You’re in the right place if you want to push your skills to the next level after mastering the basics of JavaScript interview questions. This article dives deep into the questions you can expect in intermediate-level JavaScript interviews.
However, excelling in interviews is not the only aspect. Understanding closures and asynchronous programming is crucial for becoming an effective and confident developer.
1. Asynchronous JavaScript
Imagine you’re building a weather application. You can’t halt the entire app to fetch weather data. Here, asynchronous programming methods like callbacks, promises, and async/await come to your rescue.
Question: What is ‘callback hell’ in the context of using callbacks, and what are its disadvantages?
Answer: “Callback hell” is used when you have many nested callbacks in our code. Think of it as a layer cake of functions where each layer depends on the one above it. This makes our code look like a pyramid, often called the “Pyramid of Doom.” The downside? It makes our code hard to read and even harder to debug. Plus, each new layer adds complexity, making future changes a headache.
Question: How is a Promise different from a callback, and what are the three states of a Promise?
Answer: Promises to improve upon callbacks by offering better code readability and easier error handling. A Promise can be in one of three states: pending, fulfilled, or rejected.
Question: How does async/await help in writing asynchronous code?
Answer: Async/await makes our code easier to read and write. It’s based on Promises but looks like traditional synchronous code.
2. Advanced Array Methods
When dealing with arrays, methods like map, filter, and reduce are your best friends. These methods can be beneficial in scenarios like sorting and filtering products on an e-commerce website.
// Example using map
const numbers = [1, 2, 3];
const squared = numbers.map(x => x * x);
Question: Briefly overview the map, filter, and reduce methods.
Answer: The map method transforms each item into an array and produces a new array of the same length. The filter method returns a new array containing only items that pass a specified condition. The reduce method processes each element in the array with an accumulator to reduce the array to a single value.
3. Closures
Closures are unique types of functions that allow for data hiding and encapsulation. They can be extremely useful in scenarios such as user authentication, where you may want to create private variables.
Question: What is a closure, and how can it be used to create private data?
Answer: Consider closure as a function that can “remember” its outer scope, enabling you to create variables that are not accessible externally. This capability makes closures invaluable for data hiding and encapsulation.
4. this Keyword
Understanding this keyword is essential for manipulating object data and working with event handlers. Its value can change based on the context in which it’s used.
Question: How does this keyword work in JavaScript, and what methods change its value?
Answer: In JavaScript, this refers to the object it belongs to. Methods like bind(), call(), or apply() explicitly set its value.
5. Prototypal Inheritance
Inheritance is a cornerstone of object-oriented programming, but JavaScript does it differently. Here, the paradigm is “prototypal inheritance,” a concept crucial for anyone working on larger projects or libraries.
For example, if you’re developing a game, you might have different enemies that share some traits but also have unique abilities. With prototypal inheritance, managing this kind of complexity becomes easy.
Question: How does inheritance work in JavaScript?
Answer: In JavaScript, objects can inherit properties from other objects through prototypal inheritance. This sets up a chain of inheritance that diverges from class-based inheritance in languages like Java.
6. ES6 Features
ES6 brings a ton of helpful features that make JavaScript development smoother and more efficient. Features like destructuring can simplify your code and improve readability.
Question: What is destructuring in JavaScript, and how is it useful?
Answer: Destructuring lets you unpack values from arrays or objects into distinct variables. It makes code cleaner and easy to understand.
Question: How do the spread and rest operators differ in JavaScript?
Answer: The spread operator unfolds array elements or object properties. The rest operator does the opposite: it gathers elements into a new array or object.
7. Error Handling
Errors are inevitable in programming. Understanding how to handle them gracefully can significantly impact user experience.
Question: How does the try-catch block help in handling exceptions in JavaScript?
// Example of throwing a custom error
throw new Error("This is a custom error");
Answer: The try-catch block allows developers to test a block of code for errors. If an error occurs in the try block, the code can jump to the catch block without crashing the application.
Question: How can you throw a custom error in JavaScript?
Answer: Custom errors can be created using the throw statement, which allows you to define your error messages and types.
8. Event Delegation
Question: Why is event delegation beneficial, and how does it work?
Answer: Event delegation allows setting an event listener on parent rather than child elements. It takes advantage of the fact that events bubble up the DOM. This improves performance and ensures that dynamically added elements will still trigger the event.
9. Module Patterns
Question: How can you use the import and export statements in ES6?
Answer: ES6 lets us break our code into modules. We can export and import variables, functions, or objects in other files. This helps in code organization and reuse.
10. Throttling and Debouncing
Question: What are throttling and debouncing, and why are they essential in event handling?
Answer: Throttling and debouncing are performance techniques. Throttling allows a function to run only once in a set time. Debouncing delays the function until a certain time has passed since the last call.
11. Web Storage
Deciding where and how to store data is crucial when creating a web application. This is where localStorage and sessionStorage come into play.
Question: How do localStorage and sessionStorage differ?
Answer: localStorage persists data even when the browser is closed and reopened, making it suitable for long-term storage. On the other hand, sessionStorage only keeps data for the duration of the page session.
12. JavaScript Engines
Question: What is a JavaScript engine, and can you name a few popular ones?
Answer: A JavaScript engine translates your code into machine language. Popular ones include V8 for Chrome, SpiderMonkey for Firefox, and Chakra for Edge.
Conclusion
Congratulations, you’ve made it to the end of our guide on JavaScript Intermediate Interview Questions for 2023! Understanding these complex topics is your next step toward becoming an expert JavaScript developer.
I hope you found this guide helpful, and we wish you the best of luck on your journey toward mastering JavaScript Interview preparation.
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Top comments (2)
Incorrect. ALL functions have an associated closure. Closures are not functions, they are the combination of a function bundled together with its surrounding state
Thank you for pointing that out. You're right in emphasizing that closures are not functions themselves but rather the combination of a function and its lexical environment, capturing the surrounding state. My intent was to highlight the concept in a simplified manner for readers unfamiliar with it, but I appreciate the clarification. It's essential to maintain accuracy, and I value your input. I'll ensure to be more precise in future explanations. Thanks again for the constructive feedback! 👩💻📘