a fresh approach to mixins

mixins.js

//generic circle functions
var asCircle = function() {
    this.area = function() {
        return Math.PI * this.radius * this.radius;
    }
    this.grow = function() {
        this.radius++;
    }
    this.shrink = function() {
        this.radius--;
    }
    return this;
}

//most simple case: applying to object literal
var circle1 = asCircle.call({radius:3});
console.log(circle1.area());

//we can also apply to constructors
var Circle = function(radius) {
    this.radius = radius;
}

asCircle.call(Circle.prototype);

var circle1 = new Circle(5);
console.log(circle1.area());

//here's an object for button functions
var asButton = function() {
    this.hover = function(bool) {
        bool ? mylib.appendClass('hover') : mylib.removeClass('hover');
    }
    this.press = function(bool) {
        bool ? mylib.appendClass('pressed') : mylib.removeClass('pressed');
    }
    this.fire = function() {
        this.action();
    }
    return this;
}

//mix these two components to make round button
var RoundButton = function(radius, label) {
    this.radius = radius;
    this.label = label;
}

asButton.call(RoundButton.prototype);
asCircle.call(RoundButton.prototype);

var button1 = new RoundButton(4, 'yes!', function() {console.log('you said yes!')});
button1.fire(); //'you said yes!'

//make rectangle and oval descriptors
var asRectangle = function() {
    this.area = function() {
        return this.length * this.width;
    }
    this.grow = function() {
        this.length++, this.width++;
    }
    this.shrink = function() {
        this.length--, this.width--;
    }
    return this;
}

var asOval = function() {
    this.area = function() {
        return Math.PI * this.longRadius * this.shortRadius;
    }
    this.ratio = function() {
        return this.longRadius/this.shortRadius;
    }
    this.grow = function() {
        this.shortRadius = this.shortRadius + (1/this.ratio());
        this.longRadius++;
    }
    this.shrink = function() {
        this.shortRadius = this.shortRadius - (1/this.ratio());
        this.longRadius--;
    }
    return this;
}

//make oval button
var OvalButton = function(longRadius, shortRadius, label, action) {
    this.longRadius = longRadius;
    this.shortRadius = shortRadius;
    this.label = label;
    this.action = action;
}

asButton.call(OvalButton.prototype);
asOval.call(OvalButton.prototype);

var button3 = new OvalButton(3, 2, 'send', function() {console.log('message sent')});
console.log(button3.area());
button3.grow();
console.log(button3.area());
button3.fire();

//an example of parameterized mixin (not possible with traditional mixin method)
var asButton = function(options) {
    var hoverClass = options && options.hoverClass || 'hover';
    var pressedClass = options && options.pressedClass || 'pressed';
    this.hover = function(bool) {
        bool ? mylib.appendClass(hoverClass) : mylib.removeClass(hoverClass);
    }
    this.press = function(bool) {
        bool ? mylib.appendClass(pressedClass) : mylib.removeClass(pressedClass);
    }
    this.fire = function() {
        this.action();
    }
    return this;
}

var RectangularButton = function(height, width, label, action) {
    this.height = height;
    this.width = width;
    this.label = label;
    this.action = action;
}

asButton.call(RectangularButton.prototype, {pressedClass: 'down'});
asRectangle.call(RectangularButton.prototype);

var button4 = new RectangularButton(5, 7, 'save', function() {console.log('saved')});
button4.hover(true);
button4.press(true);

Angus,
I think the RoundButton function is missing 'action' argument (Line 44). It should be,
var RoundButton = function(radius, label, action) {
this.radius = radius;
this.label = label;
this.action = action;
}