How Can I Efficiently Remove Duplicate Values from JavaScript Arrays?

Removing Duplicate Values from Arrays in JavaScript

Are you dealing with an array that may contain duplicates and need to create a new array with only the unique values? Here's a comprehensive guide with multiple approaches to solve this common issue.

TL;DR: Using ES6 Set and Spread Syntax

const uniq = [...new Set(array)];

"Smart" but Naïve Approach (Quadratic Time)

const uniqueArray = a.filter((item, pos) => a.indexOf(item) === pos);

Hashtables to the Rescue (Linear Time)

const uniq = (a) => {
  const seen = {};
  return a.filter((item) => {
    return seen.hasOwnProperty(item) ? false : (seen[item] = true);
  });
};

The Best of Both Worlds (Linear Time)

const uniq = (a) => {
  const prims = { boolean: {}, number: {}, string: {} }, objs = [];
  return a.filter((item) => {
    const type = typeof item;
    if (type in prims) {
      return prims[type].hasOwnProperty(item) ? false : (prims[type][item] = true);
    } else {
      return objs.indexOf(item) >= 0 ? false : objs.push(item);
    }
  });
};

Sort and Filter (Linear Time)

const uniq = (a) => {
  return a.sort().filter((item, pos, ary) => !pos || item !== ary[pos - 1]);
};

Unique by Key (Filtering Based on a Callback)

const uniqBy = (a, key) => {
  const seen = {};
  return a.filter((item) => {
    const k = key(item);
    return seen.hasOwnProperty(k) ? false : (seen[k] = true);
  })
};

Preserving the First or Last Occurrence (Using Set or Map)

const uniqByKeepFirst = (a, key) => {
  const seen = new Set();
  return a.filter((item) => {
    const k = key(item);
    return seen.has(k) ? false : seen.add(k);
  });
};

const uniqByKeepLast = (a, key) => {
  return [...new Map(a.map((x) => [key(x), x]))].values();
};

Choose the approach that aligns with your array type and performance requirements.

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