Find Index of SINGLE odd / even number in array
You only need to iterate the first 3 numbers of the array.

for(let i = 0; i &lt; 3; i++) {


You are iterating through the array twice. You should only iterate once. You can do this by setting the values in the first loop:

let evenCount = 0;
let oddCount = 0;
let evenNumber = 0;
let oddNumber = 0;

for(let i = 0; i &lt; 3; i++) {
    if(integers[i] % 2 === 0) {
      evenCount++;
      evenNumber = integers[i];
    } else {
      oddCount++;
      oddNumber = integers[i];
    }
}

return evenCount &gt; 1 ? oddNumber : evenNumber;