The importance of constructors and prototypes in JavaScript

I came across a code snippet that involves defining a constructor like this:

var Resource = function(data) {
    angular.extend(this, data);
}

Afterward, a method is defined for it as well.

Resource.query = function(url) {
    console.log(url);
}

I'm curious about how this works. I am aware that functions are considered objects too. Is this similar to the code below? And if yes, what happens to the constructor function?

var data = {};
data.query = function(url) {
    console.log(url);
}

Additionally, why not simply define it on the prototype instead?

Resource.prototype.query = function(url) {
    console.log(url);
}

You can find the code snippet here.

Answer №1

If you want specific instances to have certain methods, you would define those methods on the prototype. On the other hand, if a method is closely related to a class but doesn't really fit as an instance method, it can be implemented as a static method within the constructor.

In the code examples below, consider which one makes more sense to you:

1-

var user = new User();

user.findUser('somequery').then(...);

2-

User.findUser('somequery').then(...);

Answer №2

It's worth noting that functions are objects too. Does this mean they function the same as other properties on objects?

Your point is valid - a function behaves like any other property within an object.

Why not establish it on the prototype instead?

The reason being, when a method is placed on the prototype, every object created with that constructor will inherit the method in their prototype chain. This isn't the case if the method is defined directly on the constructor itself.

For example:

var Resource = function() {}
Resource.one = function() {
    console.log("one");
}
Resource.prototype.two = function() {
    console.log("two");
}

var r = new Resource();
r.two(); // => "two"
r.one(); // => TypeError: r.one is not a function

Additionally:

Resource.one(); // => "one"
Resource.two(); // => TypeError: Resource.two is not a function

While you may refer to one as a static method, in reality, the language does not differentiate it from any other function (unlike methods labeled using static in Java).

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