JavaScript Developer Hiring Guide

Hiring Guide for JavaScript Engineers

Ask the right questions to secure the right JavaScript talent among an increasingly shrinking pool of talent.

JavaScript, first developed by Netscape Communications Corporation in 1995, is a high-level, interpreted programming language primarily used for enhancing web pages to provide interactive user experiences. It is a key pillar of the World Wide Web, alongside HTML and CSS. JavaScript supports object-oriented programming and allows the creation of complex applications within a browser environment. Its versatility has led to its use in other environments as well, such as server-side scripting with Node.js. As per Stack Overflow's Developer Survey 2020, JavaScript remains the most commonly used programming language for the eighth year running.

First 20 minutes

General JavaScript knowledge and experience

The next 20 minutes of the interview should attempt to focus more specifically on the development questions used, and the level of depth and skill the engineer possesses.

What is JavaScript hoisting?

Hoisting is JavaScript's behavior of moving declarations to the top of the current scope. Hoisting applies to variable declarations and function declarations. Because of this, JavaScript functions and variables can be called or used before they are declared.

What is a closure in JavaScript?

A closure is a function that has access to its own scope, the outer function’s scope, and the global scope. It also has access to its own variables, the outer function’s variables, and the global variables.

What is the difference between '==' and '===' in JavaScript?

'==' checks for equality in value but not in type. '===' is a stricter equality test and returns false if either the value or the type of the two variables are different.

How does JavaScript handle type conversions?

JavaScript is a loosely typed language, meaning it performs automatic type conversion. This can be both implicit and explicit. Implicit type conversion or coercion is when JavaScript automatically converts a value from one type to another. Explicit type conversion is when the developer manually changes the data type.

What are JavaScript data types?

JavaScript has six primitive data types: String, Number, Boolean, Null, Undefined, and Symbol. Additionally, it has one complex data type: Object, which includes arrays and functions.

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What youre looking for early-on

Does the candidate have experience with testing and debugging in JavaScript?

Testing and debugging are crucial parts of the development process. A candidate should be comfortable with tools and techniques for testing their code and fixing bugs.

Does the candidate write clean, maintainable code?

The ability to write clean, well-documented code is important for maintainability and collaboration with other team members.

How well does the candidate understand asynchronous programming and promises?

Asynchronous programming is a key part of JavaScript. A good understanding of this concept is necessary for writing efficient, non-blocking code.

Is the candidate familiar with JavaScript frameworks and libraries?

Knowledge of frameworks and libraries like React, Angular, or Vue.js can be a strong indicator of a candidate's experience and versatility.

Can the candidate solve problems using JavaScript?

Problem-solving skills are essential for any developer. They should be able to demonstrate this by explaining how they would approach a hypothetical problem.

Does the candidate have a strong understanding of JavaScript fundamentals?

This is crucial as a deep understanding of the language's basics will allow them to write efficient and effective code.

Next 20 minutes

Specific JavaScript development questions

The next 20 minutes of the interview should attempt to focus more specifically on the development questions used, and the level of depth and skill the engineer possesses.

How would you create an object in JavaScript?

Objects in JavaScript can be created using object literal syntax, object constructor syntax with the 'new' keyword, or Object.create method.

What is the use of 'this' keyword in JavaScript?

'This' keyword refers to the object from where it was called. It provides a reference to the current object and is used to access the properties and methods of that object.

What is the difference between a method and a function in JavaScript?

In JavaScript, a function is a block of code designed to perform a particular task. A method, on the other hand, is a function defined as a property of an object.

What is the difference between 'null' and 'undefined' in JavaScript?

'Undefined' means a variable has been declared but has not yet been assigned a value. 'Null' is an assignment value that means no value or no object. It implies absence of value.

How would you handle exceptions in JavaScript?

Exceptions in JavaScript are handled using try, catch, and finally blocks. The try block contains the code that might throw an exception, the catch block executes if an exception is thrown, and the finally block executes after the try and catch blocks, regardless of the result.

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The ideal back-end app developer

What you’re looking to see on the JavaScript engineer at this point.

At this stage, a skilled JavaScript engineer should demonstrate strong problem-solving abilities, a deep understanding of JavaScript concepts like closures and prototypes, and proficiency in frameworks like React or Node.js. Red flags include inability to explain code logic, lack of testing knowledge and poor understanding of asynchronous programming.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with JavaScript.

What does the following JavaScript code do?

let a = 10; 
let b = a; 
b = 20;

This code creates two variables 'a' and 'b'. 'a' is assigned a value of 10. Then 'b' is assigned the value of 'a'. Finally, 'b' is assigned a new value of 20. This does not affect the value of 'a' since numbers are primitive types in JavaScript and are passed by value, not by reference.

What will be the output of the following JavaScript code?

let name = 'John'; 
console.log(`Hello ${name}`);

This code will output 'Hello John' to the console. This is due to JavaScript's template literals syntax, which allows the embedding of expressions within string literals.

What does the following JavaScript code do?

let arr = [1, 2, 3, 4, 5]; 
let sum = arr.reduce((a, b) => a + b, 0);

This code calculates the sum of all numbers in the array 'arr'. It uses the 'reduce' method, which applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single output value.

What does the following JavaScript code do?

let promise = new Promise((resolve, reject) => { 
setTimeout(() => resolve('Done!'), 1000); 

This code creates a new Promise that resolves with the string 'Done!' after 1 second. Promises are used for asynchronous computations and represent a value which may be available now, or in the future, or never.

What does the following JavaScript code do?

class Person { 
constructor(name) { = name; 
greet() { 
return `Hello, ${}`; 

This code defines a 'Person' class with a constructor and a 'greet' method. The constructor is called when a new object is created from the class and sets the 'name' property of the object. The 'greet' method returns a greeting string that includes the 'name' property of the object.

What will be the output of the following JavaScript code?

let obj = { 
foo: function() { 
bar: 'Hello World' 
let test =; 

This code will output 'undefined'. This is because when the method 'foo' is assigned to the variable 'test' and then invoked as a function, its context is no longer 'obj', so '' refers to a 'bar' property that does not exist in the global context.

Wrap-up questions

Final candidate for JavaScript role questions

The final few interview questions for a JavaScript candidate should typically focus on a combination of technical skills, personal goals, growth potential, team dynamics, and company culture.

What are JavaScript service workers?

Service workers are a type of web worker. They act as a proxy server that sits between web applications, and the browser and the network. They are used to enable functionality like offline experiences, background syncs, and push notifications. They are a powerful tool for building Progressive Web Apps and other modern web experiences.

How would you use Promises in JavaScript?

Promises in JavaScript represent an operation that hasn't completed yet, but is expected in the future. A Promise is in one of these states: pending, fulfilled, or rejected. Once a promise is fulfilled or rejected, it is considered settled and can't transition to any other state or have its value changed. Promises are used to handle asynchronous operations, providing a more robust pattern for handling asynchronous operations compared to callbacks.

Describe the difference between synchronous and asynchronous programming in JavaScript.

Synchronous programming means the code is executed sequentially from top-to-bottom, blocking execution until each operation completes. Asynchronous programming means the engine runs in an event loop. When a blocking operation is needed, the request is started, and the code keeps running without blocking for the result. When the response is ready, an interrupt is fired, which causes an event handler to be run, where the control flow continues.

What is event bubbling and capturing in JavaScript?

Event bubbling and capturing are two ways of event propagation in JavaScript. In bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements. In capturing, the event is first captured by the outermost element and propagated to the inner elements.

What is prototypal inheritance in JavaScript?

Prototypal inheritance is a type of object-oriented inheritance model that JavaScript uses. Objects can inherit properties from other objects, known as prototypes. This is different from classical inheritance where classes inherit from other classes.

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JavaScript application related

Product Perfect's JavaScript development capabilities

Beyond hiring for your JavaScript engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in JavaScript projects, and can engage in multiple capacities.