Essential JavaScript Interview Questions for One-Year Experienced Candidates

Passing through the interview process for a front-end engineering role with one year of experience can be a daunting task. Interviewers will primarily assess your understanding of JavaScript fundamentals, data structures, and algorithms, along with your problem-solving skills. This article provides a comprehensive overview of the key JavaScript questions that are commonly asked during such interviews, helping you prepare thoroughly and boost your chances of landing a dream job.

Understanding JavaScript Fundamentals

Having an in-depth knowledge of JavaScript is crucial for candidates with one year of experience. Here are some essential JavaScript questions that you should be prepared to answer:

1. What is a Promise? Why would you use promises over callbacks?

A Promise is a JavaScript object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. Instead of using callbacks, which can lead to the callback hell (nested callbacks), Promises provide a better way to handle asynchronous operations by allowing you to chain operations with the .then() and .catch() methods.

2. What are the signatures of the .map and .reduce functions of an Array?

The .map function creates a new array populated with the results of calling a provided function on every element in the calling array. Its signature is:

[].map(callback(thisArg), thisArg)

The .reduce function executes a reducer function (that you provide) on each element of the array, resulting in a single output value. Its signature is:

[].reduce(callback(accumulator, currentValue, currentIndex, array), initialValue)

3. What does the .bind method of a function do and what is its signature?

The .bind method creates a new function by lifting the this value to a specified value. The signature is:

(thisArg, arg1, arg2, ...)

4. Explain the main difference between .call and .apply.

The .call method calls a function with a given this value and arguments provided individually as separate parameters. On the other hand, the .apply method calls a function with a given this value and arguments provided as an array.

5. Explain the difference between and

The (loose equality) operator can convert different types as necessary for a comparison, whereas the (strict equality) operator does not attempt to convert types and requires both values to be of the same type and have the same value for a true comparison result.

Understanding JavaScript Concurrency and Single-Threading

Knowledge of how JavaScript handles concurrent tasks is critical. Here are some questions to familiarize yourself with:

6. Is JavaScript single-threaded or multi-threaded?

JavaScript is single-threaded, which means that the event loop allows only one command to be executed at a time. However, thanks to asynchronous programming, JavaScript can perform multiple tasks via callbacks, Promises, and async/await.

7. What guarantees does an arrow function offer? What would you do if arrow functions didn’t exist?

Arrow functions provide lexical this and allow concise function syntax. If arrow functions weren’t available, you would need to use regular function expressions or methods like .bind, .call, or .apply to achieve similar results.

Working with Modern JavaScript Features

The ECMAScript 6 (ES6) introduced several new features that are now widely used. Here are some of them:

8. What is the Spread operator and Rest operator?

The operator is used to pass an arbitrary number of arguments to a function. The spread operator allows you to spread an iterable (like an array) into individual elements or use them as arguments.

9. What is Template Literals?

Template literals are string literals that allow you to embed expressions within string literals. They are defined with backticks ` instead of double or single quotes. You can use `${expression}` to embed expressions within template literals.

10. What are new Data Types in ES6?

ES6 introduced several new types such as BigInt for handling large integers, Promise for asynchronous operations, and Symbol for generating unique identifiers. These additions help structure code effectively and handle complex data.

Benefits of Pure Functions and Immutable Structures

Pure functions and immutable structures are critical concepts in functional programming, making code more predictable and easier to reason about. Here's a brief explanation of their benefits:

11. What are the benefits of a pure function?

A pure function has no side effects and produces the same output for the same input. This predictability makes it easier to reason about code and test functions. Pure functions can also be easily cached and reused, improving performance.

12. What are the advantages and drawbacks of using immutable structures?

Immutable structures are those that cannot be changed once created. Advantages include easier debugging and error handling, better compatibility with functional programming, and easier isolation of side effects. However, they can be less performant in situations requiring frequent changes, as immutable data must always create new versions.

Advanced JavaScript Techniques

Understanding advanced JavaScript techniques is key to demonstrating your expertise and problem-solving skills during interviews:

13. What is Currying and/or Partial Application?

Currying is the process of transforming a function with multiple arguments into a sequence of functions with a single argument. Partial application is a technique where you fix a certain number of parameters with default values, creating a new function.

14. What does setTime do?

The setTimeout function is used to call a function or specified piece of code once the specified time has passed. The syntax is:

setTimeout(function, timeout, arg1, arg2, ...)

15. What does super keyword do when it is used?

The super keyword is used when a subclass calls a method or variable in its superclass. It helps in accessing the methods and variables from the parent class.

Practical Coding Problems

Tackling coding problems using JavaScript is an essential part of the interview. Here are some common coding challenges you may face:

1. Debounce / Throttle

Implement functions that limit the frequency of certain events by delay or throttling the event.

2. Middleware Stack

Create a middleware stack for a server or a framework like Express.

3. Exclusion Lists

Implement a function to exclude certain elements from an array based on a criteria.

4. Longest Unique Substring - Sliding Windows vs Brute Force Loops

Determine the longest substring that contains no repeated characters using the sliding window technique.

5. Until All N Arguments are Provided

Write a function that returns a function for consuming more arguments when all N arguments are provided, demonstrating currying.

6. Round Number to Two Decimal Places

Create a function that rounds a number to two decimal places.

7. Chain API via Chainobj with an Automatic Value Added

Chain API calls together with an automatic value addition.

8. Array.flatten for Multidimensional Arrays

Create a function that flattens a multidimensional array.

Prepare thoroughly for your interviews by not only mastering these concepts but also continuously honing your problem-solving skills. To get the best offer from top companies like FAANG, a lot of practice is essential. Consider joining a comprehensive preparation course that covers all aspects of front-end engineering interviews, where you can interact with experienced instructors from leading technology companies. Register for the FREE webinar and start your journey today.