eslint/docs/src/rules/no-const-assign.md

   1 ---
   2 title: no-const-assign
   3 layout: doc
   4 rule_type: problem
   5 ---
   6
   7
   8
   9 We cannot modify variables that are declared using `const` keyword.
  10 It will raise a runtime error.
  11
  12 Under non ES2015 environment, it might be ignored merely.
  13
  14 ## Rule Details
  15
  16 This rule is aimed to flag modifying variables that are declared using `const` keyword.
  17
  18 Examples of **incorrect** code for this rule:
  19
  20 ::: incorrect
  21
  22 ```js
  23 /*eslint no-const-assign: "error"*/
  24 /*eslint-env es6*/
  25
  26 const a = 0;
  27 a = 1;
  28 ```
  29
  30 :::
  31
  32 ::: incorrect
  33
  34 ```js
  35 /*eslint no-const-assign: "error"*/
  36 /*eslint-env es6*/
  37
  38 const a = 0;
  39 a += 1;
  40 ```
  41
  42 :::
  43
  44 ::: incorrect
  45
  46 ```js
  47 /*eslint no-const-assign: "error"*/
  48 /*eslint-env es6*/
  49
  50 const a = 0;
  51 ++a;
  52 ```
  53
  54 :::
  55
  56 Examples of **correct** code for this rule:
  57
  58 ::: correct
  59
  60 ```js
  61 /*eslint no-const-assign: "error"*/
  62 /*eslint-env es6*/
  63
  64 const a = 0;
  65 console.log(a);
  66 ```
  67
  68 :::
  69
  70 ::: correct
  71
  72 ```js
  73 /*eslint no-const-assign: "error"*/
  74 /*eslint-env es6*/
  75
  76 for (const a in [1, 2, 3]) { // `a` is re-defined (not modified) on each loop step.
  77     console.log(a);
  78 }
  79 ```
  80
  81 :::
  82
  83 ::: correct
  84
  85 ```js
  86 /*eslint no-const-assign: "error"*/
  87 /*eslint-env es6*/
  88
  89 for (const a of [1, 2, 3]) { // `a` is re-defined (not modified) on each loop step.
  90     console.log(a);
  91 }
  92 ```
  93
  94 :::
  95
  96 ## When Not To Use It
  97
  98 If you don't want to be notified about modifying variables that are declared using `const` keyword, you can safely disable this rule.