eslint/docs/rules/no-plusplus.md

   1 # disallow the unary operators `++` and `--` (no-plusplus)
   2
   3 Because the unary `++` and `--` operators are subject to automatic semicolon insertion, differences in whitespace can change semantics of source code.
   4
   5 ```js
   6 var i = 10;
   7 var j = 20;
   8
   9 i ++
  10 j
  11 // i = 11, j = 20
  12 ```
  13
  14 ```js
  15 var i = 10;
  16 var j = 20;
  17
  18 i
  19 ++
  20 j
  21 // i = 10, j = 21
  22 ```
  23
  24 ## Rule Details
  25
  26 This rule disallows the unary operators `++` and `--`.
  27
  28 Examples of **incorrect** code for this rule:
  29
  30 ```js
  31 /*eslint no-plusplus: "error"*/
  32
  33 var foo = 0;
  34 foo++;
  35
  36 var bar = 42;
  37 bar--;
  38
  39 for (i = 0; i < l; i++) {
  40     return;
  41 }
  42 ```
  43
  44 Examples of **correct** code for this rule:
  45
  46 ```js
  47 /*eslint no-plusplus: "error"*/
  48
  49 var foo = 0;
  50 foo += 1;
  51
  52 var bar = 42;
  53 bar -= 1;
  54
  55 for (i = 0; i < l; i += 1) {
  56     return;
  57 }
  58 ```
  59
  60 ## Options
  61
  62 This rule has an object option.
  63
  64 * `"allowForLoopAfterthoughts": true` allows unary operators `++` and `--` in the afterthought (final expression) of a `for` loop.
  65
  66 ### allowForLoopAfterthoughts
  67
  68 Examples of **correct** code for this rule with the `{ "allowForLoopAfterthoughts": true }` option:
  69
  70 ```js
  71 /*eslint no-plusplus: ["error", { "allowForLoopAfterthoughts": true }]*/
  72
  73 for (i = 0; i < l; i++) {
  74     doSomething(i);
  75 }
  76
  77 for (i = l; i >= 0; i--) {
  78     doSomething(i);
  79 }
  80
  81 for (i = 0, j = l; i < l; i++, j--) {
  82     doSomething(i, j);
  83 }
  84 ```
  85
  86 Examples of **incorrect** code for this rule with the `{ "allowForLoopAfterthoughts": true }` option:
  87
  88 ```js
  89 /*eslint no-plusplus: ["error", { "allowForLoopAfterthoughts": true }]*/
  90
  91 for (i = 0; i < l; j = i++) {
  92     doSomething(i, j);
  93 }
  94
  95 for (i = l; i--;) {
  96     doSomething(i);
  97 }
  98
  99 for (i = 0; i < l;) i++;
 100 ```