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
  18 Examples of **correct** code for this rule:
  19
  20 ```js
  21 /*eslint no-redeclare: "error"*/
  22
  23 var a = 3;
  24 // ...
  25 a = 10;
  26 ```
  27
  28 ## Options
  29
  30 This rule takes one optional argument, an object with a boolean property `"builtinGlobals"`. It defaults to `true`.
  31 If set to `true`, this rule also checks redeclaration of built-in globals, such as `Object`, `Array`, `Number`...
  32
  33 ### builtinGlobals
  34
  35 The `"builtinGlobals"` option will check for redeclaration of built-in globals in global scope.
  36
  37 Examples of **incorrect** code for the `{ "builtinGlobals": true }` option:
  38
  39 ```js
  40 /*eslint no-redeclare: ["error", { "builtinGlobals": true }]*/
  41
  42 var Object = 0;
  43 ```
  44
  45 Examples of **incorrect** code for the `{ "builtinGlobals": true }` option and the `browser` environment:
  46
  47 ```js
  48 /*eslint no-redeclare: ["error", { "builtinGlobals": true }]*/
  49 /*eslint-env browser*/
  50
  51 var top = 0;
  52 ```
  53
  54 The `browser` environment has many built-in global variables (for example, `top`). Some of built-in global variables cannot be redeclared.
  55
  56 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.
  57
  58 ## Related Rules
  59
  60 * [no-shadow](no-shadow.md)