== vs === : Loose vs Strict Equality
One of the most-asked JS interview questions. The short version: always use ===. Here's why loose equality is dangerous.
What each does
=== (strict) compares type and value with no conversion. == (loose) coerces the operands to a common type first, which produces results almost nobody can predict reliably.
0 == ""; // true
0 == "0"; // true
"" == "0"; // false
null == undefined; // true
null === undefined;// falseThe one useful == idiom
The only place loose equality earns its keep is `x == null`, which is true for both null and undefined — a concise way to catch both at once. Everywhere else, use ===.
Crack the JS output questions
Coercion, truthiness and the 'what does this print?' traps — all covered with clear reasoning in the JavaScript Interview Kit.
⚡ Get the JavaScript Interview Kit → ₹299Frequently asked questions
- Is === faster than ==?
- Marginally, because it skips coercion — but the real reason to prefer it is correctness and predictability, not speed.
- What does [] == ![] return?
- true — a notorious coercion trick. ![] is false, [] coerces to '', both become 0, so 0 == 0. It's exactly why == is avoided.