Javascript Coding Interview Questions

What Are JavaScript Coding Interview Questions?

The purpose of JavaScript coding interview questions is to evaluate a candidate’s skill level in JavaScript and their ability to solve problems. These questions frequently test familiarity with JavaScript syntax, data structures, and built-in functions, alongside practical coding tasks like debugging and algorithm optimization. Since JavaScript plays a crucial role in web development, interviewers often focus on your understanding of event handling, asynchronous programming, and the Document Object Model (DOM). Being well-prepared for these questions can greatly improve your prospects for front-end or full-stack developer positions.

10 Common JavaScript Coding Interview Questions with Answers

What distinguishes == from === in JavaScript?

Answer:== is the equality operator that checks if two values are equal after performing type conversion, while === is the strict equality operator that checks if two values are both equal and of the same type. It’s a best practice to use === to avoid unexpected type coercion.

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 console.log(2 == '2');  // true (type coercion occurs) console.log(2 === '2'); // false (different types) 

What are closures in JavaScript, and how are they used?

Answer:A closure in JavaScript is defined as a function that holds onto variables from its enclosing scope, remaining accessible even after the outer function has exited. This characteristic is advantageous for creating private variables and functions, as well as for managing callback functions.

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

What is hoisting in JavaScript?

Answer:In JavaScript, hoisting is the behavior that lifts adjustable and function statements to the top of their respective scope during the compile phase. Only the declarations are hoisted, while the initializations stay in place. This means you can use a function or variable before its declaration; however, it's wise to declare all variables and functions at the beginning of their scope to avoid any misunderstandings.

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console.log(x); // undefined (due to hoisting)var x = 5;

Explain event delegation in JavaScript.

Answer:With event delegation, you attach a single event listener to a parent element, which then manages events originating from its child elements. This technique reduces the need for multiple event listeners on child elements and utilizes event bubbling, where events move up the DOM, enabling efficient event management from the parent level.

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document.getElementById('parent').addEventListener('click', function(event) { if (event.target.tagName === 'BUTTON') { console.log('Button clicked:', event.target.textContent);}});

What are the key differences among var, let, and const in JavaScript?

Answer:var has function scope and is hoisted. It can be re-declared and updated.
let has block scope and is not hoisted. It can be modernized but not re-declared within the same scope.
const has block scope and is not hoisted. It cannot be updated or re-declared after its initial assignment, but if assigned to an object, the properties of the object can be changed.

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 var a = 1; let b = 2; const c = 3; a = 4; // allowed b = 5; // allowed c = 6; // Error: Assignment to constant variable 

What are promises in JavaScript?

Answer:A promise represents an object indicating the eventual completion or failure of an asynchronous task. There are three states for promises: pending, fulfilled, and rejected. They allow for cleaner and more readable asynchronous code compared to callback functions.

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const myPromise = new Promise((resolve, reject) => {setTimeout(() => resolve('Success!'), 1000);});myPromise.then(result => console.log(result)) // 'Success!' after 1 second.catch(error => console.log(error));

What is the difference between call(), apply(), and bind() in JavaScript?

Answer:call() and apply() are used to appeal to a purpose with a specific context. The difference is that call() accepts arguments individually, while apply() accepts them as an array.
bind() creates a new function that, when called, has a specific this context, allowing you to pass arguments permanently.

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function greet(greeting) { console.log(`${greeting}, ${this.name}`);}const person = { name: 'John' };greet.call(person, 'Hello');  // 'Hello, John'greet.apply(person, ['Hi']);  // 'Hi, John'const boundGreet = greet.bind(person); boundGreet('Hey');  // 'Hey, John'

What is the distinction between null and undefined in JavaScript?

Answer:undefined means that a variable has been declared but not assigned a value, while null is an assigned value that represents “no value.” They are both falsy values, but they are used in different contexts.

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

Explain asynchronous programming in JavaScript.

Answer:Asynchronous programming allows code to run without blocking the execution of other tasks. JavaScript is single-threaded, but asynchronous operations (like network requests or timers) can be executed without freezing the main thread, thanks to the event loop. Promises, async/await, and callbacks are common ways to handle asynchronous behavior in JavaScript.

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async function fetchData() {try {let response = await fetch('https://api.example.com/data'); let data = await response.json();console.log(data);} catch (error) {console.error('Error fetching data:', error); }}

What are arrow functions, and how are they different from regular functions?

Answer:In JavaScript, arrow functions present a compact way to define functions. They do not have their own this context; instead, they inherit the this value from their surrounding scope, making them ideal for use in callbacks or methods that need to preserve this. They also do not have access to the arguments object, unlike regular functions.

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// Regular functionfunction add(a, b) { return a + b; }  // Arrow function const add = (a, b) => a + b; 

Conclusion

JavaScript coding interviews often address a variety of topics, from foundational syntax and functions to advanced themes like closures, asynchronous programming, and event handling. Preparing for these typical questions and practicing your coding abilities can help you gain confidence and boost your chances of succeeding in your JavaScript interview. Grasping both the basics and advanced elements of JavaScript is vital for doing well in a coding interview and attaining your desired position in web development.