no-constant-binary-expression

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. Additionally, when configured with the checkRelationalComparisons option, it can identify constant relational comparisons (such as < or >=).

Examples of incorrect code for this rule:

::: incorrect

/*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;

:::

Examples of correct code for this rule:

::: correct

/*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;

:::

Options

This rule has an object option:

• "checkRelationalComparisons": true (default false) checks for relational comparisons (<, <=, >, >=) where both sides are literal values.

checkRelationalComparisons

Examples of incorrect code for this rule with the { "checkRelationalComparisons": true } option:

::: incorrect

/*eslint no-constant-binary-expression: ["error", { "checkRelationalComparisons": true } ]*/

const value1 = 1 < 2;

const value2 = "a" >= "b";

const value3 = true > false;

const hasStreak = profile.streak ?? 0 > 1;

:::

Examples of correct code for this rule with the { "checkRelationalComparisons": true } option:

::: correct

/*eslint no-constant-binary-expression: ["error", { "checkRelationalComparisons": true } ]*/

const value1 = 1 < x;

const value2 = a >= b;

const hasStreak = (profile.streak ?? 0) > 1;

:::