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

== compares values after converting types, while === compares both value and type with no conversion — and === is the one you should use.

Both operators ask "are these equal?", but they differ in how strict they are:

• == (loose equality) first coerces the two values to a common type, then compares. So 5 == '5' is true, because the string '5' becomes the number 5.
• === (strict equality) compares value and type, with no coercion. 5 === '5' is false, because a number is not a string.

5 == '5'            // true  — '5' coerced to number 5
5 === '5'           // false — different types

0 == false          // true  — false coerced to 0
0 === false         // false — number vs boolean

null == undefined   // true  — a special loose-equality rule
null === undefined  // false — different types

Loose == has a handful of surprising rules (like the ones above) that quietly cause bugs. The remedy is simple: default to === so comparisons mean exactly what they say. The same distinction applies to inequality: != (loose) versus !== (strict) — prefer !==.

Go deeper:

• javascript.info: Comparisons — how type conversion drives ==, plus the counterintuitive null/undefined edge cases.