JavaScript Interview Questions

Deep dive into the core mechanics of JavaScript that interviewers love to test.

1. What is a "Closure"?

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In simpler terms, a closure gives you access to an outer function's scope from an inner function.

function makeCounter() {
  let count = 0;
  return function() {
    return ++count;
  };
}
const counter = makeCounter();
console.log(counter()); // 1
console.log(counter()); // 2

2. What is "Hoisting"?

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope during the compilation phase.

Variables (var): Hoisted and initialized as undefined.
Variables (let, const): Hoisted but not initialized, leading to the "Temporal Dead Zone."
Function Declarations: Completely hoisted.

3. Difference between `==` and `===`?

== (Abstract Equality): Performs type conversion (coercion) before comparing.
=== (Strict Equality): Does not perform type conversion. It only returns true if both the value and the type are identical. Always use ===.

4. What is the Event Loop?

The Event Loop is the mechanism that allows JavaScript to perform non-blocking I/O operations despite being single-threaded. It constantly checks if the Call Stack is empty and moves tasks from the Callback Queue/Microtask Queue to the stack.

5. What are Promises?

A Promise is an object representing the eventual completion (or failure) of an asynchronous operation and its resulting value. It has three states:

Pending: Initial state.
Fulfilled: Operation completed successfully.
Rejected: Operation failed.

6. What is `this` keyword in JavaScript?

The value of this depends on how the function is called:

Globally: Points to the window/global object.
Inside a method: Points to the object that owns the method.
In an arrow function: Points to the this value of the enclosing lexical scope.

Next Step: Check out Complex Syntax.