Top JavaScript Interview Questions (With Answers) for 2024

JavaScript is crucial for web development and frequently used in technical interviews. This guide covers key JavaScript interview topics to help you feel well-prepared and confident.

1. Key JavaScript Data Types

JavaScript offers several types:

Primitive Types:

• Number: For numerical values, including both integers and decimals.
• String: For sequences of characters.
• Boolean: For true/false values.
• Undefined: When a variable has been declared but lacks a value.
• Null: Represents an empty or unknown value.
• Symbol: Provides unique identifiers for properties.
• BigInt: Allows handling of numbers beyond the safe integer limit in JavaScript.
• Object: A non-primitive data type that lets you store collections of properties and complex data structures, such as arrays and functions.

2. var, let, and const: Key Differences

In JavaScript, variable declarations vary by keyword:

• var: Supports function-level scope, which can lead to unintentional errors when working outside of functions.
• let: Block-scoped, preventing accidental redeclaration or leaks outside of the specific block.
• const: Also block-scoped, with the key restriction of preventing reassignment after the initial declaration.

3. What Are JavaScript Closures?

A closure is a function that captures variables from its surrounding context, keeping these accessible even after the outer function has executed. This can be helpful in cases where you want to preserve a variable’s state.

Example:

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

4. What is Event Delegation?

Event delegation is an approach that lets you assign a single event listener to a parent element, which then handles events for multiple child elements as they are interacted with. This can be especially useful in dynamic content situations, where elements are added or removed frequently, as it reduces the need for individual listeners on each child.

Example:

document.querySelector('#list').addEventListener('click', function(event) {
    if (event.target && event.target.tagName === 'BUTTON') {
        console.log('Button clicked!');
    }
});

5. Comparing == and ===

The differences between == and === come down to type coercion:

• == performs type conversion when comparing two values.

  console.log(1 == '1'); // true
  

• === checks both value and type directly.

  console.log(1 === '1'); // false
  

6. How Does this Work in JavaScript?

In JavaScript, the behavior of this varies based on the context in which it’s used. When accessed in the global scope, this points to the global object, providing access to globally defined variables and functions. However, when used within functions or methods, this can change to refer to the calling object or the context set at runtime, particularly if bind, call, or apply methods are used.

Example:

const person = {
    name: 'Alice',
    greet() {
        console.log(this.name);
    }
};
person.greet(); // Outputs: Alice

7. Prototypal Inheritance Basics

JavaScript uses a unique form of inheritance called prototypal inheritance. Instead of inheriting directly from classes as in other languages, JavaScript objects can reference other objects, allowing them to access the properties and methods of the “prototype” object they are linked to. This model allows for efficient code reuse without duplicating methods across objects.

Example:

function Animal(name) {
    this.name = name;
}
Animal.prototype.sound = function() {
    return `${this.name} makes a noise.`;
};
const dog = new Animal('Dog');
console.log(dog.sound()); // Outputs: Dog makes a noise.

8. JavaScript Promises

Promises offer a way to handle asynchronous operations, returning a value that may be resolved (successful) or rejected (failed). Promises simplify managing tasks that don’t complete immediately, like server requests.

Example:

const myPromise = new Promise((resolve, reject) => {
    const success = true;
    if (success) {
        resolve('Done');
    } else {
        reject('Failed');
    }
});
myPromise
    .then(result => console.log(result))  // Done
    .catch(error => console.log(error));

9. Event Loop in JavaScript

The JavaScript event loop enables asynchronous operations by processing events when the call stack is clear. It manages the scheduling of function execution for non-blocking tasks, allowing JavaScript’s single-threaded nature to handle asynchronous tasks.

10. Difference Between call(), apply(), and bind()

These methods manage function this binding in different ways:

• call(): Immediately calls a function, allowing individual arguments to be passed.
  
  

  function greet() { console.log(this.name); }
  const person = { name: 'Emma' };
  greet.call(person); // Emma
  

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

  greet.apply(person, []); // Emma
  

• bind(): Returns a new function bound to a specified object, without calling it immediately.
  
  

  const boundGreet = greet.bind(person);
  boundGreet(); // Emma
  

Conclusion

Mastering these JavaScript concepts will boost your confidence and readiness for interviews. From variable scoping to understanding asynchronous behavior, knowing these essentials is key to navigating questions with ease and making a great impression as a developer.