Find all duplicates in an array in JavaScript

This post will discuss how to find all duplicates in an array in JavaScript.

1. Using `Array.prototype.indexOf()` function

The idea is to compare the index of all items in an array with an index of their first occurrence. If both indices don’t match for any item in the array, you can say that the current item is duplicated. To return a new array with duplicates, use the filter() method.

The following code example shows how to implement this using the indexOf() method:

const arr = [ 5, 3, 4, 2, 3, 7, 5, 6 ];

const findDuplicates = arr => arr.filter((item, index) => arr.indexOf(item) !== index)

const duplicates = findDuplicates(arr);

console.log(duplicates);

Output: [ 3, 5 ]

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The above solution can result in duplicate values in the output. To handle this, you can convert the result into a Set, which stores unique values.

function findDuplicates(arr) {

const filtered = arr.filter((item, index) => arr.indexOf(item) !== index);

return [...new Set(filtered)]

}

const arr = [ 5, 3, 4, 2, 3, 7, 5, 6 ];

const duplicates = findDuplicates(arr);

console.log(duplicates);

Output: [ 3, 5 ]

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2. Using `Set.prototype.has()` function

Alternatively, to improve performance, you can use the ES6 Set data structure for efficiently filtering the array.

The following solution finds and returns the duplicates using the has() method. This works because each value in the Set has to be unique.

function findDuplicates(arr) {

const distinct = new Set(arr); // to improve performance

const filtered = arr.filter(item => {

// remove the element from the set on very first encounter

if (distinct.has(item)) {

distinct.delete(item);

}

// return the element on subsequent encounters

else {

return item;

}

});

return [...new Set(filtered)]

}

const arr = [ 5, 3, 4, 2, 3, 7, 5, 6 ];

const duplicates = findDuplicates(arr);

console.log(duplicates);

Output: [ 3, 5 ]

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That’s all about finding all duplicates in an array in JavaScript.

Find all duplicates in an array in JavaScript

1. Using Array.prototype.indexOf() function

2. Using Set.prototype.has() function

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1. Using `Array.prototype.indexOf()` function

2. Using `Set.prototype.has()` function