Equality Operators (=== and ==)

                a === b / a == b
              

Purpose

=== (strict equality) compares type and value with no conversion. == (loose equality) converts one or both sides to a common type first, following a specific set of rules, then compares. === is the recommended default because its behavior needs no memorized rule list.

Syntax

5 === 5; // true
5 === '5'; // false — different types, no coercion
5 == '5'; // true — '5' is coerced to 5 first

Behavior notes

Selected == coercion rules worth knowing exist, even if you default to ===:

Comparison	Result	Rule
0 == false	true	Boolean is coerced to a number.
"" == false	true	Both sides coerce toward 0/false.
null == undefined	true	Special-cased; true only for each other.
null == 0	false	null does not coerce to 0.
NaN == NaN	false	NaN never equals anything, == or ===.

Object.is(a, b) is a third comparison, rarely needed, that differs from === only for NaN (Object.is(NaN, NaN) is true) and signed zero (Object.is(0, -0) is false).

Common mistakes

• Treating == as “a more forgiving ===” rather than a specific, sometimes counterintuitive rule table — null == 0 being false surprises people expecting a general leniency.
• Using === (or ==) to check for NaN. Use Number.isNaN(x) instead.

Runtime notes

Both operators and their coercion rules are defined by ECMAScript and behave identically everywhere.