What are the most commonly asked JavaScript interview topics?

The top 6 core interview topics are: 1. Closures & Lexical Scope; 2. Event Loop & Microtask/Macrotask Queue; 3. The 'this' keyword & call/apply/bind; 4. Prototypal Inheritance; 5. Promises & Async/Await; 6. var vs let vs const & Hoisting.

How should a BCA or CS student answer technical viva questions effectively?

Follow the 3-step technical answer framework: 1. State the concise formal definition; 2. Provide a real-world analogy or memory model explanation (Stack vs Heap); 3. Walk through a brief 2-line code example illustrating the concept.

JavaScript 100+ Interview Questions & Answers: University Viva & Job Mastery Guide

Welcome to the definitive JavaScript Interview & University Viva Voce Preparation Guide. Curated specifically for BCA Students, Computer Science Undergraduates, Full-Stack Job Seekers, and Tech Bloggers, this high-yield resource distills the most frequently tested JavaScript concepts into clear, authoritative answers.

Category 1: Core Fundamentals & Language Architecture

Q1. Is JavaScript an interpreted, compiled, or hybrid language?

Answer: JavaScript is a Just-In-Time (JIT) compiled language. In modern engines like Google Chrome's V8, source code is parsed into an Abstract Syntax Tree (AST), converted to bytecode by the Ignition interpreter for instant execution, and concurrently profiled by the TurboFan JIT compiler which compiles frequently executed ("hot") code into optimized native machine code.

Q2. Differentiate between == (loose equality) and === (strict equality).

Answer: == compares values with implicit type coercion (converting types if they differ, e.g., 5 == "5" is true). === performs strict equality checking without coercion, requiring both the value and the data type to match exactly (5 === "5" is false). In production, always use ===.

Q3. What are the 7 Primitive data types in JavaScript?

Answer: The 7 primitive types are: Number, String, Boolean, Undefined, Null, BigInt, and Symbol. Primitives are immutable and stored directly in the Call Stack by value. All other entities are Reference Objects stored in the Memory Heap.

Q4. What is the difference between null and undefined?

Answer: undefined signifies that a variable has been declared but not yet assigned any value. null represents an intentional assignment signifying the absence of any object value. Note that typeof undefined === "undefined", while typeof null === "object" due to a legacy 1995 V8 type tag bug.

Category 2: Execution Context, Hoisting & Scoping

Q5. What is Hoisting and what is the Temporal Dead Zone (TDZ)?

Answer: Hoisting is JavaScript's compile-phase behavior of allocating memory for variable and function declarations before code execution. var is initialized with undefined. let and const are allocated in the Temporal Dead Zone (TDZ) and remain uninitialized until their physical declaration line, throwing a ReferenceError if accessed prematurely.

Q6. What is a Closure and what is a practical use case?

Answer: A closure is the combination of an inner function bundled with references to its outer lexical environment. It allows an inner function to access an outer function's variables even after the outer function has completed execution and popped off the Call Stack. Use cases: Data privacy (private variables), function currying, and memoization caches.

Q7. How do call(), apply(), and bind() differ?

Answer: All three explicitly bind the this keyword. call(thisArg, arg1, arg2) executes immediately with comma-separated arguments. apply(thisArg, [argsArray]) executes immediately with arguments in an array. bind(thisArg) returns a brand-new function with this permanently bound without executing immediately.

Category 3: Asynchronous JavaScript & The Event Loop

Q8. How does single-threaded JavaScript handle concurrent async tasks?

Answer: Through the Event Loop concurrency model. Synchronous code executes in the single-threaded Call Stack. Asynchronous tasks (timers, fetch) are offloaded to browser Web API background threads. Once finished, callbacks are pushed to task queues (Microtask Queue for Promises; Macrotask Queue for Timers). When the Call Stack is empty, the Event Loop pushes pending callbacks to the stack.

Q9. What is the execution priority between Microtasks and Macrotasks?

Answer: The Microtask Queue has higher priority. The Event Loop drains the entire Microtask Queue (Promise callbacks, async/await continuations) before executing even a single task from the Macrotask Queue (setTimeout, setInterval).

Q10. Why does fetch() not reject on HTTP 404 or 500 status codes?

Answer: The Promise returned by fetch() only rejects on true network failures (DNS lookup failure, loss of internet connection, CORS blockage). For HTTP 404 Not Found or 500 Server Errors, the promise resolves normally. Developers must check if (!response.ok) manually.

Category 4: Tricky Output Predictor Questions

Q11. Predict Output: console.log(typeof NaN); and console.log(NaN === NaN);

Answer:
typeof NaN returns "number".
NaN === NaN returns false (NaN is the only value in JavaScript never equal to itself. Use Number.isNaN() to check).

Q12. Predict Output: console.log(0.1 + 0.2 === 0.3);

Answer: Returns false! Due to binary IEEE 754 floating-point precision representation, 0.1 + 0.2 = 0.30000000000000004. To compare safely, use Math.abs((0.1 + 0.2) - 0.3) < Number.EPSILON.