Discussion on: JavaScript: How to Remove Duplicate Values from Arrays

[Evan Kennedy]

The approach below is O(n) and should work without ES6. The issue with some of the approaches above is they are using Array.indexOf or Array.includes, which is basically another loop through the array. You basically take a performance hit when the searched values are located near the end of the array. The only downside to the method below is that it's memory intensive for large arrays. You're essentially giving up memory for performance.

const originalArray = [1, 2, 3, 4, 1, 2, 3, 4];

let key = {};

const uniqueArray = originalArray.filter((item) => {
  if (key[item]) return false;

  key[item] = true;
  return true;
});

[willsmart]

A similar function using Set is

function withoutDuplicates(inputArray) {
  const trackerSet=new Set();
  return inputArray.filter(
    v => !trackerSet.has(v) && trackerSet.add(v)
  ) // add returns the Set instance, which is truthy. You may wish to add a more explicit truthy return
}

These have the main advantage over Array.from(new Set(inputArray)) in that there's no need to iterate the Set after filling it with values, but I've done a little profiling and my snippet has pretty much the same characteristics and timing as Array.from(new Set(inputArray)) (just a little bit slower), even for very large arrays.

tbh, I think the big win is that they aren't leaning on JS Set's latent insertion-order stability, making the code more WYSIWYG, and more portable since in many other C-style languages Sets don't have that guarantee.

[Will Harris]

Thanks Evan! That's also a great way to approach the problem.