What is the difference between == and ===?

The short interview answer is that === checks type and value, while == converts operands to a common type before comparing. The follow-up is whether there is ever a good reason to use ==.

0 == false; // true
0 === false; // false

"" == false; // true
"" === false; // false

null == undefined; // true
null === undefined; // false

NaN === NaN; // false
Object.is(NaN, NaN); // true

The coercion rules worth knowing

• If the types differ, == converts booleans and strings to numbers, then compares.
• null == undefined is true, but neither equals 0, "" or false under ==.
• Objects are converted with ToPrimitive before comparison, which is why [] == false is true.
• NaN is not equal to anything, including itself, under both == and ===.

The one idiomatic use of == is the null check: value == null matches both null and undefined and nothing else. Otherwise, prefer ===.

if (value == null) {
  // true for null or undefined
}

if (Object.is(value, NaN)) {
  // explicit NaN check
}

Common mistakes

• Saying == always converts to a boolean. It converts to a common type, not necessarily boolean.
• Using === for NaN and expecting true. Use Number.isNaN() or Object.is().
• Treating Object.is() as identical to ===; it differs for NaN and signed zero.