BlogTop 15 JavaScript Developer Interview Questions
Top 15 JavaScript Developer Interview Questions
Kamil Rizwan
1 Jan 2024

JavaScript is a scripted programming language that enables you to add interactivity and complex features on web pages. Just like HTML (Hypertext Markup Language) and CSS (Cascading Style Sheets), JavaScript is also a key component of web development. However, JavaScript allows developers to create dynamic web pages which include features such as animations, interactive maps, 2D/3D graphics and scrolling jukeboxes. It is supported by all web browsers and can be used in server-side environments on platforms like Node.js. The versatility that JavaScript provides, makes it an essential web development tool. 

Being an essential tool for programming, the demand of JavaScript developers has increased over the years. To qualify yourself as a JavaScript developer, you need to understand the important questions that the recruiter might ask you. This blog talks about the top 15 JavaScript interview questions to hire JavaScript developers. 

1- What is JavaScript and how does it differ from Java?

JavaScript and Java are distinct programming languages with different origins and purposes. JavaScript is a compelling scripting language mainly used for front-end web development. It is commonly used in web applications like browsers. To become a front-end designer, you need to understand HTML and CSS alongside JavaScript because JavaScript is a complementary programming language. 

On the other hand, Java is an object-based programming language. It is known for its versatility as it can be used for web development, mobile applications, desktop applications and much more. Java is a self-contained language used for back-end applications. 

Despite having similar names and some shared syntax, both languages serve different purposes and are in demand for 2023. Developers with either of these skills can land for a job in the tech world. To excel in a JavaScript interview, you need to understand the important differences between JavaScript and Java as well as JavaScript problem solving questions.

2- What are JavaScript data types?

JavaScript has two main categories of data types: primitive types and reference types. Primitive types include Number (for both integers and floating-point numbers), String (sequences of characters), Boolean (true or false values), Undefined (for declared but unassigned variables), Null (representing intentional absence of value), and Symbol (unique and immutable values, introduced in ES6). Reference types encompass Object (collections of key-value pairs), Array (ordered lists of values), Function (callable objects), Date (specific moments in time), RegExp (patterns for matching character combinations in strings), and user-defined objects. JavaScript’s dynamic typing allows variables to hold values of any type and change their type during program execution, providing flexibility in programming. 

Additionally, special values like NaN, Infinity, and -Infinity are used in numeric operations, and the typeof operator helps determine the type of variables or expressions. Understanding and effectively using these data types is crucial for proficient JavaScript programming.

3- Some Advantages of JavaScript

JavaScript offers multiple advantages that make it stand out from other languages. Here are some advantages:

  • Client-Side Interactivity:

JavaScript offers a faster response time and smoother user experience allowing dynamic and interactive user interfaces. 

  • Versatility: 

JavaScript can be used for multiple ranges of applications and web development such as mobile app development, desktop applications and much more.

  • Platform Independent:

JavaScript is a highly portable language as it can run on any device with a web browser. 

  • Rich Ecosystem:

JavaScript has a vast ecosystem of libraries, frameworks, and tools that can significantly speed up development.

  • Community and Support:

JavaScript has a large active developer community which means that there are plenty of resources and tutorials available to help developers. 

  • Integration with HTML/CSS:

JavaScript is an essential tool for creating interactive web pages with integrating with HTML and CSS. 

  • Cross-Browser Compatibility: 

To handle compatibility issues modern JavaScript frameworks are designed, ensuring consistency. 

  • Quick Prototyping:

JavaScript’s dynamic nature and the availability of frameworks and libraries enable rapid prototyping of ideas.

  • Continuous Development and Improvement: 

JavaScript is an evolving language, with regular updates and new features being introduced. This ensures that developers have access to modern tools and techniques to build cutting-edge applications.

4- What are arrow functions?

Arrow function in JavaScript allows you to create functions in a cleaner way and provide syntax for writing. They have a shorter syntax, inherit this from the surrounding context, lack their own object arguments, can’t be used as constructors, and don’t have a super keyword. They are especially useful for short, one-line functions and those that don’t rely heavily on this.

5- Explain string in JavaScript

A string in JavaScript is a sequence of one or more characters that consist of multiple letters and numbers enclosed in single (”) or double (“”) quotes. Strings are fixed which means their values can’t be changed once created. Finding length, extracting substrings and changing cases are some properties for string manipulation in JavaScript.

6- What are undeclared and undefined variables?

An undeclared variable is one that hasn’t been formally defined in the code. If you try to use it, JavaScript will throw a reference error. An undefined variable, on the other hand, is one that has been declared but hasn’t been assigned a value yet. By default, it holds the value undefined. It’s important to declare and assign values to variables before using them to avoid errors.

7- What is a closure in JavaScript?

In JavaScript, a closure is a function that is combined with the lexical context in which it was declared. This makes it possible for the function to continue using variables and parameters from its external scope even after the execution of that scope has concluded. In other words, even when a function is executed outside of its lexical scope, it can still “remember” and access its scope thanks to a closure. JavaScript closures are an incredibly useful feature that are frequently used in functional programming to implement modules, generate private variables, and retain state. They are essential to writing modular, effective code.

8- Explain the concept of hoisting

With JavaScript, hoisting is the procedure that moves function and variable declarations to the top of their contained scope during build time. This enables you to make use of them prior to their official declaration. But the initializations are not hoisted; just the declarations are. Hence, undefined will be the initial value of variables that are initialized later. Functions allow you to call them before they are declared because they have both a name and a body hoisted. To prevent unexpected behavior in your code, hoisting is something you should be aware of.

9- What is a callback function in JavaScript?

A callback function is a function that is called when another function finishes and receives an argument. It is essential for managing asynchronous operations like retrieving data from a server or reacting to events since it enables autonomous execution. Programming that is responsive and effective is made possible by this strategy.

10- Explain the concept of “this” keyword in JavaScript

A function’s calling context is indicated by the keyword “this.” The manner and location in which the function is called determines its value. This refers to the global object (a browser window) in the global scope. Whether a function is invoked as a constructor, a method of an object, or with call or apply to explicitly set this will determine how it is used. This is inheritable from the enclosing scope of arrow functions. Comprehending this is essential for managing contexts in JavaScript correctly.

11- What is the difference between function declaration and function expression?

A function declaration is defined using the function keyword and can be called before it’s defined due to hoisting. Example: function add(a, b) { return a + b; }.

Whereas, a function expression is defined by assigning a function to a variable and can only be called after it’s defined. Example: const add = function(a, b) { return a + b; };. Named function expressions provide a name that can be used for self-reference within the function. Example: const multiply = function multiply(x, y) { return x * y; };.

12- What is the difference between synchronous and asynchronous code in JavaScript?

Synchronous code executes tasks one after the other, blocking the program until each task is finished. Asynchronous code allows tasks to run independently, so multiple tasks can occur simultaneously, improving responsiveness. This is crucial for tasks like network requests or waiting for user input, where waiting for completion could lead to delays. JavaScript uses mechanisms like callbacks, promises, and async/await for asynchronous programming.

13- Explain the difference between ‘let’, ‘const’, and ‘var’ in JavaScript

While using JavaScript you can declare variables with ‘var’, ‘let’ and ‘const; keywords. All of these variables have different functions such as:

‘var’ is function-scoped, and can be re-declared and updated. ‘let’ is block-scoped, can be updated but not re-declared. ‘const’ is block-scoped, and cannot be reassigned after declaration, but for objects/arrays, their properties/elements can still be modified. Use ‘const’ for values that shouldn’t change, ‘let’ for variables that may need to be updated, and use ‘var’ sparingly due to its function scope and hoisting behavior.

14- Does JavaScript support automatic type conversion?

JavaScript does support automatic type conversion, also known as type coercion. This means that JavaScript will sometimes automatically convert a value from one data type to another in certain situations, without explicit instructions from the programmer. It’s important for developers to be aware of this behavior to avoid potential pitfalls in their code.

15- What are some best practices for writing clean and efficient JavaScript code?

Writing an efficient code in JavaScript is not a complex task, all it requires are some useful skills and practices that help a developer. The practices include:

  • Use Meaningful Variable and Function Names
  • Follow a Consistent Coding Style
  • Limit the Use of Global Variables
  • Avoid Magic Numbers and Strings
  • Use Strict Mode
  • Avoid Unnecessary Nesting
  • Optimize Loops and Iterations
  • Minimize DOM Manipulation
  • Avoid Blocking the Event Loop
  • Handle Errors Gracefully
  • Avoid Code Duplication
  • Keep Functions Small and Focused
  • Comments and Documentation
  • Use Modern JavaScript Features

Conclusion

As a conclusion, to hire JavaScript developers, these 15 questions would identify if the candidate has the right knowledge about JavaScript. These questions cover a wide range of important concepts, from basic language fundamentals to more advanced topics like closures, promises, and asynchronous programming. By thoroughly understanding these concepts and practicing your problem-solving skills, you’ll be well-equipped to tackle any JavaScript-related challenges that come your way in an interview setting.

To assess the candidate more in-depth, recruiters use the modern hiring methods such as pre-screening skill based assessments that identify candidate’s skill sets. These assessments include JavaScript problem solving questions which helps the candidate further for JavaScript interview coding exercises.