eslint/docs/src/rules/no-undef.md

   1 ---
   2 title: no-undef
   3 layout: doc
   4 rule_type: problem
   5 related_rules:
   6 - no-global-assign
   7 - no-redeclare
   8 ---
   9
  10
  11
  12 This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the `var` keyword in a `for` loop initializer).
  13
  14 ## Rule Details
  15
  16 Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a `/*global ...*/` comment, or specified in the [`globals` key in the configuration file](../user-guide/configuring/language-options#using-configuration-files-1). A common use case for these is if you intentionally use globals that are defined elsewhere (e.g. in a script sourced from HTML).
  17
  18 Examples of **incorrect** code for this rule:
  19
  20 ::: incorrect
  21
  22 ```js
  23 /*eslint no-undef: "error"*/
  24
  25 var foo = someFunction();
  26 var bar = a + 1;
  27 ```
  28
  29 :::
  30
  31 Examples of **correct** code for this rule with `global` declaration:
  32
  33 ::: correct
  34
  35 ```js
  36 /*global someFunction, a*/
  37 /*eslint no-undef: "error"*/
  38
  39 var foo = someFunction();
  40 var bar = a + 1;
  41 ```
  42
  43 :::
  44
  45 Note that this rule does not disallow assignments to read-only global variables.
  46 See [no-global-assign](no-global-assign) if you also want to disallow those assignments.
  47
  48 This rule also does not disallow redeclarations of global variables.
  49 See [no-redeclare](no-redeclare) if you also want to disallow those redeclarations.
  50
  51 ## Options
  52
  53 * `typeof` set to true will warn for variables used inside typeof check (Default false).
  54
  55 ### typeof
  56
  57 Examples of **correct** code for the default `{ "typeof": false }` option:
  58
  59 ::: correct
  60
  61 ```js
  62 /*eslint no-undef: "error"*/
  63
  64 if (typeof UndefinedIdentifier === "undefined") {
  65     // do something ...
  66 }
  67 ```
  68
  69 :::
  70
  71 You can use this option if you want to prevent `typeof` check on a variable which has not been declared.
  72
  73 Examples of **incorrect** code for the `{ "typeof": true }` option:
  74
  75 ::: incorrect
  76
  77 ```js
  78 /*eslint no-undef: ["error", { "typeof": true }] */
  79
  80 if(typeof a === "string"){}
  81 ```
  82
  83 :::
  84
  85 Examples of **correct** code for the `{ "typeof": true }` option with `global` declaration:
  86
  87 ::: correct
  88
  89 ```js
  90 /*global a*/
  91 /*eslint no-undef: ["error", { "typeof": true }] */
  92
  93 if(typeof a === "string"){}
  94 ```
  95
  96 :::
  97
  98 ## Environments
  99
 100 For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in [Specifying Environments](../user-guide/configuring/language-options#specifying-environments).  A few examples are given below.
 101
 102 ### browser
 103
 104 Examples of **correct** code for this rule with `browser` environment:
 105
 106 ::: correct
 107
 108 ```js
 109 /*eslint no-undef: "error"*/
 110 /*eslint-env browser*/
 111
 112 setTimeout(function() {
 113     alert("Hello");
 114 });
 115 ```
 116
 117 :::
 118
 119 ### Node.js
 120
 121 Examples of **correct** code for this rule with `node` environment:
 122
 123 ::: correct
 124
 125 ```js
 126 /*eslint no-undef: "error"*/
 127 /*eslint-env node*/
 128
 129 var fs = require("fs");
 130 module.exports = function() {
 131     console.log(fs);
 132 };
 133 ```
 134
 135 :::
 136
 137 ## When Not To Use It
 138
 139 If explicit declaration of global variables is not to your taste.
 140
 141 ## Compatibility
 142
 143 This rule provides compatibility with treatment of global variables in [JSHint](http://jshint.com/) and [JSLint](http://www.jslint.com).