Implement myReduce (with and without an initial value)

Implement myReduce(arr, callback, initialValue)

The trickiest of the array polyfills, because initialValue is optional, and "omitted" is different from "passed as undefined":

• If initialValue was passed (3 arguments), start the accumulator there and iterate over every element from index 0.
• If it was not passed (2 arguments), use arr[0] as the initial accumulator and iterate from index 1.
• If it was not passed and the array is empty, throw a TypeError.

myReduce([1, 2, 3], (acc, n) => acc + n, 0); // 6
myReduce([1, 2, 3], (acc, n) => acc + n);    // 6 (starts from arr[0])
myReduce([], (acc, n) => acc + n);           // throws TypeError

Hint: arguments.length (this must be a regular function, not an arrow function) tells you whether the caller actually passed a third argument.