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JavaScript Understanding "this" in JavaScript

Zijun Liao
Zijun Liao
13,409 Points

declare class in Javascript - class vs function

I notice that sometimes people use function keyword to declare a class like the second example, and sometimes use class.

Are there any different, and when I should use class or function?


2 Answers

Alex Koumparos
Alex Koumparos
Python Development Techdegree Student 36,887 Points

Hi Zijun,

class is syntactic sugar introduced in ECMAScript 2015 (aka ECMAScript 6). It is intended to provide a way for developers who are familiar with class-based OO to work more efficiently with JS's prototype-based OO.

If you are already familiar with languages like Java, C++, Python, etc, the class syntax will definitely look more like the syntax you know from those languages. If you're not already familiar with OO, then you're generally better off using the 'older' syntax, since this syntax more accurately reflects JavaScript's underlying behaviour.

It's important to understand that this syntax doesn't change JavaScript's OO model. When you create a new object using the class keyword you are creating exactly the same kind of object as you would using the older syntax. This can cause unpleasant surprises for people expecting the 'class' objects to work just like in other languages. All the peculiarities of JS's prototype-based object model are still present. The most fundamental point being that in true class-based OO languages, instantiation of an object is a copy action: the original class is not subject to modification by the instance. However, in JS, when you create an object from another object (even if that other object was defined using the class keyword) you are not making an independent instance, you are making a delegation link between the prototype object and the new object. This means that there are circumstances where modifications to your new object can change the 'parent' object.

There is an excellent discussion of the differences between JavaScript's prototype-based OO model and other OO languages in Kyle Simpson's You Don't Know JS, particularly Chapters 4–6 and Appendix A. The discussion also covers how the class syntax works behind the scenes. All of which can be read online for free.

When writing your own code, use whichever way you prefer, but you're going to keep running into other people's code written both ways, so you'll have to get comfortable with both approaches.

Hope that helps.

Happy coding,


Class is the newer simpler syntax.