Objects and Methods

In this section we discuss how methods interact with the objects they belong to, and look into the infamous this language construct.

Relevant Links

• this in MDN

Notes

A key characteristic of objects is that all objects “of the same class” tend to have access to a common set of functions, which are typically called the object methods.

In other languages, these would be placed in a “class”. Javascript doesn’t use classes. Instead, objects essentially inherit properties from other objects.

We will see how that happens more extensively soon, but for now let us try a silly example:

var a = {
    name: "Pete",
    greet: function() { return "Hello! I am " + a.name; }
};
a.greet();

Now let us suppose we want to create a second object. It will need its own name, but perhaps we could reuse the greet function. Here is an attempt at doing so:

var b = { name: "Myka" };
b.greet = a.greet;
b.greet();

Oops, that did not work! greet still only knows about Pete. a.name still references the a object.

What we need is a special way to refer to the “current object”, the object that the function was executed from. This is what the keyword “this” is for.

In a call like o.f(...), any references to this in the function f will refer to the object o.

So here is a version of the above that works:

var a = {
    name: "Pete",
    greet: function() { return "Hello! I am " + this.name; }
};
var b = { name: "Myka" };
b.greet = a.greet;

Of course it’s not always a great idea to create objects in quite this way, each of them having the methods they need as properties. But it a good example to illustrate some of the mechanics of this.

To understand Javascript’s object model further, we will need to study prototypes.