Reproducing the reduce() method

reduce.js

function reduce(arr, callbackFunc, initVal) {
  /*
      1. make sure array is being passed
      2. if initVal is not passed, accumulator uses the first element in array
         then just use the array starting with 2nd element
         if initVal is passed, accumulator is set to initVal
  */
  if (Array.isArray(arr)) {
  	let accum;
    if (initVal === undefined) {
      accum = arr[0];
      arr = arr.slice(1);
    } else {
      accum = initVal;
    }
  
     for (let i = 0; i < arr.length; i++) {
       accum = callbackFunc(accum, arr[i], i, arr);
     }
    return accum;
  }
}