How to find duplicates in an array using JavaScript

There are multiple methods available to check if an array contains duplicate values in JavaScript. You can use the indexOf() method, the Set object, or iteration to identify repeated items in an array.

Set Object

Set is a special data structure introduced in ES6 that stores a collection of unique values. Since each value in a Set has to be unique, passing any duplicate item will be removed automatically:

const numbers = [1, 2, 3, 2, 4, 5, 5, 6];

const unique = Array.from(new Set(numbers));

console.log(unique);
// [ 1, 2, 3, 4, 5, 6 ]

The Array.from() method, we used above, converts the Set back to an array. This is required because a Set is not an array. You could also use spread operator if you want for conversion:

const unique = [...new Set(numbers)];

To check if there were duplicate items in the original array, just compare the length of both arrays:

const numbers = [1, 2, 3, 2, 4, 5, 5, 6];

const unique = Array.from(new Set(numbers));

if(numbers.length === unique.length) {
    console.log(`Array doesn't contain duplicates.`);
} else {
    console.log(`Array contains duplicates.`);
}
// Output: Array contains duplicates.

To find out exactly which elements are duplicates, you could make use of the unique array above, and remove each item from the original array as shown below:

const numbers = [1, 2, 3, 2, 4, 5, 5, 6];

const set = new Set(numbers);

const duplicates = numbers.filter(item => {
    if (set.has(item)) {
        set.delete(item);
    } else {
        return item;
    }
});

console.log(duplicates);
// [ 2, 5 ]

indexOf() Method

In this method, we compare the index of the first occurrence of an element with all the elements in an array. If they do not match, it implies that the element is a duplicate:

const numbers = [1, 2, 3, 2, 4, 5, 5, 6];

const duplicates = numbers.filter((item, index) => index !== numbers.indexOf(item));

console.log(duplicates);
// [ 2, 5 ]

The above solution works perfectly as long as you only want to check if the array contains repeated items. However, the output array may contain duplicate items if those items occur more than twice in the array:

const numbers = [1, 2, 3, 2, 2, 4, 5, 5, 6];

const duplicates = numbers.filter((item, index) => index !== numbers.indexOf(item));

console.log(duplicates);
// [ 2, 2, 5 ]

some() Method

In JavaScript, the some() method returns true if one or more elements pass a certain condition.

Just like the filter() method, the some() method iterates over all elements in an array to evaluate the given condition.

In the callback function, we again use the indexOf() method to compare the current element index with other elements in the array. If both the indexes are the same, it means that the current item is not duplicate:

const numbers = [1, 2, 3, 2, 4, 5, 5, 6];

const isDuplicate = numbers.some((item, index) => index !== numbers.indexOf(item));

if (!isDuplicate) {
    console.log(`Array doesn't contain duplicates.`);
} else {
    console.log(`Array contains duplicates.`);
}
// Output: Array contains duplicates.

for Loop

Finally, the last method to find duplicates in an array is to use the for loop.

Here is an example that compares each element of the array with all other elements of the array to check if two values are the same using nested for loop:

const numbers = [1, 2, 3, 2, 4, 5, 5, 6];

let isDuplicate = false;

// Outer for loop
for (let i = 0; i < numbers.length; i++) {
    // Inner for loop
    for (let j = 0; j < numbers.length; j++) {
        // Skip self comparison
        if (i !== j) {
            // Check for duplicate
            if (numbers[i] === numbers[j]) {
                isDuplicate = true;
                // Terminate inner loop
                break;
            }
        }
        // Terminate outer loop
        if (isDuplicate) {
            break;
        }
    }
}

if (!isDuplicate) {
    console.log(`Array doesn't contain duplicates.`);
} else {
    console.log(`Array contains duplicates.`);
}
// Output: Array contains duplicates.

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