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);
});

Answer 1

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]));

• Sam Sirry: Looks like the return is always true. The question requires knowing whether all the items are unique.
• Michael Geary: @SamSirry The code looks correct to me. If you run the snippet the first test returns false as expected.
• Christie: Thank you @CertainPerformance for the prompt answer. Appreciate it!
• Sam Sirry: Sorry, I misread the code. Probably something was wrong with morning coffee... Thank you for your insight @MichaelGeary