Question

When I run the following code I get told, that talk is not a function.
Why?

function cat(name) {
    talk = function() {
        alert(" say meeow!" )
    }
} 

cat("felix");
cat.talk()

Answer 1

What you’re trying to do is create an object for which the function is a constructor, but what the code is actually doing is setting the variable talk to a function. You want:

function cat(name) {
    this.talk = function() {
        alert(" say meeow!" )
    }
} 

var myCat = new cat("felix");
myCat.talk()

edit:

Relevant javascript tech talk: http://www.youtube.com/watch?v=ljNi8nS5TtQ

He talks about constructing objects with functions at about 30 minutes in. The code he posts is:

function Circle(radius){
    this.radius = radius;
    this.area = function(){
        return this.radius * this.radius * Math.PI;
    };
}
var instance = {};
Circle.call(instance, 5);
instance.area(); // ==> 78.5398
var instance2 = new Circle(5);
instance2.area() // ==> 78.5398
instance instanceof Circle // ==> false
instance2 instanceof Circle // ==> true

And the relevant quote:

The new keyword is just a shorthand that is saying “make a new object
and call the constructor on it … the new keyword has no other
meaning”

In other words, he’s saying that when using the new keyword, you’re defining your variable as an object and calling the function in the context of that object (this points to your object).

The extra thing that the new keyword does is set the prototype of the newly made object to the prototype of the constructor. So if we do:

function Circle(radius){
    this.radius = radius;
    this.area = function(){
        return this.radius * this.radius * Math.PI;
    };
}
var instance = {};
Circle.call(instance, 5);
instance.__proto__ = Circle.prototype; // we set the prototype of the new object to that of the constructor
instance.area(); // ==> 78.5398
var instance2 = new Circle(5);
instance2.area() // ==> 78.5398
instance instanceof Circle // ==> true // this is now true 
instance2 instanceof Circle // ==> true

instance instanceof Circle is now true.