Get all non-unique values (i.e.: duplicate/more than one occurrence) in an array

Question

I need to check a JavaScript array to see if there are any duplicate values. What\'s the easiest way to do this? I just need to find what the duplicated values are - I don\'

Answer 1

Here's the simplest solution I could think of:

    const arr = [-1, 2, 2, 2, 0, 0, 0, 500, -1, 'a', 'a', 'a']

    const filtered = arr.filter((el, index) => arr.indexOf(el) !== index)
    // => filtered = [ 2, 2, 0, 0, -1, 'a', 'a' ]

    const duplicates = [...new Set(filtered)]

    console.log(duplicates)
    // => [ 2, 0, -1, 'a' ]

That's it.

Note:

1. It works with any numbers including 0, strings and negative numbers e.g. -1 - Related question: Get all unique values in a JavaScript array (remove duplicates)

2. The original array arr is preserved (filter returns the new array instead of modifying the original)

3. The filtered array contains all duplicates; it can also contain more than 1 same value (e.g. our filtered array here is [ 2, 2, 0, 0, -1, 'a', 'a' ])

4. If you want to get only values that are duplicated (you don't want to have multiple duplicates with the same value) you can use [...new Set(filtered)] (ES6 has an object Set which can store only unique values)

Hope this helps.