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

Question

Accepted Answer

**`===`** (strict equality) compares value **and** type with no conversion. **`==`** (loose equality) performs **type coercion** first, which leads to surprising results.

```js
0 == "0";    // true  — "0" coerced to 0
0 == "";     // true  — "" coerced to 0
0 == false;  // true  — false coerced to 0
null == undefined; // true  — special rule
"" == false; // true

0 === "0";   // false — different types, no coercion
NaN === NaN; // false — NaN is never equal to anything
```

## Why == is dangerous

The coercion rules are non-obvious and lead to bugs. For example, `[] == ![]` is `true` (a notorious quirk). You rarely want JavaScript silently converting types behind your back during a comparison.

## The rule

**Always use `===`** (and `!==`). The one common, intentional exception is checking for null *or* undefined together:

```js
if (value == null) { ... } // true for BOTH null and undefined — a deliberate idiom
```

For everything else, strict equality keeps comparisons predictable. To compare objects you need a deep-equality check (they compare by reference, so `{a:1} === {a:1}` is `false`).