Question library

A good interview explanation gives the listener a usable understanding of a concept, not just a memorized definition. Start with what it is, why it exists and how it works at a basic level. Then connect the idea to an example, a mistake or a trade-off so the interviewer can follow your reasoning.

What should I study first?

Preparation works best when later topics build on earlier ones. JavaScript scope and async behavior support Node and React; database rules support backend correctness and system design. Follow the sequence, but spend extra time where you cannot yet explain a concept without reading the answer.

How do I answer a debugging question?

A debugging answer is an investigation plan that narrows a symptom to a cause using evidence. It should explain what you would measure, what the result would mean and what you would do next. Separate reducing user impact now from proving and preventing the underlying defect.

What if I do not know an answer?

An honest answer can still show strong engineering judgment when part of a topic is unfamiliar. Clearly separate facts you know, assumptions you are making and details you would verify. This lets the interviewer assess your reasoning without being misled by confident guesses.

How do I explain my own work?

A project answer should connect a real business problem with your specific contribution and its outcome. The problem → responsibility → action → result structure keeps the story clear without hiding technical details. Add the reason for an important decision and the trade-off you accepted.

How can I reduce dependence on AI?

Reducing AI dependence means being able to understand, test and change a solution without treating generated output as authority. Use AI as a source of candidate ideas, then establish correctness through your own reasoning and evidence. The goal is independent judgment, not avoiding useful tools entirely.

What is an execution context and call stack?

An execution context is the information JavaScript uses while running a piece of code, including its variables, scope and this value. The call stack tracks which function is currently running and where execution should return. Each function call adds a context, and returning removes it.

What is scope? What does lexical mean?

Scope is the part of a program where a variable can be accessed. JavaScript uses lexical scope, which means the available outer variables depend on where a function was defined, not where it is later called. The engine searches outward through surrounding scopes when a name is not found locally.

What is the difference between var, let and const?

var, let and const declare variables, but they have different scope and initialization rules. let and const are block-scoped, making it easier to keep a variable inside the code that needs it. const prevents reassignment of the binding; it does not make an object immutable.

What are hoisting and the TDZ (Temporal Dead Zone)?

Hoisting is JavaScript's behavior of processing declarations before executing the code in a scope. Because of this, a function declaration can usually be called before its line appears, while variables behave differently depending on how they are declared.

Are objects passed by reference?

JavaScript passes every argument by value. For an object, the copied value is a reference to that object, so the caller and function can still reach the same underlying object. This explains why changing a property affects the caller, but assigning a new object to the parameter does not.

What is the difference between == and ===?

== is loose equality: it may convert values to compatible types before comparing them. === is strict equality: it compares without that conversion. Strict equality usually makes application checks easier to understand because strings, numbers and booleans are not silently treated as the same value.

What are null, undefined and NaN?

undefined usually means a value has not been supplied, while null is commonly used to mark an intentionally empty value. NaN means a numeric calculation or conversion did not produce a valid number. These are different cases, so an API should not treat them as interchangeable without a clear rule.

JavaScript & TypeScript·Functions & objects

What is a closure?

A closure is a function together with access to the variables in the scope where it was created. That access remains available even after the outer function has finished running. Closures are useful for private state, callbacks and functions configured with some remembered value.

JavaScript & TypeScript·Functions & objects

How does this work? How are arrow functions different?

this is the receiver used when a regular function runs, and its value usually depends on how the function is called. Arrow functions do not create their own this; they use the surrounding one. This matters when a method is passed as a callback and loses its original receiver.

JavaScript & TypeScript·Functions & objects

How do prototypes and classes work?

A prototype is an object from which another object can inherit properties and methods. When a property is missing on an object, JavaScript follows its prototype chain until it finds the property or reaches null. Classes provide structured syntax for constructing objects and sharing methods through that mechanism.

JavaScript & TypeScript·Async JavaScript

How does async JavaScript run?

Asynchronous JavaScript lets a program start work that may finish later without waiting synchronously for the result. A promise represents eventual completion, and async/await provides a readable way to work with promises. The runtime queues continuations and runs them when the current synchronous work has finished and the relevant checkpoints are reached.

JavaScript & TypeScript·Async JavaScript

Which Promise method should I use?

Promise combinators combine several promises into one result. The correct method depends on whether you need every success, every outcome, the first outcome or the first success. They observe promises; they do not automatically cancel ongoing operations when the combined result settles.