JavaScript Interview Prep
Polyfills — bind, map, filter, reduce, Promise.all, debounce, throttle
Master writing core JavaScript polyfills from scratch. Implement bind, map, filter, reduce, Promise.all, debounce, and throttle without built-in helpers.
1. Introduction
A Polyfill is code that implements a feature on web browsers that do not natively support it. Writing polyfills from scratch is one of the most effective ways to prove deep mastery of JavaScript during senior technical interviews.
2. Essential Polyfill Implementations
1. Polyfill for Function.prototype.bind:
2. Polyfill for Array.prototype.map:
3. Polyfill for Array.prototype.filter:
4. Polyfill for Array.prototype.reduce:
5. Polyfill for Promise.all:
6. Polyfill for debounce:
7. Polyfill for throttle:
3. Quick Quiz
Q1: Why must Promise.all polyfills use results[index] = value instead of results.push(value)?
A) Because push() causes memory leaks
B) Because promises resolve asynchronously in arbitrary order; indexing guarantees that the output array preserves the input order
Answer: B — Indexing preserves the exact input order regardless of when individual promises finish resolving.
4. Production Considerations
- • Avoid Mutating Native Prototypes: In application code, avoid overwriting
Array.prototypeorFunction.prototypedirectly unless writing compatibility polyfills for older browser engines. Modifying shared global prototypes can cause collision bugs with third-party libraries.