72. Array.prototype.reduce (Polyfill)

Implement Array.prototype.myReduce — a polyfill that behaves like native Array.prototype.reduce.

Reduce applies a reducer function to each element and returns a single accumulated result.

Requirements

1) Core behavior

• Signature: arr.myReduce(callbackFn, initialValue?)
• Callback receives: (accumulator, currentValue, index, array)
• Return the final accumulator.

2) initialValue rules (most important)

• If initialValue is provided: start at index 0 with that accumulator.
• If initialValue is not provided:
• Find the first existing element and use it as accumulator.
• Start reducing from the next index.

3) Edge cases

• If the array has no existing elements and no initialValue, throw TypeError.
• Skip holes in sparse arrays.
• Capture length at start (pushes beyond initial length are not visited).