Hiring guide for JavaScript Engineers

JavaScript Developer Hiring Guide

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.

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

First 20 minutes

General JavaScript app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in JavaScript application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

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.
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 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.
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 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.
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What you’re looking for early on

Does the candidate have a strong understanding of JavaScript fundamentals?
Can the candidate solve problems using JavaScript?
Is the candidate familiar with JavaScript frameworks and libraries?
How well does the candidate understand asynchronous programming and promises?

Next 20 minutes

Specific JavaScript development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

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.
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.
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 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.
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.
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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 point, a skilled JavaScript engineer should demonstrate strong problem-solving abilities, proficiency in JavaScript programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

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.

Wrap-up questions

Final candidate for JavaScript Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle JavaScript application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

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.
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.
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.

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.