no-constant-binary-expression

Disallow 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. 
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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`. 
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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 === []; const shortCircuit1 = condition1 && false && condition2; const shortCircuit2 = condition1 || true || condition2; const shortCircuit3 = condition1 ?? "non-nullish" ?? condition2; 
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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; 
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Related Rules

• no-constant-condition

Version

This rule was introduced in ESLint v8.14.0.

Further Reading

Interesting bugs caught by no-constant-binary-expression - ESLint - Pluggable JavaScript Linter
eslint.org

Resources

• Rule source
• Tests source