Mystery behind JS array Equality ( === )

Lets us understand the mystery

Equality operators (== or ===) both work differently for objects (Type of array is an object in JS ) when compared to other primitive types.

const a = 1;
const b = 1;

console.log(a == b) //true
console.log(a === b) //true 
console.log(1 === 1) //true
console.log(1 == 1) //true

As you can see when we compare the two numbers, we are able to get true as expected and straight forward.

const a = [1];
const b = [1];

console.log(a == b) //false
console.log(a === b) //false
console.log([1] === [1]) //false
console.log([1] == [1]) //false
console.log(a == [1]) //false
console.log(a === [1]) //false
console.log(a == a) //true
console.log(a === a) //true

We just wrapped the same number to array, suddenly we are getting false , which is not the usual expectation.

As mentioned earlier, we are actually comparing the references not values, incase of object

typeof [1] — object

typeof 1 — number

So be careful when you are comparing array (and object) in javascript.

So how to compare the values of the array

If order of the elements should be matched, we can compare by converting to string

JSON.stringify(a) === JSON.stringify(b)

If elements can be of any order in both the arrays, we can do the following

a.length === b.length && a.every((value, index) => value === b[index])

we have to iterate with every element in the array and ensure every element on each side is equal