Equality

JavaScript · Reference cheat sheet

📋 Overview

JavaScript has strict equality (=== / !==), loose equality (== / !=) with coercion, and Object.is for SameValue semantics. Prefer === almost always; use Object.is for NaN and ±0 edge cases; avoid == except intentional nullish checks.

🔧 Core concepts

Op	Rule
`===`	Same type + value; no coercion; `NaN !== NaN`
`==`	Coerces then compares (complex rules)
`Object.is(a,b)`	SameValue: `NaN` equals `NaN`; `-0` ≠ `+0`
`SameValueZero`	Used by `Map`/`Set`: `NaN` equals `NaN`, ±0 equal

null == undefined is true; null === undefined is false.
Objects: equality is by reference, not deep content.

1 === "1"; // false
1 == "1"; // true
Object.is(NaN, NaN); // true

💡 Examples

// Prefer strict
if (status === 200) {
  /* ... */
}

// Intentional nullish (either)
if (value == null) {
  // value is null or undefined
}

// NaN checks
Number.isNaN(x); // preferred over x === NaN
Object.is(x, NaN);

// Zeros
+0 === -0; // true
Object.is(+0, -0); // false

// Objects
const a = { x: 1 };
const b = { x: 1 };
a === b; // false
a === a; // true

// Arrays / boxed
[] == false; // true (avoid!)
[1] == true; // false weirdness — never use == with objects

// Map keying uses SameValueZero
const m = new Map();
m.set(NaN, "yes");
m.get(NaN); // "yes"

// Deep equality — use a library or structured approach
function shallowEq(a, b) {
  return Object.keys(a).length === Object.keys(b).length &&
    Object.keys(a).every((k) => a[k] === b[k]);
}

⚠️ Pitfalls

== with mixed types is a footgun ("" == 0, null == undefined, arrays).
NaN === NaN is false — use Number.isNaN / Object.is.
Document / iframe boundaries: different Object constructors break instanceof and sometimes expectations.
Floating point: 0.1 + 0.2 !== 0.3 — compare with epsilon, not ===.

type_coercion.md — how == coerces
boolean.md — truthiness
nullish_coalescing.md — nullish ops
object / map — SameValueZero keys
number.md — NaN and floats

Equality

Equality

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

On this page