eslint/docs/rules/no-constant-condition.md

   1 # disallow constant expressions in conditions (no-constant-condition)
   2
   3 A constant expression (for example, a literal) as a test condition might be a typo or development trigger for a specific behavior. For example, the following code looks as if it is not ready for production.
   4
   5 ```js
   6 if (false) {
   7     doSomethingUnfinished();
   8 }
   9 ```
  10
  11 ## Rule Details
  12
  13 This rule disallows constant expressions in the test condition of:
  14
  15 * `if`, `for`, `while`, or `do...while` statement
  16 * `?:` ternary expression
  17
  18 Examples of **incorrect** code for this rule:
  19
  20 ```js
  21 /*eslint no-constant-condition: "error"*/
  22
  23 if (false) {
  24     doSomethingUnfinished();
  25 }
  26
  27 if (void x) {
  28     doSomethingUnfinished();
  29 }
  30
  31 if (x &&= false) {
  32     doSomethingNever();
  33 }
  34
  35 if (class {}) {
  36     doSomethingAlways();
  37 }
  38
  39 if (new Boolean(x)) {
  40     doSomethingAlways();
  41 }
  42
  43 if (x ||= true) {
  44     doSomethingAlways();
  45 }
  46
  47 for (;-2;) {
  48     doSomethingForever();
  49 }
  50
  51 while (typeof x) {
  52     doSomethingForever();
  53 }
  54
  55 do {
  56     doSomethingForever();
  57 } while (x = -1);
  58
  59 var result = 0 ? a : b;
  60 ```
  61
  62 Examples of **correct** code for this rule:
  63
  64 ```js
  65 /*eslint no-constant-condition: "error"*/
  66
  67 if (x === 0) {
  68     doSomething();
  69 }
  70
  71 for (;;) {
  72     doSomethingForever();
  73 }
  74
  75 while (typeof x === "undefined") {
  76     doSomething();
  77 }
  78
  79 do {
  80     doSomething();
  81 } while (x);
  82
  83 var result = x !== 0 ? a : b;
  84 ```
  85
  86 ## Options
  87
  88 ### checkLoops
  89
  90 Set to `true` by default. Setting this option to `false` allows constant expressions in loops.
  91
  92 Examples of **correct** code for when `checkLoops` is `false`:
  93
  94 ```js
  95 /*eslint no-constant-condition: ["error", { "checkLoops": false }]*/
  96
  97 while (true) {
  98     doSomething();
  99     if (condition()) {
 100         break;
 101     }
 102 };
 103
 104 for (;true;) {
 105     doSomething();
 106     if (condition()) {
 107         break;
 108     }
 109 };
 110
 111 do {
 112     doSomething();
 113     if (condition()) {
 114         break;
 115     }
 116 } while (true)
 117 ```