Top 15 JavaScript Interview Questions and Answers for 2024 (Categorized by Difficulty)

JavaScript interviews cover a broad range of topics, requiring both theoretical understanding and practical application. Below, the questions are categorized into Easy, Medium, and Hard levels, and the answers are provided in varying lengths to match the complexity of the question. Let's explore these concepts in-depth with examples wherever necessary.

Easy Level Questions

1. What is JavaScript?

JavaScript is a lightweight, interpreted programming language that is widely used for building interactive and dynamic web pages. It is one of the three core technologies of the web (alongside HTML and CSS). JavaScript runs in browsers and on servers using environments like Node.js. It's used for adding interactivity to web pages, such as form validation, animations, and handling user events.

2. What are data types in JavaScript?

JavaScript has several data types, categorized into primitive types and non-primitive types.

Primitive types:

• String: Represents text data. Example: "Hello".
• Number: Represents numeric data. Example: 42, 3.14.
• Boolean: Represents true or false. Example: true, false.
• Undefined: A variable that has been declared but not assigned a value. Example: let x;.
• Null: Represents the absence of any object value.
• Symbol: A unique and immutable data type introduced in ES6.
• BigInt: Represents numbers larger than Number.MAX_SAFE_INTEGER introduced in ES2020.

Non-primitive type:

• Object: Used to store collections of data or more complex entities like arrays or functions.

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

• var: Function-scoped. It can be re-declared and updated. Variables declared using var are hoisted, meaning they can be accessed before the actual declaration in the code.

var x = 10;
function demo() {
  var y = 20;
  console.log(y); // Works
}

• let: Block-scoped. It can be updated but not re-declared in the same scope. It is not hoisted in the same way as var.

let a = 10;
if (true) {
  let a = 20; // Different variable than the outer one
  console.log(a); // 20
}
console.log(a); // 10

• const: Block-scoped like let, but cannot be updated or re-declared. It must be initialized at the time of declaration.

const b = 30;
b = 40; // Error: Assignment to constant variable

4. What is hoisting in JavaScript?

Hoisting is JavaScript’s default behavior of moving declarations to the top of the current scope (to the top of the script or the top of a function). This means you can use variables or functions before they are declared in the code.

For example, with var:

console.log(x); // undefined, because var declarations are hoisted
var x = 5;

However, let and const are hoisted differently and result in a ReferenceError if used before their declaration.

5. What is the this keyword in JavaScript?

The this keyword refers to the object that the function is a property of. The value of this depends on the context in which it is used:

• In a method, this refers to the owner object.
• In a function, this refers to the global object (in non-strict mode) or undefined (in strict mode).
• In event handlers, this refers to the element that received the event.

Example:

const person = {
  name: "John",
  greet: function() {
    console.log(this.name);
  }
};

person.greet(); // "John"

Medium Level Questions

6. What is a closure in JavaScript?

A closure is a function that has access to its own scope, the scope of the outer function, and the global scope. Closures allow a function to "remember" the environment in which it was created, even after the outer function has completed execution.

Example:

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

const newFunction = outerFunction("outside");
newFunction("inside");
// Output: 
// Outer: outside
// Inner: inside

In this example, innerFunction forms a closure that includes the outerVariable from outerFunction, even after outerFunction has returned.

7. What is the difference between == and === in JavaScript?

• == (Loose Equality): Compares two values for equality, after performing type conversion if necessary. For example, 5 == "5" is true because JavaScript converts the string "5" to the number 5 before comparing.
• === (Strict Equality): Compares two values for both value and type. For example, 5 === "5" is false because the types (number and string) are different.

Example:

console.log(5 == "5");  // true
console.log(5 === "5"); // false

It is recommended to use === to avoid unexpected results from type coercion.

8. What is the event loop in JavaScript?

The event loop is responsible for handling asynchronous operations in JavaScript. JavaScript is single-threaded, meaning only one operation is processed at a time. However, the event loop allows JavaScript to handle non-blocking operations (such as timers, network requests) by pushing asynchronous callbacks into the event queue and executing them once the call stack is empty.

Example:

console.log("Start");

setTimeout(() => {
  console.log("Timeout");
}, 0);

console.log("End");

Output:

Start
End
Timeout

In this example, setTimeout is an asynchronous operation, and the callback is executed after the synchronous code has finished executing.

9. What is a promise in JavaScript?

A promise is an object that represents the eventual completion (or failure) of an asynchronous operation. Promises can be in one of three states:

• Pending: The initial state, neither fulfilled nor rejected.
• Fulfilled: The operation completed successfully.
• Rejected: The operation failed.

You can chain .then() and .catch() methods to handle the resolution or rejection of a promise.

Example:

const promise = new Promise((resolve, reject) => {
  setTimeout(() => resolve("Success!"), 1000);
});

promise.then(result => console.log(result)).catch(error => console.log(error));

In this example, the promise resolves after 1 second, logging "Success!" to the console.

10. What is the difference between call(), apply(), and bind()?

• call(): Invokes a function with a specified this value and arguments passed individually.

const obj = { name: "Alice" };
function greet() {
  console.log(this.name);
}
greet.call(obj); // Alice

• apply(): Similar to call(), but arguments are passed as an array.

const numbers = [1, 2, 3];
const max = Math.max.apply(null, numbers); // 3

• bind(): Returns a new function, with this permanently bound to the specified object. It does not invoke the function immediately.

const obj = { name: "Alice" };
const greet = function() {
  console.log(this.name);
};
const boundGreet = greet.bind(obj);
boundGreet(); // Alice

Hard Level Questions

11. What are higher-order functions in JavaScript?

A higher-order function is a function that either takes another function as an argument or returns a function as a result. Higher-order functions are a key feature of JavaScript and are widely used in functional programming.

Example:

function higherOrder(fn) {
  return function(x) {
    return fn(x);
  };
}

function multiplyBy2(x) {
  return x * 2;
}

const multiply = higherOrder(multiplyBy2);
console.log(multiply(5)); // 10

In this example, higherOrder is a higher-order function because it takes another function (multiplyBy2) as an argument and returns a new function that multiplies a given number by 2.

12. Explain the difference between synchronous and asynchronous programming in JavaScript.

Synchronous programming executes one operation at a time. Each operation waits for the previous one to complete before executing. This can result in blocking behavior, where long-running operations (like file reading or network requests) prevent other tasks from being executed.

Asynchronous programming allows multiple operations to be executed concurrently. Instead of waiting for each operation to complete

, asynchronous tasks are pushed to the event loop, allowing other tasks to execute. Asynchronous code can be handled using callbacks, promises, or async/await.

Example (Synchronous):

function task1() {
  console.log("Task 1");
}

function task2() {
  console.log("Task 2");
}

task1();
task2();

Output:

Task 1
Task 2

Example (Asynchronous):

function task1() {
  setTimeout(() => {
    console.log("Task 1");
  }, 1000);
}

function task2() {
  console.log("Task 2");
}

task1();
task2();

Output:

Task 2
Task 1

In the asynchronous example, task2 is executed before task1, even though task1 was called first. This is because setTimeout is asynchronous and does not block the execution of other tasks.

13. What is the difference between deep copy and shallow copy in JavaScript?

A shallow copy of an object copies the reference to the original object, meaning changes to the copy will affect the original object if they share nested objects. A deep copy creates an entirely new object that is completely independent of the original object, even for nested objects.

Shallow Copy Example:

const original = { name: "Alice", address: { city: "NY" } };
const shallowCopy = { ...original };

shallowCopy.address.city = "LA";
console.log(original.address.city); // LA (original object is also affected)

Deep Copy Example:

const original = { name: "Alice", address: { city: "NY" } };
const deepCopy = JSON.parse(JSON.stringify(original));

deepCopy.address.city = "LA";
console.log(original.address.city); // NY (original object is not affected)

14. How does async/await work in JavaScript?

async/await is a syntactic sugar built on top of promises, making asynchronous code easier to write and understand. When a function is declared with the async keyword, it always returns a promise. Inside an async function, the await keyword pauses the execution until the promise resolves or rejects.

Example:

async function fetchData() {
  try {
    const response = await fetch("https://api.example.com/data");
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.log(error);
  }
}

fetchData();

In this example, await pauses the execution of fetchData until the promise returned by fetch is resolved, making the asynchronous code look more like synchronous code. If the promise is rejected, the catch block handles the error.

15. What is the concept of currying in JavaScript?

Currying is a technique in functional programming where a function is transformed into a sequence of functions, each taking a single argument. Instead of calling a function with multiple arguments at once, currying allows you to call the function with one argument at a time.

Example:

function add(a) {
  return function(b) {
    return a + b;
  };
}

const add5 = add(5);
console.log(add5(3)); // 8

In this example, add(5) returns a new function that takes a second argument and adds it to 5. This technique is useful for creating reusable functions with partially applied arguments.

Finally

JavaScript is a dynamic and versatile language with a lot of depth. Mastering these easy, medium, and hard level questions will prepare you for a wide range of interview scenarios. Understanding concepts such as closures, event loops, promises, and higher-order functions can give you the edge you need to excel in a JavaScript interview.

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