Reduce by hand

function reduce(arr, reduceCallback, initialValue) {
  if (!Array.isArray(arr) || !arr.length || typeof reduceCallback !== 'function') 
  {
    return [];
  } else {
    let hasInitialValue = initialValue !== undefined;
    let value = hasInitialValue ? initialValue : arr[0];
    for (let i = hasInitialValue ? 0 : 1, len = arr.length; i < len; i++) {
      value = reduceCallback(value, arr[i], i, arr); 
    }
    return value;
  }
}