binary_search.js

const arr = [2,3,5,6,23,34,45,86];
const toFind = 23;

function indexOf (arr, num) {
    let low = 0,
        high = arr.length-1;
  
    while (low <= high) {
      let mid = parseInt((high + low) / 2)
      let current = arr[mid]
      if (current > toFind) { // lower part
        high = mid - 1;
      } else if (current < toFind) { // upper part
        low = mid + 1;
      } else {
        return mid;
      }
    }
  
    return -1
}

console.log(indexOf(arr, toFind));