Understanding the Different Behavior of Extended Native Classes in JavaScript

Learn why JavaScript exhibits `different behaviors` when extending native classes using constructors, and how to properly handle arguments in inherited classes.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Different behavior when extend natives class with constructor and passing arguments to super

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the Different Behavior of Extended Native Classes in JavaScript

When working with JavaScript, developers often extend native classes to create more specialized functionality. However, this can sometimes lead to surprising behaviors, especially when dealing with constructors and arguments. In this post, we'll explore the peculiarities of extending the Array class in JavaScript, focusing on the differences in behavior when calling constructors explicitly versus relying on the default constructor behavior.

The Problem: Inconsistent Behavior in Inheritance

When subclassing native classes like Array, the way you define the constructor can have a significant impact on how instances behave. Let's take a look at two separate cases involving the MyArray class, which extends the built-in Array class.

Case 1: Passing Arguments via Constructor

In this scenario, we define a constructor that explicitly takes an argument and passes it to the superclass (Array):

[[See Video to Reveal this Text or Code Snippet]]

Here, the MyArray constructor receives an argument from argument, but it only passes the first item to the super constructor. As a result, when we log arr, we only see ['first'], illustrating the unexpected behavior caused by argument handling.

Case 2: Automatic Constructor Behavior

In contrast, when we don't explicitly define a constructor in the subclass, JavaScript provides a default constructor. This default version handles all arguments and forwards them to the superclass:

[[See Video to Reveal this Text or Code Snippet]]

Here, arr correctly outputs both items, as the default constructor uses the syntax super(...args) to send all arguments to the Array constructor.

The Explanation: How Constructors Work in Subclasses

Default Behavior of Constructors

When you don't provide a custom constructor, JavaScript automatically defines one that looks like this:

[[See Video to Reveal this Text or Code Snippet]]

This means that all arguments—regardless of their number or type—are forwarded to the superclass. This is a key factor in the differing behaviors observed in the examples above. By contrast, the constructor we've defined manually only captures the first argument, ignoring any others that might have been passed.

Adjusting the Custom Constructor

To fix the issue while still accommodating the first argument (if needed), we can modify the custom constructor to use rest parameters. Here's how you can do that:

[[See Video to Reveal this Text or Code Snippet]]

This approach ensures that all arguments are correctly handled while allowing you to implement unique behavior for the first argument.

Important Considerations: The Array Constructor's Quirks

When subclassing Array, it's crucial to recognize that its constructor behaves differently based on both the number and type of arguments:

If the constructor receives only one argument that is a number, it initializes the array to that length, creating an empty array.

If it receives multiple arguments or a single argument that is not a number, it populates the array with those arguments as elements.

Understanding these behaviors will help you avoid pitfalls and design more effective subclasses!

Conclusion

In conclusion, extending native classes in JavaScript provides great power but demands a careful approach to argument handling in constructors. By examining the different behaviors of constructors when extending Array, we've highlighted how to ensure that your subclasses function correctly and predictably. Always be mindful of how you define your constructors and consider how many arguments you want to handle; with a little attention to detail, you c