JavaScript isNaN: Test a Number Without Accidental Coercion

The task

Inspect invalid arithmetic, numeric-looking text and an empty string. Check NaN with a dedicated predicate rather than a comparison to itself.

Runnable Node 22 example

const assert = require('node:assert/strict');

const broken = 0 / 0;
assert.equal(Number.isNaN(broken), true);
assert.equal(broken === broken, false);
assert.equal(Number.isNaN('oops'), false);
assert.equal(isNaN('oops'), true);
assert.equal(isNaN(''), false);
assert.equal(Number.isNaN(undefined), false);
assert.equal(isNaN(undefined), true);
console.log(`numericNaN=${Number.isNaN(broken)}; textCoerced=${isNaN('oops')}`);

Expected output and explanation

The failed numeric operation produces NaN, and the explicit predicate detects it. Equality with the same value is false, which explains why value === NaN cannot serve as the intended test.

The string oops is not itself the Number NaN, so Number.isNaN returns false. The coercing global operation reports true after trying its numeric conversion. Empty text takes yet another path and is not rejected by that global check. Conversion and validation should therefore remain explicit steps.

Pitfall to check

Not-NaN does not mean finite or valid for your measurement. Infinity passes a NaN-only test. Use Number.isFinite and any required unit or range checks when that is the actual contract.