JSFiddle - React, Tailwind, and code Playground

JavaScript

// 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.
    //...
}