eslint/docs/rules/no-redeclare.md

   1 # disallow variable redeclaration (no-redeclare)
   2
   3 In JavaScript, it's possible to redeclare the same variable name using `var`. This can lead to confusion as to where the variable is actually declared and initialized.
   4
   5 ## Rule Details
   6
   7 This rule is aimed at eliminating variables that have multiple declarations in the same scope.
   8
   9 Examples of **incorrect** code for this rule:
  10
  11 ```js
  12 /*eslint no-redeclare: "error"*/
  13
  14 var a = 3;
  15 var a = 10;
  16
  17 class C {
  18     foo() {
  19         var b = 3;
  20         var b = 10;
  21     }
  22
  23     static {
  24         var c = 3;
  25         var c = 10;
  26     }
  27 }
  28 ```
  29
  30 Examples of **correct** code for this rule:
  31
  32 ```js
  33 /*eslint no-redeclare: "error"*/
  34
  35 var a = 3;
  36 a = 10;
  37
  38 class C {
  39     foo() {
  40         var b = 3;
  41         b = 10;
  42     }
  43
  44     static {
  45         var c = 3;
  46         c = 10;
  47     }
  48 }
  49
  50 ```
  51
  52 ## Options
  53
  54 This rule takes one optional argument, an object with a boolean property `"builtinGlobals"`. It defaults to `true`.
  55 If set to `true`, this rule also checks redeclaration of built-in globals, such as `Object`, `Array`, `Number`...
  56
  57 ### builtinGlobals
  58
  59 The `"builtinGlobals"` option will check for redeclaration of built-in globals in global scope.
  60
  61 Examples of **incorrect** code for the `{ "builtinGlobals": true }` option:
  62
  63 ```js
  64 /*eslint no-redeclare: ["error", { "builtinGlobals": true }]*/
  65
  66 var Object = 0;
  67 ```
  68
  69 Examples of **incorrect** code for the `{ "builtinGlobals": true }` option and the `browser` environment:
  70
  71 ```js
  72 /*eslint no-redeclare: ["error", { "builtinGlobals": true }]*/
  73 /*eslint-env browser*/
  74
  75 var top = 0;
  76 ```
  77
  78 The `browser` environment has many built-in global variables (for example, `top`). Some of built-in global variables cannot be redeclared.
  79
  80 Note that when using the `node` or `commonjs` environments (or `ecmaFeatures.globalReturn`, if using the default parser), the top scope of a program is not actually the global scope, but rather a "module" scope. When this is the case, declaring a variable named after a builtin global is not a redeclaration, but rather a shadowing of the global variable. In that case, the [`no-shadow`](no-shadow.md) rule with the `"builtinGlobals"` option should be used.
  81
  82 ## Related Rules
  83
  84 * [no-shadow](no-shadow.md)