Return the first duplicate number from an array in JavaScript

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. The condition is that we have to do this in constant space (i.e., without utilizing extra memory).

Let’s devise 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 duplicates.

Example

const firstDuplicate = arr => {    for(let i = 0; i < arr.length; i++){       if(arr.lastIndexOf(arr[i]) !== i){          return i;       };    };    return -1; } console.log(firstDuplicate([3, 5, 6, 8, 5, 3])); // 0 console.log(firstDuplicate([0, 1, 2, 3, 4, 4, 5])); // 4 console.log(firstDuplicate([0, 1, 1, 2, 3, 4, 4, 5])); // 1 console.log(firstDuplicate([0, 1, 2, 3, 4, 9, 5])); // -1

Output

The output in the console will be −

0 4 1 -1