no-constant-binary-expression
Disallows expressions where the operation doesn't affect the value.
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.
These errors are especially common in complex expressions where operator precedence is easy to misjudge. For example:
// One might think this would evaluate as `a + (b ?? c)`: const x = a + b ?? c; // But it actually evaluates as `(a + b) ?? c`. Since `a + b` can never be null, // the `?? c` has no effect.
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.
// Programmers coming from a language where objects are compared by value might expect this to work: const isEmpty = x === []; // However, this will always result in `isEmpty` being `false`.
Rule Details
This rule identifies ==
and ===
comparisons which, based on the semantics of the JavaScript language, will always evaluate to true
or false
.
It also identifies ||
, &&
and ??
logical expressions which will either always or never short-circuit.
Examples of incorrect code for this rule:
/*eslint no-constant-binary-expression: "error"*/ const value1 = +x == null; const value2 = condition ? x : {} || DEFAULT; const value3 = !foo == null; const value4 = new Boolean(foo) === true; const objIsEmpty = someObj === {}; const arrIsEmpty = someArr === [];
Examples of correct code for this rule:
/*eslint no-constant-binary-expression: "error"*/ const value1 = x == null; const value2 = (condition ? x : {}) || DEFAULT; const value3 = !(foo == null); const value4 = Boolean(foo) === true; const objIsEmpty = Object.keys(someObj).length === 0; const arrIsEmpty = someArr.length === 0;
Related Rules
Version
This rule was introduced in ESLint 8.14.0.
Resources
© OpenJS Foundation and other contributors
Licensed under the MIT License.
https://eslint.org/docs/rules/no-constant-binary-expression