Lodash : Get duplicate values from an array

In JavaScript, using the Lodash library to extract duplicate values from an array is a common requirement.

First, we can utilize the Lodash _.countBy function to count the occurrences of each element in the array, then combine it with _.keys and _.filter to identify elements that appear more than once.

Here is a specific example:

Assume we have an array: [1, 2, 2, 3, 4, 4, 5], and we want to find the duplicate elements within this array.

javascript
import _ from 'lodash';

// Original array
const originalArray = [1, 2, 2, 3, 4, 4, 5];

// Use _.countBy to count occurrences of each value
const countElements = _.countBy(originalArray);
// countElements result is: { '1': 1, '2': 2, '3': 1, '4': 2, '5': 1 }

// Filter elements with occurrences greater than 1
const duplicates = _.filter(_.keys(countElements), (key) => countElements[key] > 1);
// duplicates result is: ['2', '4'], indicating that 2 and 4 are duplicates

// Convert the string array to a numeric array
const duplicateNumbers = duplicates.map(Number);
// duplicateNumbers result is: [2, 4]

In this example, we first apply the _.countBy function to count occurrences of each element, then use _.keys to retrieve all keys (corresponding to the original array values), followed by _.filter to select keys with occurrences exceeding 1, and finally convert the resulting string array to a numeric array.

This approach is not only clear but also leverages Lodash's efficient functions, resulting in concise and easily understandable code.