In JavaScript, an array is an object with an index as a key and an array value as the value of a specific key of the array object. Sometimes, we need to check if two arrays are the same.
The first solution that comes to my mind is to use the equal sign operator to compare like array1 == array2. oops! This won't work because an array is an object and in JavaScript we can't compare two objects directly. So, we have to compare each element of the array.
In this tutorial, we will learn different methods to compare two JavaScript array objects.
Use the sort() method of JavaScript and compare every element
The sort() method allows us to sort array values in JavaScript. After that, we can use a for loop to compare the elements at each index in the array. If the elements at any index do not match, we can say that the two array objects are different.
Syntax
Users can use the sort() method and for loop to compare two array objects according to the following syntax.
// sort arrays first
arra1.sort();
array2.sort();
if (array1.length != array2.length) {
// arrays are not the same
} else {
for () {
// if elements at index i are not the same, return false
}
}
}
// both arrays are the same
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Algorithm
Users can operate according to the following algorithm.
Step 1 − Use the sort() method to sort both arrays.
Step 2 - Compare the lengths of the two arrays; if the lengths are not the same, return false, indicating that the two arrays are not the same.
Step 3 − If the lengths of the two arrays are the same, use a for loop to traverse the two arrays.
Step 4 − Compare the elements of both arrays at every index, and if the elements at the index don’t match, return false.
Step 5 - Two arrays are identical if all elements in both arrays match.
Example
In the following example, we create two arrays of numbers and sort them using the sort() method. We then use a for loop to compare each element of the two arrays.
In the output, users can see that both arrays are the same as both arrays contain the same values.
<html>
<body>
<h3>Using the <i>array.sort()</i> method and <i>for</i> loop to check if two arrays are the same using JavaScript </h2>
<button onclick = "checkArray()"> Compare arrays </button>
<div id = "output"> </div>
<script>
let output = document.getElementById('output');
let array1 = [32, 32, 54, 1, 2, 3, 4];
let array2 = [1, 2, 3, 4, 32, 54, 32];
output.innerHTML += "The first array values are " + array1 + "<br>";
output.innerHTML += "The second array values are " + array2 + "<br>";
function checkArray() {
array1.sort();
array2.sort();
if (array1.length != array2.length) {
output.innerHTML += "Both arrays are not same!";
return false;
} else {
for (let i = 0; i < array1.length; i++) {
if (array1[i] != array2[i]) {
output.innerHTML += "Both arrays are not same!";
return false;
}
}
}
output.innerHTML += "Both arrays are the same!";
return true;
}
</script>
</body>
</html>
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Use forEach loop and indexOf() method
We can use the forEach loop to iterate through every array element. The indexOf() method finds the first occurrence of the element in the array and returns -1 if the reference array doesn’t contain the element.
Syntax
Users can use the forEach loop and indexOf() method to compare two array objects according to the following syntax.
if (array2.length != array1.length) {
// array are not the same
return false;
} else {
array1.forEach((element) => {
if (array2.indexOf(element) == -1) {
return false;
}
})
}
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Algorithm
In this algorithm, we don’t need to sort the arrays like the first approach.
Step 1 - Check if the lengths of the two arrays are the same; if not, return false.
Step 2 − If the lengths are the same, use the forEach() loop to iterate through every element.
Step 3 − For each element of array 1, use the indexOf() method to check whether it exists in array 2.
Step 4 - If the indexOf() method returns -1 for any element, it means that the two arrays are not the same.
Example
In the example below, when the user clicks the button, it will invoke a compareArray() function. The compareArray() function compares the two arrays of string elements.
In the output, the user can observe that the two arrays are not the same because the values of the two arrays are different.
<html>
<body>
<h3>Using the <i>forEach() loop and indexOf() method</i> to check if two arrays are the same using JavaScript </h3>
<button onclick = "compareArray()"> Compare arrays</button>
<div id = "output"> </div>
<script>
let output = document.getElementById('output');
let array1 = ["Hello", "Hi", "How"];
let array2 = ["Are", "You", "!"];
output.innerHTML += "The first array values are " + array1 + " <br>";
output.innerHTML += "The second array values are " + array2 + " <br>";
function compareArray() {
var isSame = true;
if (array2.length != array1.length) {
output.innerHTML += "Both arrays are not same!";
return false;
} else {
array2.forEach((element) => {
if (array1.indexOf(element) == -1) {
isSame = false;
}
})
}
if (isSame) {
output.innerHTML += "Both arrays are the same!";
} else {
output.innerHTML += "Both arrays are not same!";
}
return true;
}
</script>
</body>
</html>
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We have learned two different ways to compare two arrays in JavaScript. Users can use the first method to compare arrays containing duplicate values, while the second method is only suitable for comparing arrays containing unique values.
Also, users can use the JSON.stringify() method to compare the array of objects and sorted arrays.
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