If you have been keeping up with the JavaScript ecosystem, you have likely come across the saying that, “Everything in JavaScript is an object.” Even though it is not entirely accurate, it shows the important role of Objects in JavaScript. Objects in JavaScript are at the core of how JavaScript organizes and manipulates data.

Constructors in JavaScript allow you to create and initialise objects with predefined properties and behaviors. Whether you are working with classes or plain object blueprints, constructors are essential for setting up default values or configurations when creating new instances.

In this detailed blog, we will look into JavaScript constructors, exploring their shift from traditional constructor functions to modern ES6 classes. By the end of this blog, you will have a fair understanding of how constructors work and how to use them effectively in your projects.

The Basics of JavaScript Constructors

A constructor in JavaScript is a special function used to create and initialise objects. It provides a way to wrap your logic into an object, ensuring each instance has its own set of properties and behaviors.

 

Constructors simplify the creation of multiple similar objects. You can create it once and reuse it accordingly. In JavaScript, constructors come in two main forms: Constructor Functions and ES6 Classes.

 

Let’s look into both approaches to understand their differences and use cases.

 

Understanding Object Constructor Functions in JavaScript

An object constructor function is a special function used to create and initialise objects. Constructor functions allow you to define a blueprint for objects, and when the function is called using the new keyword, it creates a new instance of an object with properties and methods defined within the constructor.

Let’s take an example. Suppose we have to create a car object as shown below. We can define all the properties that we need inside the object.

What if we need to store another model of another brand? One way is to create new objects again and then add their properties.

Although the above approach is doable, this isn’t an ideal approach as we can have thousands of combinations possible. We need a more maintainable way and reduce the amount of boilerplate code.  Here what you can do is, create an object constructor to construct the objects and then pass the required data to it.

Here as you can see, we created an object constructor and we were able to create multiple instances of it without repeating the code.

 

Constructor function can make object creation more dynamic, modular, and reusable as you can see in the above example.

Types of Constructors in JavaScript

In the earlier section, we looked at an example of a functional constructor. JavaScript has several types of constructors, each serving a specific purpose in object creation and initialization. 

 

ES6 Classes

 

Earlier there was no concept of classes in JavaScript. The only way to create constructors was by using object constructors. In 2015, ES6 introduced class syntax. Classes are the modern way to define constructors and create objects. 

Although Classes in JavaScript are syntactic sugar over constructor functions, they offer a more readable and structured approach.

Built-in Constructors

JavaScript also has several built-in constructors that allow you to create predefined objects. These constructors are part of the JavaScript standard library and are used for common object types such as arrays, dates, and regular expressions.

Some common built-in constructors:

• Object: Creates a general-purpose object.
• Array: Creates an array.
• Date: Creates a date object.
• Boolean: Creates a new boolean object.
• RegExp: Creates a regular expression object.
• Function: Creates a new function.
• Error: Creates an error object.

There are also some other ways to create constructors in JavaScript. Just like extending the prototypes, you can add properties and methods that are shared across all instances.

Class constructors and methods

Class constructors and methods are the backbone of JavaScript’s object-oriented programming model, allowing developers to create and manage objects efficiently. 

A class can have only one constructor method, which is automatically invoked when a new instance of the class is created. This method is primarily used to initialize the object’s properties, setting up default values or configurations for the instance.

Methods

A Class method is a function that is defined within the class and can perform actions or computations related to the class objects. They are created with the same syntax as object methods.

Static Methods

Static methods are functions that belong to the class itself rather than its instances. They are particularly useful for utility or helper functions that operate independently of instance-specific data, making them ideal for tasks directly related to the class’s behaviour or functionality.

Getters and Setters

Classes can also include getter and setter methods, providing a structured way to access and modify an object’s properties. 

Getters allow you to define how specific property values are retrieved, while setters enable you to control how those values are updated. Getters and setters can help you encapsulate property logic, making your code cleaner and more maintainable.

Advantages of using ES6 classes over traditional Constructor functions

Most developers now prefer writing ES6 classes rather than depending on traditional constructor functions. The reason is that ES6 classes provide a much better way and other comparative advantages. Let’s now look into it:

Improved Readability and Syntax

ES6 classes provide a cleaner, more developer-friendly syntax that is easier to read and understand, especially for developers coming from other object-oriented programming languages background. 

The Class syntax hides the complexity of prototype manipulation behind a more familiar object-oriented structure, making the code easier to understand and maintain.

Simplified Inheritance

The extends keyword in ES6 classes makes it easy to set up inheritance hierarchies, making the process more intuitive and less error-prone compared to the older prototype-based approach. 

ES6 syntax significantly reduces the complexity of creating and managing object hierarchies, allowing developers to focus more on functionality rather than boilerplate code.

Reduced Boilerplate

ES6 classes remove unnecessary boilerplate code that was required when defining constructor functions and their prototypes. This reduction in code leads to increased developer productivity and easier maintenance for anyone who is starting to work on that codebase.

Enhanced Features

ES6 classes introduce several new features that were difficult or impossible to implement with traditional constructor functions:

• Super calls: The super keyword allows for easier and more intuitive supercalls to parent class methods.
• Static methods: Classes support static methods, which are called on the class itself rather than on instances.
• Private fields and methods: ES6 classes support private members, improving encapsulation and data hiding.

Familiarity for New Developers

If someone novice to JavaScript started to learn this, if they have worked with any object-oriented programming languages it makes it easier for them to understand and work with JavaScript’s object model. 

Class vs Object Constructor Function: A Comparative Analysis

While both class constructors and Object constructors achieve similar results, there are some key differences in their syntax, behaviour, and features. Let’s compare the two approaches to better understand their distinctions and when to use each.

Strict Mode

Classes are always executed in strict mode by default, which enforces stricter parsing and error-handling rules. This means that certain actions that might have been silently ignored or failed in non-strict mode will throw errors in strict mode. For example:

Here since you haven’t declared the variable, it will throw you an error instead of some ambiguous value.

Constructor functions, on the other hand, are not automatically executed in strict mode. Developers need to explicitly add the “use strict” directive to enable strict mode.

Hoisting

Function declarations in JavaScript are hoisted, meaning they can be used before they are defined in the code. For example:

Here you can use the myFunction() function before the actual declaration.

Class declarations are not hoisted. They must be defined before they can be used or else they will throw an error

Method Definition

In classes, methods are defined directly within the class body:

For constructor functions, methods are typically defined on the prototype to ensure they are shared across all instances.

Inheritance

Classes provide a more intuitive syntax for inheritance using the extends keyword.

With constructor functions, inheritance requires manual prototype chaining.

These differences highlight how classes provide a more structured and safer approach to object-oriented programming in JavaScript. If you want simplicity you can go with Object constructors, but if you want to write safe and robust code, Class-based approach has its advantages.

Common Pitfalls and Best Practices

When working with constructors in JavaScript, developers often encounter several common pitfalls. Let’s explore some of the most frequent mistakes and how to avoid them:

Forgetting to use the new keyword

One of the most common mistakes is forgetting to use the new keyword when creating objects from constructor functions. This can lead to unexpected behaviour and bugs.

To overcome this, you can use the “use strict” keyword to give you an early error.

Misusing ‘this’ inside constructors

Another common pitfall is misunderstanding how ‘this’ works inside constructors, especially when dealing with callbacks or closures.

Here the this keyword does not refer to the instance of Counter. Instead, it refers to the global context. To fix this you can use an arrow function or bind the correct context.

Wrapping it up

Thank you for taking the time to read this comprehensive blog post on JavaScript constructors. We have covered a wide range of topics, from fundamental concepts to more advanced applications of constructors in JavaScript.

Throughout this article, we have looked into the basic principles of constructors, different types of constructors and their use cases, and best practices for implementing constructors in your code. You can now start implementing these to get a better grasp of constructors in JavaScript.

Happy learning,