eslint/docs/src/rules/no-constant-binary-expression.md

   1 ---
   2 title: no-constant-binary-expression
   3 layout: doc
   4 rule_type: problem
   5 related_rules:
   6 - no-constant-condition
   7 further_reading:
   8 - https://eslint.org/blog/2022/07/interesting-bugs-caught-by-no-constant-binary-expression/
   9 ---
  10
  11
  12 Comparisons which will always evaluate to true or false and logical expressions (`||`, `&&`, `??`) which either always short-circuit or never short-circuit are both likely indications of programmer error.
  13
  14 These errors are especially common in complex expressions where operator precedence is easy to misjudge. For example:
  15
  16 ```js
  17 // One might think this would evaluate as `a + (b ?? c)`:
  18 const x = a + b ?? c;
  19
  20 // But it actually evaluates as `(a + b) ?? c`. Since `a + b` can never be null,
  21 // the `?? c` has no effect.
  22 ```
  23
  24 Additionally, this rule detects comparisons to newly constructed objects/arrays/functions/etc. In JavaScript, where objects are compared by reference, a newly constructed object can _never_ `===` any other value. This can be surprising for programmers coming from languages where objects are compared by value.
  25
  26 ```js
  27 // Programmers coming from a language where objects are compared by value might expect this to work:
  28 const isEmpty = x === [];
  29
  30 // However, this will always result in `isEmpty` being `false`.
  31 ```
  32
  33 ## Rule Details
  34
  35 This rule identifies `==` and `===` comparisons which, based on the semantics of the JavaScript language, will always evaluate to `true` or `false`.
  36
  37 It also identifies `||`, `&&` and `??` logical expressions which will either always or never short-circuit.
  38
  39 Examples of **incorrect** code for this rule:
  40
  41 ::: incorrect
  42
  43 ```js
  44 /*eslint no-constant-binary-expression: "error"*/
  45
  46 const value1 = +x == null;
  47
  48 const value2 = condition ? x : {} || DEFAULT;
  49
  50 const value3 = !foo == null;
  51
  52 const value4 = new Boolean(foo) === true;
  53
  54 const objIsEmpty = someObj === {};
  55
  56 const arrIsEmpty = someArr === [];
  57 ```
  58
  59 :::
  60
  61 Examples of **correct** code for this rule:
  62
  63 ::: correct
  64
  65 ```js
  66 /*eslint no-constant-binary-expression: "error"*/
  67
  68 const value1 = x == null;
  69
  70 const value2 = (condition ? x : {}) || DEFAULT;
  71
  72 const value3 = !(foo == null);
  73
  74 const value4 = Boolean(foo) === true;
  75
  76 const objIsEmpty = Object.keys(someObj).length === 0;
  77
  78 const arrIsEmpty = someArr.length === 0;
  79 ```
  80
  81 :::