eslint/docs/src/rules/no-unmodified-loop-condition.md

   1 ---
   2 title: no-unmodified-loop-condition
   3 layout: doc
   4 rule_type: problem
   5 ---
   6
   7
   8 Variables in a loop condition often are modified in the loop.
   9 If not, it's possibly a mistake.
  10
  11 ```js
  12 while (node) {
  13     doSomething(node);
  14 }
  15 ```
  16
  17 ```js
  18 while (node) {
  19     doSomething(node);
  20     node = node.parent;
  21 }
  22 ```
  23
  24 ## Rule Details
  25
  26 This rule finds references which are inside of loop conditions, then checks the
  27 variables of those references are modified in the loop.
  28
  29 If a reference is inside of a binary expression or a ternary expression, this rule checks the result of
  30 the expression instead.
  31 If a reference is inside of a dynamic expression (e.g. `CallExpression`,
  32 `YieldExpression`, ...), this rule ignores it.
  33
  34 Examples of **incorrect** code for this rule:
  35
  36 ::: incorrect
  37
  38 ```js
  39 /*eslint no-unmodified-loop-condition: "error"*/
  40
  41 var node = something;
  42
  43 while (node) {
  44     doSomething(node);
  45 }
  46 node = other;
  47
  48 for (var j = 0; j < items.length; ++i) {
  49     doSomething(items[j]);
  50 }
  51
  52 while (node !== root) {
  53     doSomething(node);
  54 }
  55 ```
  56
  57 :::
  58
  59 Examples of **correct** code for this rule:
  60
  61 ::: correct
  62
  63 ```js
  64 /*eslint no-unmodified-loop-condition: "error"*/
  65
  66 while (node) {
  67     doSomething(node);
  68     node = node.parent;
  69 }
  70
  71 for (var j = 0; j < items.length; ++j) {
  72     doSomething(items[j]);
  73 }
  74
  75 // OK, the result of this binary expression is changed in this loop.
  76 while (node !== root) {
  77     doSomething(node);
  78     node = node.parent;
  79 }
  80
  81 // OK, the result of this ternary expression is changed in this loop.
  82 while (node ? A : B) {
  83     doSomething(node);
  84     node = node.parent;
  85 }
  86
  87 // A property might be a getter which has side effect...
  88 // Or "doSomething" can modify "obj.foo".
  89 while (obj.foo) {
  90     doSomething(obj);
  91 }
  92
  93 // A function call can return various values.
  94 while (check(obj)) {
  95     doSomething(obj);
  96 }
  97 ```
  98
  99 :::
 100
 101 ## When Not To Use It
 102
 103 If you don't want to notified about references inside of loop conditions, then it's safe to disable this rule.