Equality

Two operators, one recommendation

=== compares without converting: same type and same value, or false. == converts first, then compares. Use === everywhere, and read this chapter so the other one holds no surprises.

console.log(5 === 5);
console.log(5 === '5');
console.log(5 == '5');

true
false
true

What == does

When the types differ, == applies a conversion table. The results are consistent, and almost nobody carries the table in their head.

console.log(0 == '');
console.log(0 == '0');
console.log('' == '0');
console.log(null == undefined);
console.log(null == 0);

true
true
false
true
false

Note the first three: 0 equals the empty string and equals "0", but those two are not equal to each other. Equality that is not transitive is a poor foundation for reasoning about code.

The one useful case

== null is true for both null and undefined and nothing else, which makes it a compact test for absent.

function check(value) {
  return value == null ? 'absent' : 'present';
}

console.log(check(null), check(undefined), check(0), check(''));

absent absent present present

Even here, value === null || value === undefined says the same thing without relying on the reader knowing the table.

Objects compare by identity

Neither operator looks inside an object. Two objects are equal only when they are the same object.

const a = { id: 1 };
const b = { id: 1 };

console.log(a === b);
console.log(a == b);
console.log(a === { ...a });

false
false
false

The two special cases

NaN is not equal to itself, and 0 and -0 are equal to each other. Object.is is a third comparison that treats both the way you would expect.

console.log(NaN === NaN);
console.log(Object.is(NaN, NaN));
console.log(0 === -0);
console.log(Object.is(0, -0));

false
true
true
false