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JavaScript JavaScript Foundations Objects Prototypes: Part 2

Posted by Britton Zirkle
Britton Zirkle
Courses Plus Student 15,005 Points

Why is the Prototype necessary?

What is the purpose of using the prototype when you can use constructor notation to define the properties and methods you wish to use in instances of the object? for example

var personPrototype = {
     name: 'Anonymous',
     greet: function (name, mood) {
          name = name || "You";
          mood = mood || "good";
          console.log("Hello, " + name + " I am " + this.name + " and I am in
          a " + mood + " mood!"); 
     },
     species: 'Homo Sapien'
};

function Person (name) {
     this.name = name;
}

Person.prototype = personPrototype;

jim = new Person("Jim");

as opposed to 

function Person (name) {
     this.name = name;
     this.species = 'Homo Sapien';
     this.greet = function (name, mood) {
          name = name || "You";
          mood = mood || "good";
          console.log("Hello, " + name + " I am " + this.name + " and I am in
          a " + mood + " mood!"); 
     };
}

var jim = new Person('Jim');

Unless I am missing something, using the constructor notation seems to save a lot of code and time.

2 Answers

eck
eck
43,038 Points

You're right, you can declare your properties and methods in the constructor, and your code will work as expected. From my understanding though, you have small performance improvements when using the prototype to store common data and methods.

Check this stack overflow thread that basically answers the same question you have asked.

Britton Zirkle
Britton Zirkle
Courses Plus Student 15,005 Points

Erik,

Thanks for the info. You are correct. I also found out that using prototype allows it to act more like a class in which a change in the prototype will cascade to all objects created from that prototype which is pretty cool.

Olga Filatova
Olga Filatova
8,045 Points
Olga Filatova
Olga Filatova
8,045 Points

Not only it's good for performance and cascading changes but also when using prototypes you can set one prototype to different classes or prototypes.

For example here we can declare prototypes like "american", "briton", "ukrainian", etc. linking to prototype "person" and for each inheriting prototype declare their own methods and properties. And so on, and so on.

Imagine which code will become larger in such circumstances.

In JS for example String, Number, Array, etc. prototypes inherit from Object prototype.

And winning on performance is not little. Especially in big JS projects. In having link to prototype in object or whole bunch of repeating code inserted in every little variable is a big differnence. DRY. Have mercy on RAM and your user. :)