Top 10 JavaScript Interview Questions You Need to Know

Introduction:

JavaScript remains one of the most popular programming languages in web development. Whether you’re a junior developer or a seasoned professional, mastering JavaScript is crucial for acing interviews. In this blog, we’ll cover ten essential JavaScript interview questions that every developer should be prepared to answer.

1. What Are Closures in JavaScript?

Closures are a fundamental concept in JavaScript. They occur when a function retains access to its lexical scope, even when the function is executed outside that scope.

function outerFunction() {
  let outerVariable = 'I am from outer scope';

  function innerFunction() {
    console.log(outerVariable); // Accesses outerVariable
  }

  return innerFunction;
}

const closureFunction = outerFunction();
closureFunction(); // Outputs: I am from outer scope

Why It Matters: Closures are key to understanding data encapsulation and function scope in JavaScript.

2. Explain the Event Loop and How It Works

The Event Loop is a mechanism that allows JavaScript to perform non-blocking operations despite being single-threaded. It works by placing operations in a queue and executing them when the call stack is empty.

Key Points:

Call Stack: Handles function execution.
Callback Queue: Holds callbacks that are ready to be executed.
Event Loop: Moves tasks from the callback queue to the call stack.
Why It Matters: Understanding the event loop is essential for writing efficient and responsive JavaScript code.

3. What Is the Difference Between == and ===?

== (Equality Operator): Compares values with type coercion.
=== (Strict Equality Operator): Compares values without type coercion (strict equality).

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

Why It Matters: Using === avoids unexpected bugs caused by type coercion, making code more predictable and reliable.

4. What Are Promises and How Do They Work?

Promises are objects representing the eventual completion or failure of an asynchronous operation. They provide a cleaner way to handle asynchronous code compared to callbacks.

Key Methods:

then(onFulfilled, onRejected): Handles resolved and rejected states.
catch(onRejected): Handles errors.
finally(onFinally): Executes code regardless of the promise’s outcome.

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

promise.then(result => console.log(result)); // Outputs: Resolved!

5. What Is the this Keyword in JavaScript?

The this keyword refers to the object that is executing the current function. Its value depends on the context in which a function is called.

Common Use Cases:

Global Context: this refers to the global object.
Object Method: this refers to the object.
Constructor Function: this refers to the instance created.

const obj = {
  name: 'Alice',
  greet: function() {
    console.log(this.name);
  }
};

obj.greet(); // Outputs: Alice

6. What Are Arrow Functions and How Do They Differ from Regular Functions?

Arrow Functions provide a more concise syntax and have lexical this binding, meaning they inherit this from the surrounding context.

const add = (a, b) => a + b;

Difference:

Regular Function: Has its own this.
Arrow Function: Inherits this from the parent scope.

Why It Matters: Arrow functions simplify code and prevent common issues with this binding in callbacks. Arrow functions simplify code and prevent common issues with this binding in callbacks.

7. How Does Prototypal Inheritance Work in JavaScript?

Prototypal Inheritance allows objects to inherit properties and methods from other objects. JavaScript uses prototypes to implement inheritance.

const animal = {
  speak() {
    console.log('Animal speaks');
  }
};

const dog = Object.create(animal);
dog.bark = function() {
  console.log('Woof');
};

dog.speak(); // Outputs: Animal speaks
dog.bark();  // Outputs: Woof

8. What Is the Difference Between null and undefined?

null: Represents an intentional absence of value.

undefined: Represents a variable that has been declared but not initialized.

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

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

9. What Are map, filter, and reduce Methods?

These are higher-order functions used for array manipulation:

map: Creates a new array by applying a function to each element.
filter: Creates a new array with elements that pass a test.
reduce: Reduces an array to a single value by applying a function.

const numbers = [1, 2, 3, 4, 5];

const doubled = numbers.map(n => n * 2);
const evens = numbers.filter(n => n % 2 === 0);
const sum = numbers.reduce((total, n) => total + n, 0);

console.log(doubled); // [2, 4, 6, 8, 10]
console.log(evens);   // [2, 4]
console.log(sum);     // 15

10. How Do You Optimize JavaScript Performance?

Performance Optimization Techniques:

Minify and Bundle: Use tools like Webpack to minify and bundle JavaScript files.

Lazy Loading: Load resources only when needed.

Debouncing/Throttling: Control the rate of function execution in response to events.

Avoid Memory Leaks: Ensure proper cleanup of event listeners and intervals.

Conclusion

Mastering these ten JavaScript interview questions will prepare you for various scenarios and challenges you might face in technical interviews. Deepening your understanding of these topics will not only help you in interviews but also improve your overall JavaScript skills.

"Power comes in response to a need, not a desire. You have to create that need."