Equality of Values in JavaScript

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Understanding the equality of values helps prevent bugs. You often need to know when you are dealing with the same value or different values.

There are three kinds of equality in JavaScript:

Same Value Equality

Same value equality tells us whether two values are the same value. You can use Object.is(a, b) to compare any two values, primitive or object.

Let’s look at an example using primitive values:

let dwarves = 7;
let continents = "7";
let worldWonders = 3 + 4;

console.log(Object.is(dwarves, continents)); // false
console.log(Object.is(continents, worldWonders)); // false
console.log(Object.is(worldWonders, dwarves)); // true

Let’s look at an example using object values and remember that {} creates a new object value. Also, remember that = means “point the wire on the left side to the value on the right side”.

let banana = {};
let cherry = banana;
let chocolate = cherry;
cherry = {};

console.log(Object.is(banana, cherry)); // false
console.log(Object.is(cherry, chocolate)); // false
console.log(Object.is(chocolate, banana)); // true

Strict Equality

Strict equality compares two values using the === operator and there is also a corresponding opposite !== operator.

console.log(2 === 2); // true
console.log(2 !== 2); // false
console.log({} === {}); // false

In the previous examples, a === b behaves in the same way as Object.is(a, b). However, there are two rare cases where the behavior of === is different:

Strict Equality with NaN

NaN is a special value in JavaScript that means “not a number” and appears when performing invalid calculations such as 0 / 0.

let width = 0 / 0; // NaN
let height = width * 2; // NaN

console.log(width === height); // false
console.log(Object.is(width, height)); // true

Remember that NaN === NaN is always false, even though NaN is the same value as NaN.

Also, if you want to check if a value is NaN, instead of value === NaN, here are some ways to check if the value is NaN:

Number.isNaN(value);
Object.is(value, NaN);
value !== value;

Strict Equality with -0 and 0

Both 0 === -0 and -0 === 0 are always true, even though 0 is a different value from -0:

let width = 0; // 0
let height = -width; // -0

console.log(width === height); // true
console.log(Object.is(width, height)); // false

Loose Equality

The rules of loose equality (or “abstract equality”) are a bit confusing. In fact, it is uncommon in modern codebases, and many coding standards prohibit the use of == and != in code.