const insertionSort = (arr) => {
  for (let i = 0; i < arr.length; i++) {
    let el = arr[i];
    let j;
    // loop over elements to the left of currentEl
    for (j = i - 1; j >= 0 && arr[j] > el; j--) {
      // keep inserting current element until it gets to its
      // proper place
      arr[j + 1] = arr[j];
    }
    // insert current element at the correct index
    arr[j + 1] = el;
  }
  return arr;
}

const foo = [5, 3, 1, 2, 4];

insertionSort(foo); // returns [1, 2, 3, 4, 5]