```javascript
function reduce(array, callback, initialValue) {
  let accumulator = initialValue;
  for (let i = 0; i < array.length; i++) {
    accumulator = callback(accumulator, array[i]);
  }
  return accumulator;
}
// Uncomment these to check your work!
const nums = [4, 1, 3];
const add = function(a, b) { return a + b; }
console.log(reduce(nums, add, 0)); // should log 8
```