eslint/docs/rules/no-global-assign.md

   1 # Disallow assignment to native objects or read-only global variables (no-global-assign)
   2
   3 JavaScript environments contain a number of built-in global variables, such as `window` in browsers and `process` in Node.js. In almost all cases, you don't want to assign a value to these global variables as doing so could result in losing access to important functionality. For example, you probably don't want to do this in browser code:
   4
   5 ```js
   6 window = {};
   7 ```
   8
   9 While examples such as `window` are obvious, there are often hundreds of built-in global objects provided by JavaScript environments. It can be hard to know if you're assigning to a global variable or not.
  10
  11 ## Rule Details
  12
  13 This rule disallows modifications to read-only global variables.
  14
  15 ESLint has the capability to configure global variables as read-only.
  16
  17 * [Specifying Environments](../user-guide/configuring#specifying-environments)
  18 * [Specifying Globals](../user-guide/configuring#specifying-globals)
  19
  20 Examples of **incorrect** code for this rule:
  21
  22 ```js
  23 /*eslint no-global-assign: "error"*/
  24
  25 Object = null
  26 undefined = 1
  27 ```
  28
  29 ```js
  30 /*eslint no-global-assign: "error"*/
  31 /*eslint-env browser*/
  32
  33 window = {}
  34 length = 1
  35 top = 1
  36 ```
  37
  38 ```js
  39 /*eslint no-global-assign: "error"*/
  40 /*global a:readonly*/
  41
  42 a = 1
  43 ```
  44
  45 Examples of **correct** code for this rule:
  46
  47 ```js
  48 /*eslint no-global-assign: "error"*/
  49
  50 a = 1
  51 var b = 1
  52 b = 2
  53 ```
  54
  55 ```js
  56 /*eslint no-global-assign: "error"*/
  57 /*eslint-env browser*/
  58
  59 onload = function() {}
  60 ```
  61
  62 ```js
  63 /*eslint no-global-assign: "error"*/
  64 /*global a:writable*/
  65
  66 a = 1
  67 ```
  68
  69 ## Options
  70
  71 This rule accepts an `exceptions` option, which can be used to specify a list of builtins for which reassignments will be allowed:
  72
  73 ```json
  74 {
  75     "rules": {
  76         "no-global-assign": ["error", {"exceptions": ["Object"]}]
  77     }
  78 }
  79 ```
  80
  81 ## When Not To Use It
  82
  83 If you are trying to override one of the native objects.
  84
  85 ## Related Rules
  86
  87 * [no-extend-native](no-extend-native.md)
  88 * [no-redeclare](no-redeclare.md)
  89 * [no-shadow](no-shadow.md)