How to Find Duplicate Values in a JavaScript Array?

How to Identify Duplicate Values Within an Array

The ability to identify and retrieve non-unique elements from a JavaScript array is a common programming task. Unlike removing duplicates, this process involves segregating values that appear multiple times.

Easiest Approach Using Sorting and Iteration

The simplest and efficient solution is to leverage the sorted version of the array. After sorting, compare consecutive elements:

function findDuplicates(arr) {
  let sortedArr = arr.slice().sort();
  let duplicates = [];
  for (let i = 0; i < sortedArr.length - 1; i++) {
    if (sortedArr[i] == sortedArr[i + 1]) {
      duplicates.push(sortedArr[i]);
    }
  }
  return duplicates;
}

This approach provides an efficient search with a time complexity of less than O(n2), where n is the array size.

Consider the example array [9, 9, 111, 2, 3, 4, 4, 5, 7]:

console.log(findDuplicates([9, 9, 111, 2, 3, 4, 4, 5, 7]));
// Result: [9, 4]

The function captures the duplicate values, enabling you to identify the occurrences without retrieving the specific indices or frequency of repetition.

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