71. Array.prototype.map (Polyfill)

Implement Array.prototype.myMap — a polyfill that behaves like native Array.prototype.map.

Mapping is a foundational skill in JavaScript interviews, but many implementations miss important details: sparse arrays (holes), callback arguments, thisArg binding, and mutation behavior during iteration.

Requirements

1) Core behavior

• arr.myMap(callbackFn, thisArg?) returns a new array.
• It calls callbackFn once per existing element in the array (skip holes).
• It passes (value, index, array) to the callback.
• It does not mutate the original array (unless the callback does).

2) Edge cases / correctness

• Preserve output array length equal to the input length.
• Skip missing indices in sparse arrays (holes remain holes in output).
• thisArg binds the callback this (non-arrow callbacks).
• Throw TypeError if callbackFn is not a function.
• Use the array length as captured at start (items appended beyond initial length are not visited).