Equality Operators

JavaScript has two equality/inequality operators: the strict and the abstract equality operators.

Abstract Equality

The operator looks similar to that used in languages like Java and C++:

console.log(2 == 2);
console.log(1 == 2);
console.log(3 != 2);

The problem is, when the operands are of different types, the abstract equality operator converts the types first!

console.log("2" == 2);
console.log(1 == true);
console.log(0 == false);
console.log("" == false);
console.log("0" == false); // both are converted to 0

Type conversion before checking for equality is more error prune than it can be desired.

Do not use the "abstract equality" operators == and !=.

Strict Equality

The strict equality uses an extra equal sign. It performs the equality comparison in a more exact identity sort of matching.

When the operands have the same type, their value is compared:

console.log(2 === 2);
console.log(1 === 2);
console.log(3 !== 2);

If the operands don't have the same type, then they are considered unequal:

console.log("2" === 2);
console.log(1 === true);
console.log(0 === false);
console.log("" === false);
console.log("0" === false);

While numbers, boolean values and strings compared by value, object/array references are strictly equal if they refer to the same object in memory:

const student = { name: "John Doe" };
const courseAssistant = { name: "John Doe" };
const headAssistant = courseAssistant;

console.log(student === courseAssistant);        // false
console.log(headAssistant === courseAssistant);  // true

A good read: Object equality in Javascript.

Nuances

Note that undefined and null are only equal to themselves (unless compared using abstract equality).

console.log(undefined === undefined);
console.log(null === null);
console.log(undefined === null);  // false
console.log(undefined == null);   // true

Here is a bit of madness, NaN is not equal to NaN no matter what equality operator you use

console.log(NaN == NaN);
console.log(NaN === NaN);

So JavaScript's equality operator is not an equivalence relation (since it is not reflexive).

If you want to check if a variable is NaN, you should try one of the following static methods:

let num = NaN;
console.log(Number.isNaN(num));
console.log(Object.is(num, NaN));

Object.is is a new addition to JavaScript which works similar to strict equality except for NaN and +0/-0.

console.log(+0 === -0);
console.log(Object.is(-0, +0));

JavaScript Object.is is designed to have the properties of an equivalence relation (it is reflexive, symmetric, and trasitive).