Top JavaScript Interview Questions & Answers

1. What are the different data types in JavaScript?

Answer:

JavaScript has the following primitive data types:

String – Represents textual data (e.g., 'Hello')
Number – Represents numeric values (e.g., 42, 3.14)
BigInt – Represents large integers (e.g., BigInt(9007199254740991))
Boolean – Represents true or false
Undefined – Represents an uninitialized variable
Null – Represents an empty or unknown value
Symbol – Represents unique identifiers

Additionally, JavaScript has objects, which include arrays, functions, and other complex structures.

2. What is the difference between `==` and `===` in JavaScript?

Answer:

== (loose equality) checks for value equality but allows type coercion.
```
console.log(5 == "5"); // true
```
=== (strict equality) checks both value and type.
```
console.log(5 === "5"); // false
```

Using === is recommended for better type safety.

3. What are closures in JavaScript?

Answer:

A closure is a function that remembers the variables from its outer scope even after the outer function has executed.

Example:

function outerFunction(outerVariable) {
  return function innerFunction(innerVariable) {
    console.log(`Outer: ${outerVariable}, Inner: ${innerVariable}`);
  };
}

const newFunction = outerFunction("Hello");
newFunction("World"); // Output: Outer: Hello, Inner: World

Closures are useful for data encapsulation and maintaining state.

4. What is the difference between `let`, `const`, and `var`?

Answer:

var: Function-scoped, can be redeclared and updated.
let: Block-scoped, cannot be redeclared, but can be updated.
const: Block-scoped, cannot be redeclared or updated.

Example:

var x = 10;
let y = 20;
const z = 30;

Prefer let and const over var to avoid scoping issues.

5. What is the event loop in JavaScript?

Answer:

The event loop is what enables JavaScript to be asynchronous. It continuously checks the call stack and the task queue, executing tasks in order.

Example:

console.log("Start");
setTimeout(() => console.log("Timeout"), 0);
console.log("End");

Output:

Start
End
Timeout

Even though setTimeout is set to 0ms, it executes last due to the event loop mechanism.

6. What is the difference between `null` and `undefined`?

Answer:

null is an assigned value representing an intentional absence of value.
undefined means a variable has been declared but has not been assigned a value.

Example:

let a;
console.log(a); // undefined

let b = null;
console.log(b); // null

7. How does `this` work in JavaScript?

Answer:

this refers to the object that is currently executing the function.

In a regular function: this refers to the global object (window in browsers, global in Node.js).
In a method: this refers to the object that owns the method.
In an arrow function: this is lexically bound (inherits from the surrounding scope).

Example:

const obj = {
  name: "John",
  greet: function() {
    console.log(`Hello, ${this.name}`);
  }
};
obj.greet(); // Output: Hello, John

8. What are promises in JavaScript?

Answer:

Promises are used for handling asynchronous operations. A promise can be in one of three states:

Pending – Initial state
Fulfilled – The operation was successful
Rejected – The operation failed

Example:

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => resolve("Promise Resolved!"), 2000);
});

myPromise.then((message) => console.log(message));

9. What is the difference between `map`, `forEach`, and `filter`?

Answer:

map() – Returns a new array after applying a function to each element.
forEach() – Iterates over an array but does not return anything.
filter() – Returns a new array containing elements that pass a condition.

Example:

const nums = [1, 2, 3, 4, 5];
const doubled = nums.map(num => num * 2);
const evens = nums.filter(num => num % 2 === 0);
nums.forEach(num => console.log(num));

10. What is destructuring in JavaScript?

Answer:

Destructuring allows unpacking values from arrays or objects into separate variables.

Example:

const person = { name: "Alice", age: 25 };
const { name, age } = person;
console.log(name, age); // Alice 25

Conclusion

These JavaScript interview questions cover fundamental and advanced concepts that frequently appear in technical interviews. Practicing these will help you build confidence and improve your problem-solving skills. Keep coding and good luck with your interview!

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