Pseudoclasses and prototypal inheritance in JS
This is my third post in the series of “JS: Leave the classes to those other languages”. If you haven’t checked out the first two posts, you can read them on JS: Leave the classes to those other languages and JS Prototype chain mechanism.
Real classes (classical inheritance)
Before diving into pseudoclasses it is important to point out some characteristics of real classes (classes in Java, Ruby, etc).
In traditional languages:
- Classes are blueprints
- Objects (instances) are copies of all the characteristics described by classes
giamir is an instance of
Student and when we send to it the message
giamir knows how to handle the message without asking or delegate to some other objects.
In JS this is not possible because JS doesn’t copy object properties (natively, by default).
Pseudoclasses (prototypal inheritance)
Prototypal inheritance is the most popular way to fake classes (work with pseudoclasses) in JS.
The following code implement a pseudoclass
Person and a pseudoclass
Student that “inherits” from
It’s also creating an “instance” of the pseudoclass
Student and assigned it to a variable
Here is a mental map of the prototype chain generated by the code above.
We create a function
Person and a function
Student and we want to use them as constructors. Functions in JS automatically get a property called
prototype, which is just an empty object internally linked to
We use explicit pseudo-polymorphism to delegate between
Person constructors. We call the
Person constructor in the context of the new object we are creating using the
Obviously we want the
Student.prototype object be able to use all the
Person.prototype methods. To do so we need to replace
Student.prototype with a new object that delegates to
Person.prototype instead of
Object.prototype. The easiest way to do that is using the ES5
Once that the prototype chain is in place we can create a new object
giamir using the
Student function as a constructor:
giamir is an object that delegates to
Student.prototype and when we send to it the message
getSchool for example,
giamir has to delegate the message to the object above it in the prototype chain and so on.
Inheritance or delegation?
The word “inheritance” has a very strong meaning with plenty of mental precedent. Merely adding “prototypal” in front to distinguish the actually nearly opposite behaviour in JS is inappropriate.
JS creates a link between two objects, where one object can essentially delegate property/function access to another object.
– Eric Elliott
We are ready to explore alternative patterns in JS to get rid of pseudoclasses and finally embrace the nature of the language.