JavaScript Interview Question: == vs ===

Short answer: === (strict equality) compares value and type with no conversion. == (loose equality) converts the operands to a common type first, which leads to surprising results.

1 === '1'          // false - different types
1 == '1'           // true  - '1' converted to 1

0 == ''            // true
0 == '0'           // true
'' == '0'          // false  (not transitive!)
false == '0'       // true
null == undefined  // true
null == 0          // false
NaN == NaN         // false (NaN is never equal to anything)

Rule of thumb

Always use === and !==. Linters (eqeqeq) can enforce it.

The one common exception

if (value == null) { ... }   // true for both null and undefined

Some teams allow this idiom; value ?? fallback often expresses the intent better.

Objects compare by reference

({ a: 1 }) === ({ a: 1 })   // false - two different objects
const x = { a: 1 }; x === x // true

Bonus: Object.is

Object.is(NaN, NaN)  // true
Object.is(0, -0)     // false (=== says true)

React uses Object.is to decide whether state has changed.