FAANG-Level JavaScript Interview Questions With Explanations
FAANG asks deep JavaScript questions. Here are the common ones with explanations.
FAANG-Level JavaScript Interview Questions With Explanations
FAANG companies ask deep JavaScript questions that test understanding, not just syntax. Here are the common ones with explanations.
Explain the event loop with microtasks and macrotasks
The event loop processes the call stack, then microtasks (promise callbacks), then macrotasks (setTimeout, events), one per loop. Microtasks run after each task and before the next macrotask, which is why promise callbacks run first.
How do closures enable data privacy?
Closures encapsulate variables inside a factory function, so they are not accessible from outside. This enables private state without classes or modules, and explains patterns like once and memoize.
What happens when you call setTimeout with 0?
It does not run immediately. The callback goes to the macrotask queue and runs after the current stack and all microtasks complete. setTimeout 0 defers to the next event loop tick.
What is the difference between call, apply, and bind?
call and apply call a function with a specific this and arguments (apply takes an array). bind returns a new function with this permanently set. All are about controlling this explicitly.
What is a prototype chain?
Each object has a prototype, which is another object. When you access a property, JS looks on the object, then its prototype, then its prototype's prototype, up the chain until found or reaching null. This is how inheritance works in JS.
What is garbage collection in JavaScript?
JS automatically frees memory that is no longer reachable. The garbage collector finds values no longer referenced and frees them. Memory leaks happen when values are still referenced unintentionally, like forgotten event listeners.
The Takeaway
FAANG JS questions test depth: the event loop with microtasks vs macrotasks, closures for privacy, setTimeout 0 behavior, call/apply/bind, the prototype chain, and garbage collection. These test understanding of how the language actually works.
The event loop processes the call stack, then microtasks (promise callbacks), then macrotasks (setTimeout, events), one per loop. Microtasks run after each task and before the next macrotask, which is why promise callbacks run before setTimeout callbacks.
It does not run immediately. The callback goes to the macrotask queue and runs after the current stack and all microtasks (promise callbacks) complete. setTimeout 0 defers to the next event loop tick, not to the current one.
call and apply call a function with a specific this and arguments (apply takes an array). bind returns a new function with this permanently set. All three are about controlling this explicitly, which is useful when this matters.
Each object has a prototype, which is another object. When you access a property, JS looks on the object, then its prototype, then up the chain until found or reaching null. This is how inheritance works in JS without classes.
JS automatically frees memory that is no longer reachable. The garbage collector finds values no longer referenced and frees them. Memory leaks happen when values are still referenced unintentionally, like forgotten event listeners or closures.
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