Common Mixin approach

mixins-common.js

function extend(to, from) {
  for (var key in from) {
    if (from.hasOwnProperty(key)) {
      to[key] = from[key];
    }
  }
}

function Label() { this.text = ''; };
Label.prototype.setLabel = function(text) { this.text = text || '' };
Label.prototype.getLabel = function() { return this.text;};
Label.prototype.renderLabel = function() {};

function Rectangle() {};
Rectangle.prototype.renderRectangle = function() {};

function LabeledRectangle() {};
extend(LabeledRectable.prototype, Label.prototype);
extend(LabeledRectable.prototype, Rectangle.prototype);
LabeledRectangle.prototype.render = function() {
  this.renderRectangle();
  this.renderLabel();
};