Detecting the first non-unique element in array in JavaScript

JavascriptWeb DevelopmentFront End TechnologyObject Oriented Programming

We are required to write a function that returns the index of the very first element that appears at least twice in the array. If no element appears more than once, we have to return -1. We have to do this in constant space (i.e., without utilizing extra memory).

So, let's write the solution for this problem.

We will use a for loop to iterate over the array and use the Array.prototype.lastIndexOf() method to check for duplicacy.


The code for this will be −

const arr1 = [0, 1, 1, 2, 3, 4, 4, 5];
const firstRedundant = arr => {
   for(let i = 0; i < arr.length; i++){
      if(arr.lastIndexOf(arr[i]) !== i){
         return i;
   return -1;
console.log(firstRedundant(arr1)); // 1


The output in the console will be −

Published on 20-Oct-2020 11:52:18