JavaScript Closures, Event Loop, and Promises Interview Questions
Closures, the event loop, and promises are core JS interview topics. Here are the questions and how to answer them.
JavaScript Closures, Event Loop, and Promises Interview Questions
Closures, the event loop, and promises are core JavaScript interview topics that test depth. Here are the questions and how to answer them.
What is a closure and why does it matter?
A closure is a function that remembers the variables from where it was created. It matters for data privacy, callbacks, and functional patterns like once and memoize.
What are common closure use cases?
Data privacy (encapsulating variables inside a factory function), callbacks (like setTimeout), and patterns like once, memoize, and partial application. Anything that 'remembers' state across calls uses closures.
What is the event loop and why does it matter?
The event loop lets JavaScript handle async on a single thread. It processes the call stack, then microtasks (promises), then tasks (setTimeout, events). It matters because it explains the order async code runs.
What is the difference between microtasks and macrotasks?
Microtasks (promise callbacks) run after the current task and before the next macrotask. Macrotasks (setTimeout, events) run one per loop. This ordering is why promise callbacks run before setTimeout callbacks.
What is a promise?
A promise represents the eventual result of an async operation. It has three states: pending, fulfilled, rejected. Once settled, it cannot change. Use then and catch or async/await to handle it.
What is async/await?
Syntax for promises that lets you write async code that looks synchronous. await pauses the function until the promise settles. async marks a function as returning a promise.
The Takeaway
Closures, the event loop, and promises are core JS depth topics. Know what a closure is and its use cases, the event loop and microtask vs macrotask ordering, promises and their states, and async/await. These separate strong candidates from definers.
A closure is a function that remembers the variables from where it was created. It matters for data privacy, callbacks, and functional patterns like once and memoize, since anything that 'remembers' state across calls uses closures.
Data privacy (encapsulating variables inside a factory function), callbacks like setTimeout, and patterns like once, memoize, and partial application. Anywhere you need to 'remember' state across calls without global variables uses closures.
How JavaScript handles async on a single thread. It processes the call stack, then microtasks like promises, then macrotasks like setTimeout and events. It matters because it explains the order async code runs.
Microtasks (promise callbacks) run after the current task and before the next macrotask. Macrotasks (setTimeout, events) run one per loop. This ordering is why promise callbacks run before setTimeout callbacks set at the same time.
Syntax for promises that lets you write async code that looks synchronous. await pauses the function until the promise settles, and async marks a function as returning a promise. Under the hood it is still promises, just cleaner syntax.
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