expression"` option:
  63
  64 ```js
  65 /*eslint func-style: ["error", "expression"]*/
  66
  67 function foo() {
  68     // ...
  69 }
  70 ```
  71
  72 Examples of **correct** code for this rule with the default `"expression"` option:
  73
  74 ```js
  75 /*eslint func-style: ["error", "expression"]*/
  76
  77 var foo = function() {
  78     // ...
  79 };
  80
  81 var foo = () => {};
  82
  83 // allowed as allowArrowFunctions : false is applied only for declaration
  84 ```
  85
  86 ### declaration
  87
  88 Examples of **incorrect** code for this rule with the `"declaration"` option:
  89
  90 ```js
  91 /*eslint func-style: ["error", "declaration"]*/
  92
  93 var foo = function() {
  94     // ...
  95 };
  96
  97 var foo = () => {};
  98 ```
  99
 100 Examples of **correct** code for this rule with the `"declaration"` option:
 101
 102 ```js
 103 /*eslint func-style: ["error", "declaration"]*/
 104
 105 function foo() {
 106     // ...
 107 }
 108
 109 // Methods (functions assigned to objects) are not checked by this rule
 110 SomeObject.foo = function() {
 111     // ...
 112 };
 113 ```
 114
 115 ### allowArrowFunctions
 116
 117 Examples of additional **correct** code for this rule with the `"declaration", { "allowArrowFunctions": true }` options:
 118
 119 ```js
 120 /*eslint func-style: ["error", "declaration", { "allowArrowFunctions": true }]*/
 121
 122 var foo = () => {};
 123 ```
 124
 125 ## When Not To Use It
 126
 127 If you want to allow developers to each decide how they want to write functions on their own, then you can disable this rule.
 128
 129 ## Further Reading
 130
 131 * [JavaScript Scoping and Hoisting](http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html)