Triple Equals vs Double Equals in JavaScript

In JavaScript, there are technically 4 different ways of comparing whether 2 values are equal. The 2 most common ways are the == operator and the === operator, also known as abstract equality comparison and strict equality comparison.

Strict Equality With ===

Given 2 values x and y, here's how JavaScript checks whether x === y:

1. Check the types of x and y. If they are different types, return false.
2. If x and y are numbers, first check if either x or y is NaN, and return false if one is NaN. If both x and y are either +0 or -0, return true. Otherwise, check to see if they are the same number.
3. If x and y are both null or both undefined, return true.
4. If x and y are both booleans, both strings, or both symbols, compare them by value.
5. If x and y are both objects, return true if and only if they reference the same object.

In short, here are the important details of ===:

1. No implicit type coercions. === doesn't call valueOf() or do anything else to try to convert objects to primitives.
2. No value is === NaN. You should not use === to check for NaN, use Number.isNaN() instead.
3. Object comparison is by reference - two different objects can contain the exact same keys, but === will still say they're different.

const obj1 = { answer: 42 };
const obj2 = { answer: 42 };

obj1 === obj2; // false

Abstract Equality With ==

The == operator uses a more complex abstract equality comparison algorithm to compare whether x and y are equal. Here's a brief overview:

1. If x and y are the same type, check if x === y.
2. If x and y are both either null or undefined, return true.
3. If x is a number and y is a string, convert y to a number and then compare using ===. Similarly, if x is a boolean or string, and y is a number, convert x to a number.
4. If x or y is a boolean, convert the other value of a number and compare them.
5. If x is an object and y is a symbol, string, or number, try to convert x to a primitive using valueOf() and then compare using ===.

Abstract equality comparison is responsible for many of the strange edge cases that JavaScript is so famous for.

'    ' == 0; // true
'    ' == false; // true
({ valueOf: () => 42 }) == 42; // true
({ valueOf: () => 0 }) == false; // true

In general, you should always use === rather than == unless you're sure you know what you're doing. There is one neat functional use for ==: checking for null or undefined (so-called nullish values) with a single check:

// Only true if `v === null` or `v === undefined`
v == null;

// Equivalent:
v === null || v === undefined;

Checking if v == null is a more concise way of checking if v is strictly equal to null or undefined. You may choose to do this if you're a more advanced JavaScript developer, but there is no harm in always using ===.

ESLint has a rule to disallow == unless the right hand side is null,

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