// Interfaces. var Composite = new Interface('Composite', ['add', 'remove', 'getChild']); var FormItem = new Interface('FormItem', ['save']); // CompositeForm class var CompositeForm = function(id, method, action) { // implements Composite, FormItem //... }; //... function addForm(formInstance) { Interface.ensureImplements(formInstance, Composite, FormItem); // This function will throw an error if a required method is not implemented, // halting execution of the function. // All code beneath this line will be executed only if the checks pass. //... }