Inheritance in Javascript - protoyping not in definition-part?

I am currently switching from AS3 to JavaScript.
I still have some trouble with understanding inheritance-concepts.
What I do not understand is why the following code is not working properly:

Base = function () {
    this.coolVar = "great";
}  

SmallControl = function () {

    // Inheritance:
    this.prototype = new Base();
    this.prototype.constructor = SmallControl;

    this.prototype.init = function(aMap) {
        console.log('init');
        console.log('coolVar?: ' + this.coolVar);
    }
}  
var foo = new SmallControl();  
//foo.init();         // --> TypeError: foo.init is not a function  
foo.prototype.init(); // --> works

If I put the prototype definitions outside of the "SmallControl"-Function everything works fine... but I don't understand that.

Copyright Notice：Content Author:「Roman」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/1205385/inheritance-in-javascript-protoyping-not-in-definition-part