What is the difference between ==, ===, and Object.is() in JavaScript? When does it matter in production code?

Equality Comparison Types

== (Abstract Equality — Type Coercion)

null == undefined   // true
0 == false          // true
"" == false         // true
"1" == 1            // true
[] == false         // true
[] == ![]           // true (!)

JavaScript converts types before comparison using the Abstract Equality Comparison Algorithm (ES spec §7.2.15). This leads to unexpected results.

=== (Strict Equality — No Coercion)

null === undefined  // false
0 === false         // false
"1" === 1           // false
NaN === NaN         // false (!)
+0 === -0           // true (!)

No type conversion, but has two edge cases: NaN and signed zeros.

Object.is() (SameValue — Most Precise)

Object.is(NaN, NaN)     // true
Object.is(+0, -0)       // false
Object.is(null, null)   // true

Production Impact

Checknullundefined0""false

== null✅✅ (gotcha!)❌❌❌
=== null✅❌❌❌❌
=== undefined❌✅❌❌❌

Best Practices

1. Always use === except for one valid case:
2. == null is the ONLY acceptable loose comparison — it checks both null AND undefined (useful for optional parameters):

function process(data: string | null | undefined) {
     if (data == null) return; // catches both null and undefined
   }
   

3. Object.is() for NaN checks: Object.is(value, NaN) instead of Number.isNaN() polyfill scenarios