eslint/docs/src/rules/no-unneeded-ternary.md

   1 ---
   2 title: no-unneeded-ternary
   3 layout: doc
   4 rule_type: suggestion
   5 related_rules:
   6 - no-ternary
   7 - no-nested-ternary
   8 ---
   9
  10
  11
  12 It's a common mistake in JavaScript to use a conditional expression to select between two Boolean values instead of using ! to convert the test to a Boolean.
  13 Here are some examples:
  14
  15 ```js
  16 // Bad
  17 var isYes = answer === 1 ? true : false;
  18
  19 // Good
  20 var isYes = answer === 1;
  21
  22 // Bad
  23 var isNo = answer === 1 ? false : true;
  24
  25 // Good
  26 var isNo = answer !== 1;
  27 ```
  28
  29 Another common mistake is using a single variable as both the conditional test and the consequent. In such cases, the logical `OR` can be used to provide the same functionality.
  30 Here is an example:
  31
  32 ```js
  33 // Bad
  34 foo(bar ? bar : 1);
  35
  36 // Good
  37 foo(bar || 1);
  38 ```
  39
  40 ## Rule Details
  41
  42 This rule disallow ternary operators when simpler alternatives exist.
  43
  44 Examples of **incorrect** code for this rule:
  45
  46 ::: incorrect
  47
  48 ```js
  49 /*eslint no-unneeded-ternary: "error"*/
  50
  51 var a = x === 2 ? true : false;
  52
  53 var a = x ? true : false;
  54 ```
  55
  56 :::
  57
  58 Examples of **correct** code for this rule:
  59
  60 ::: correct
  61
  62 ```js
  63 /*eslint no-unneeded-ternary: "error"*/
  64
  65 var a = x === 2 ? "Yes" : "No";
  66
  67 var a = x !== false;
  68
  69 var a = x ? "Yes" : "No";
  70
  71 var a = x ? y : x;
  72
  73 f(x ? x : 1); // default assignment - would be disallowed if defaultAssignment option set to false. See option details below.
  74 ```
  75
  76 :::
  77
  78 ## Options
  79
  80 This rule has an object option:
  81
  82 * `"defaultAssignment": true` (default) allows the conditional expression as a default assignment pattern
  83 * `"defaultAssignment": false` disallows the conditional expression as a default assignment pattern
  84
  85 ### defaultAssignment
  86
  87 When set to `true`, which it is by default, The defaultAssignment option allows expressions of the form `x ? x : expr` (where `x` is any identifier and `expr` is any expression).
  88
  89 Examples of additional **incorrect** code for this rule with the `{ "defaultAssignment": false }` option:
  90
  91 ::: incorrect
  92
  93 ```js
  94 /*eslint no-unneeded-ternary: ["error", { "defaultAssignment": false }]*/
  95
  96 var a = x ? x : 1;
  97
  98 f(x ? x : 1);
  99 ```
 100
 101 :::
 102
 103 Note that `defaultAssignment: false` still allows expressions of the form `x ? expr : x` (where the identifier is on the right hand side of the ternary).
 104
 105 ## When Not To Use It
 106
 107 You can turn this rule off if you are not concerned with unnecessary complexity in conditional expressions.