Chapter 09 — `==` vs `===` (Equality)

📖 Definition

== performs loose equality with type coercion.
=== performs strict equality — no coercion, types must match.

🔍 Explanation

== tries to make types match before comparing. The rules are notoriously inconsistent, which is why most teams ban == outright (ESLint eqeqeq).

Coercion Rules (`==`)

If types match → behave like ===.
null == undefined → true (special rule).
Number vs string → string is converted to number.
Boolean → converted to number (true → 1, false → 0).
Object vs primitive → object is converted via valueOf / toString.

💻 Code Example — Surprising Coercions

0 == "0";              // true  (string → number 0)
0 == "";               // true  (empty string → 0)
0 == false;            // true  (false → 0)
"" == false;           // true  (both → 0)

null == undefined;     // true  (special rule)
null == 0;             // false (special exception)

[] == false;           // true   ([] → "" → 0; false → 0)
[1] == 1;              // true   ([1] → "1" → 1)
[1, 2] == "1,2";       // true

NaN == NaN;            // false  (NaN never equals anything)
NaN === NaN;           // false

💻 Code Example — Strict Equality

0 === "0";             // false (different types)
0 === false;           // false
null === undefined;    // false
NaN === NaN;           // false

1 === 1;               // true
"a" === "a";           // true

💻 Code Example — Object Equality

const a = { x: 1 };
const b = { x: 1 };
const c = a;

a === b;               // false — different references
a === c;               // true  — same reference

// Deep equality requires a helper:
JSON.stringify(a) === JSON.stringify(b);   // true (works for simple shapes)

💻 Code Example — Checking for NaN

// Wrong:
x === NaN;             // always false

// Right:
Number.isNaN(x);       // ES6, type-safe
isNaN("abc");          // true  (legacy — coerces first)
Number.isNaN("abc");   // false (no coercion)

💻 Code Example — `Object.is`

Object.is(NaN, NaN);   // true
Object.is(+0, -0);     // false  (=== returns true)
Object.is(1, 1);       // true

Object.is is the same as === except for NaN and signed zeros.

💻 Code Example — Safe Null Checks

// Loose check covers both null and undefined
if (value == null) {
  // matches null OR undefined
}

// Strict equivalent
if (value === null || value === undefined) {}

// Modern (preferred)
if (value ?? false) {}             // ?? treats only null/undefined as empty

🌍 Real-World Impact

Always use === unless you specifically want value == null to catch both null and undefined.
Add "eqeqeq": "error" to ESLint config.
For deep equality, use lodash.isEqual or a library; never roll your own without testing edge cases.

🎯 Likely Interview Questions

Difference between == and ===?
Why is [] == false true? — [] converts via toString → "" → number 0. false → 0. Both equal.
How do you check for NaN? — Number.isNaN(x).
How do you compare two objects for value equality? — JSON.stringify (limited), lodash.isEqual, or a custom deep-equals.

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Chapter 09 — == vs === (Equality)