isNaNComparisons

Reports comparisons with NaN, which should use Number.isNaN() instead.

✅ This rule is included in the ts logical presets.

In JavaScript, NaN (Not-a-Number) has a unique property: it is not equal to anything, including itself. This means that comparisons like x === NaN or x !== NaN always produce unexpected results - the equality check always returns false, and the inequality check always returns true.

To reliably check if a value is NaN, use Number.isNaN() instead.

Examples

❌ Incorrect

if (value === NaN) {
  console.log("value is NaN");
}

const isNotANumber = result !== NaN;

if (value > NaN) {
  console.log("comparison result");
}

✅ Correct

if (Number.isNaN(value)) {
  console.log("value is NaN");
}

const isNotANumber = !Number.isNaN(result);

Options

This rule is not configurable.

When Not To Use It

There is no legitimate reason to compare directly with NaN. This rule should always be enabled to prevent subtle bugs.

Further Reading

• MDN: NaN
• MDN: Number.isNaN()

Equivalents in Other Linters

• Biome: useIsNan
• Deno: use-isnan
• ESLint: use-isnan
• Oxlint: eslint/use-isnan