Whre do I belong challenge, 'for ... in' loop not working

Why isn’t "for in " working in that challenge ???

function getIndexToIns(arr, num) {
 ... sort the array

  for(let i in sortedArr){//for in not working... any explanation ???
    if(sortedArr[i] >= num){
      return i

  ... rest of the code

let x = getIndexToIns([10, 20, 30, 40, 50], 35);

Basic Algorithm Scripting: Where do I Belong | freeCodeCamp.org

this link may have explanation, see Array iteration and for...in section of an article:

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