Remove duplicates from an array

The challenge

Write unique(list) that returns a new array with duplicates removed, keeping the first time each item appears.

unique([1, 2, 2, 3, 1])  // [1, 2, 3]

Try it

Write your answer below, then press Check my answer. Your code runs right here in your browser, and the tests tell you what works.

What the tests check

• unique([1, 2, 2, 3, 1]) gives [1, 2, 3]
• unique(["a", "b", "a"]) gives ["a", "b"]
• unique([]) gives []
• The original array is unchanged: (() => { const input = [1, 1]; unique(input); return input; })() gives [1, 1]

Hints

Stuck? Open one hint at a time. Try again after each one.

A solution

There’s always more than one right way. Read it, make sure you understand it, then try writing it yourself without looking.

function unique(list) {
  const result = [];
  for (const item of list) {
    if (!result.includes(item)) result.push(item);
  }
  return result;
}

Learn what you need

This exercise uses what Grade 7: Arrays: Lists of Things teaches. Read those notes first if any of it is new.