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

== performs type coercion. === checks value AND type — no coercion.

0 == ''          // true  (both coerce to falsy)
0 == '0'         // true
0 === '0'        // false
null == undefined  // true (special case)
null === undefined // false
NaN == NaN       // false (NaN never equals itself)

Always use ===. Use Number.isNaN() to check for NaN.