使用原型从对象中的对象上调用函数

Question

I'm running into problems when I'm trying to call a function on an object from within that same object. I've read that calling a function on an object from within that object is possible here , so I think it must be my use of prototype that's causing the issue. Here's an example:

function Foo() {
    this.DoStuff();
}

Foo.prototype.DoStuff = function() {
    alert("I'm frantically doing stuff!");
}

That code (or something very similar) just doesn't want to work. Any ideas why?

Answer 1

What you have should work fine. It's important to remember that the value of this depends on how you call the function. For it to work as you expect, you need to use the new operator to call the function as a constructor:

var foo = new Foo(); //`this` refers to this instance of Foo

Here's a working example .

If you call the function like normal, this refers to the global object, which doesn't have a DoStuff property, so a TypeError is thrown. Here's a broken example .

Answer 2

I'm going to guess that your prototype statement is declared after you try to use Foo().

This should work:

Foo.prototype.DoStuff = function() { alert("stuff") };
myfoo = new Foo();

This should NOT:

myfoo = new Foo();
Foo.prototype.DoStuff = function() { alert("stuff"); };

Answer 3

It depends on how you call Foo() . If you call it directly, then this refers to whatever this is in that context. If you call it with the new operator, then this becomes that instance of that object, and then it has access to DoStuff() .

var foo = new Foo(); // works!