How does Function.prototype.bind work under the hood? Can you implement a polyfill for it?

0-2 years experience

1. 1You need to pass a callback to Array.map that uses this from the surrounding object. How would you use bind to make it work, and what would happen if you omitted it?
2. 2Write a short snippet that creates a bound version of an object method and calls it later. What does this refer to inside the bound function?

2-5 years experience

1. 1Our legacy module creates many bound functions and we’re seeing a memory leak. How would you investigate the cause and what alternatives could you consider?
2. 2A teammate replaced a bound function with an arrow function, and a bug appeared when the function is instantiated with new. Explain why the bug occurs.
3. 3Implement a polyfill for Function.prototype.bind that correctly supports being called with the new operator. Walk me through your approach.

5-8 years experience

1. 1Our UI library ships a custom bind polyfill for older browsers, but bundle size and startup time have grown. How would you evaluate whether to keep the polyfill or drop support, and what trade‑offs are involved?
2. 2Design a utility that provides bind‑like behavior but also memoizes bound functions to avoid creating duplicates. What edge cases must you handle to keep semantics correct?
3. 3Explain how bind interacts with prototype inheritance when the bound function is used as a constructor. How would you test that your implementation preserves the prototype chain?

8+ years experience

1. 1We are migrating a massive codebase from ES5 to ES6+ and want to remove all uses of Function.prototype.bind. What strategy would you propose to refactor safely, and how would you mitigate regression risk?
2. 2Consider a cross‑team library that exposes a public API using bound methods. If you need to deprecate bind in favor of arrow functions, what architectural considerations and communication plan would you put in place?