10 Must Ask JavaScript Interview Questions For Hiring Managers

Introduction

JavaScript has evolved far beyond its initial conception, becoming the backbone of interactive and dynamic web applications. From frontend frameworks like React and Vue to backend environments like Node.js, JavaScript dominates modern web development.

According to the Stack Overflow 2024 Developer Survey, JavaScript remains the most popular programming language for the 12th consecutive year, used by over 63% of developers worldwide. This ubiquity makes JavaScript proficiency essential for most web development positions.

This comprehensive guide provides hiring managers with essential JavaScript interview questions designed to evaluate candidates’ understanding of core concepts, modern features, and practical application skills.

What is JavaScript?

JavaScript is a versatile, high-level programming language primarily used for creating interactive web content. Originally designed to add dynamic behavior to websites, JavaScript has expanded to support full-stack development, mobile applications, desktop applications, and even server-side programming.

Key characteristics of JavaScript include:

• Dynamic typing: Variables can hold different types of values
• Event-driven: Responds to user interactions and system events
• Prototype-based: Object-oriented programming through prototypes
• Asynchronous: Non-blocking operations through callbacks, promises, and async/await

Top 10 Must-Ask JavaScript Interview Questions

1. Explain the difference between var, let, and const.

This question tests understanding of variable declaration and scope in modern JavaScript.

Example Answer: “var has function scope and is hoisted. let and const have block scope and temporal dead zone. const cannot be reassigned but objects/arrays can be mutated. Use const by default, let when reassignment is needed, avoid var in modern code.”

2. What is hoisting in JavaScript?

Hoisting is a fundamental concept that often confuses developers.

Example Answer: “Hoisting is JavaScript’s behavior of moving variable and function declarations to the top of their scope during compilation. var declarations are hoisted and initialized with undefined. Function declarations are fully hoisted. let/const are hoisted but not accessible due to temporal dead zone.”

3. Explain closures and provide an example.

Closures are powerful features that demonstrate understanding of scope and memory management.

Example Answer: “A closure is when an inner function has access to outer function’s variables even after the outer function returns. Example:

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

### 4. What is the difference between `==` and `===`?

This tests understanding of type coercion and comparison operators.

**Example Answer**: "`==` performs type coercion and compares values after converting types. `===` checks both value and type without coercion. Example: `'5' == 5` is true, but `'5' === 5` is false. Always use `===` for predictable comparisons except when specifically needing type coercion."

### 5. Explain the `this` keyword in different contexts.

Understanding `this` binding is crucial for object-oriented JavaScript.

**Example Answer**: "`this` refers to the execution context:
- Global: `window` (browser) or `global` (Node.js)
- Method: the object owning the method
- Constructor: the new instance being created
- Arrow functions: inherit `this` from enclosing scope
- `call`/`apply`/`bind`: explicitly set `this`"

### 6. What are Promises and how do they work?

Asynchronous programming is essential in JavaScript development.

**Example Answer**: "Promises represent eventual completion/failure of asynchronous operations. They have three states: pending, fulfilled, rejected. Chain with `.then()`, `.catch()`, `.finally()`. Modern alternative is async/await:
```javascript
async function fetchData() {
  try {
    const response = await fetch('/api/data');
    return await response.json();
  } catch (error) {
    console.error(error);
  }
}
```"

### 7. Explain event bubbling and capturing.

Event handling is fundamental to interactive web applications.

**Example Answer**: "Event bubbling: events propagate from target element up to document root. Event capturing: events propagate from root down to target. Use `addEventListener(event, handler, useCapture)` where `useCapture` controls phase. `event.stopPropagation()` stops further propagation."

### 8. What are arrow functions and how do they differ from regular functions?

Arrow functions introduce different behavior and syntax.

**Example Answer**: "Arrow functions provide concise syntax and lexical `this` binding:
```javascript
// Regular function
function add(a, b) { return a + b; }
// Arrow function  
const add = (a, b) => a + b;

Differences: no own this, arguments, or prototype. Cannot be constructors. Always anonymous.”

9. Explain the prototype chain and inheritance.

Understanding prototypal inheritance is key to JavaScript’s object model.

Example Answer: “Every object has a prototype (except Object.prototype). When accessing properties, JavaScript searches the prototype chain. Inheritance achieved through prototype delegation:

function Animal(name) { this.name = name; }
Animal.prototype.speak = function() { console.log(`${this.name} makes a sound`); };
function Dog(name) { Animal.call(this, name); }
Dog.prototype = Object.create(Animal.prototype);
```"

### 10. What are the different ways to create objects in JavaScript?

Object creation patterns demonstrate understanding of JavaScript's flexibility.

**Example Answer**: "Multiple ways to create objects:
1. Object literal: `{name: 'John'}`
2. Constructor function: `new Person('John')`
3. `Object.create()`: `Object.create(proto)`
4. ES6 classes: `new Person('John')`
5. Factory function: `createPerson('John')`
Each has different use cases for inheritance and initialization."

## Modern JavaScript Concepts

For advanced positions, explore these ES6+ features:

### Destructuring Assignment
```javascript
const {name, age} = person;
const [first, second] = array;

Template Literals

const message = `Hello ${name}, you are ${age} years old`;

Modules

// export
export const utility = () => {};
export default MyClass;

// import
import MyClass, {utility} from './module.js';

Spread and Rest Operators

const newArray = [...oldArray, newItem];
const {first, ...rest} = object;

Practical Assessment Tips

When interviewing JavaScript candidates:

1. Live coding: Have candidates write and debug JavaScript code
2. Problem-solving: Present real-world scenarios requiring JavaScript solutions
3. Framework knowledge: Assess familiarity with relevant frameworks (React, Vue, Angular)
4. Async handling: Test understanding of promises, async/await, and error handling
5. Performance awareness: Discuss optimization techniques and best practices

Conclusion

JavaScript’s versatility and ubiquity make it essential for modern web development. These interview questions assess both fundamental understanding and practical application skills, helping you identify candidates who can contribute effectively to JavaScript-based projects.

The best JavaScript developers combine technical knowledge with problem-solving abilities, understanding not just language syntax but also performance implications, security considerations, and maintainable code practices.

Use Interview Zen’s technical interview platform to create comprehensive JavaScript assessments and observe candidates’ coding approaches in real-time during frontend and backend development interviews.