How to Compare Two Arrays in JavaScript

1. Array.prototype.equals to Compare Two Arrays in JavaScript
2. JSON.stringify() to Compare Arrays in JavaScript
3. Compare Two Arrays in JavaScript Using Loops

This tutorial introduces three ways to compare arrays in JavaScript. There are multiple ways of doing this, but the performance may vary.

Array.prototype.equals to Compare Two Arrays in JavaScript

JavaScript provides us the capability to add new properties and methods to the existing classes. We can use Array.prototype to add our custom method equals into the Array object.

In the below example, we will first check the length of both the arrays and then comparing each item. We are also checking if a1 and a2 are Array instances because if they are not, they are not identical. At last, we use the method to compare the first array with the second one by a1.equals(a2).

Example:

var a1 = [1, 2, 3];
var a2 = [1, 2, 3];
var a3 = [1, 2, 3, 4];

Array.prototype.equals = function(getArray) {
  if (this.length != getArray.length) return false;

  for (var i = 0; i < getArray.length; i++) {
    if (this[i] instanceof Array && getArray[i] instanceof Array) {
      if (!this[i].equals(getArray[i])) return false;
    } else if (this[i] != getArray[i]) {
      return false;
    }
  }
  return true;
};

console.log('Comparing a1 and a2', a1.equals(a2));
console.log('Comparing a1 and a3', a1.equals(a3));

Output:

Comparing a1 and a2 true
Comparing a1 and a3 false

JSON.stringify() to Compare Arrays in JavaScript

Another technique to compare two arrays is to first cast them to the string type and then compare them. JSON is used to transfer data from/to a web server, but we can use its method here. We can do this is using JSON.stringify() that converts an Array to a string. As now both the a1 and a2 are strings, we can use === to see if they are equal or not.

Example:

var a1 = [1, 2, 3];
var a2 = [1, 2, 3];
var a3 = [1, 2, 3, 4];

console.log('Comparing a1 and a2', JSON.stringify(a1) === JSON.stringify(a2));
console.log('Comparing a1 and a3', JSON.stringify(a1) === JSON.stringify(a3));

Output:

Comparing a1 and a2 true
Comparing a1 and a3 false

Compare Two Arrays in JavaScript Using Loops

Looping is the most traditional way of comparing arrays in JavaScript because it involves looping through the arrays and then comparing every single element with each other to check if they match.

To make it cleaner, we can use functions and then return boolean as a result.

Example:

var a1 = [1, 2, 3];
var a2 = [1, 2, 3];
var a3 = [1, 2, 3, 4];

const getResult = function(a1, a2) {
  var i = a1.length;
  if (i != a2.length) return false;

  while (i--) {
    if (a1[i] !== a2[i]) return false;
  }
  return true;
};

console.log('Comparing a1 and a2', getResult(a1, a2));
console.log('Comparing a1 and a3', getResult(a1, a3));

Output:

Comparing a1 and a2 true
Comparing a1 and a3 false