What is a function in JavaScript?

0-2 years experience

1. 1We have a button click handler where we need to pass a user ID, but right now it's executing immediately on page load instead of waiting for the click. How would you wrap this in a function to make sure it only runs when clicked?
2. 2Imagine you have a helper function that calculates a discount. What is the practical difference if we define this using 'function calculate()' versus 'const calculate = () => {}' when it comes to where we can call it in our file?

2-5 years experience

1. 1We are refactoring a legacy class component. In one of our custom event handlers, we are getting a 'Cannot read properties of undefined (reading state)' error because of how the handler function was declared. Why did we lose the context of 'this', and what are the tradeoffs of fixing it with an arrow function versus binding it in the constructor?
2. 2We need to limit how often a search input triggers an API call to prevent rate-limiting. How would you write a higher-order function that takes our original fetch function and returns a debounced version of it?

5-8 years experience

1. 1In a high-throughput Node.js service, we are experiencing a slow memory leak. We suspect that closures inside our event listeners are retaining references to large request payloads long after the requests have completed. How would you investigate this, and how would you restructure the functions to ensure the garbage collector can clean up those payloads?
2. 2We are designing a plugin architecture for our frontend SDK. We want third-party developers to register middleware functions. How would you design the execution pipeline to support both synchronous and asynchronous middleware functions while ensuring a single failing middleware doesn't crash the entire host application?

8+ years experience

1. 1Our organization is migrating a massive, decade-old codebase from a mix of prototype-based OOP and global functions to a strict functional programming paradigm. What architectural guidelines, linting strategies, and testing patterns would you establish to ensure this transition doesn't introduce runtime regressions across our distributed engineering teams?
2. 2We are building a custom, high-performance serverless edge runtime. We need to decide whether to expose user-defined handlers as standard JS functions, async functions, or generator-based streams. What are the cold-start, memory footprint, and execution lifecycle implications of each choice for our platform's architecture?