How to check whether the items are unique in an array using JavaScript

Question

If you have an array with primitive values as per below;

const arr = [1, 2, 3, 4, 45, 4, 66, 3];

Is there a more efficient way to check whether all the items are unique rather than iterating all items as below?

let newArr = [];
let isUnique = true;
arr.forEach(item => 
{
   if(newArr.indexOf(item) != -1)
   {
       isUnique = false;
       break;
   }
   newArray.push(item);
});

CertainPerformance · Accepted Answer · 2020-10-17 02:54:54Z

4

An indexOf in a forEach is O(n ^ 2). I'd use a Set instead - Set.has is O(1) (overall complexity of O(n)):

const allAreUnique = (arr) => {
  const set = new Set();
  for (const item of arr) {
    if (set.has(item)) return false;
    set.add(item);
  }
  return true;
};

const allAreUnique = (arr) => {
  const set = new Set();
  for (const item of arr) {
    if (set.has(item)) return false;
    set.add(item);
  }
  return true;
};
console.log(allAreUnique([1, 2, 3, 4, 45, 4, 66, 3]));
console.log(allAreUnique([1, 2, 3, 4, 45, 66]));

answered Oct 17, 2020 at 2:54

CertainPerformance

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4 Comments

Sam Sirry Over a year ago

Looks like the return is always true. The question requires knowing whether all the items are unique.

Michael Geary Over a year ago

@SamSirry The code looks correct to me. If you run the snippet the first test returns false as expected.

Christie Over a year ago

Thank you @CertainPerformance for the prompt answer. Appreciate it!

Sam Sirry Over a year ago

Sorry, I misread the code. Probably something was wrong with morning coffee... Thank you for your insight @MichaelGeary

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