JSFiddle - React, Tailwind, and code Playground

JavaScript

/*
    interface Composite {
        function add(child);
        function remove(child);
        function getChild(index);
    }
    interface FormItem {
        function save();
    }
*/

var CompositeForm = function(id, method, action) {
    this.implementsInterfaces = ['Composite', 'FormItem'];
    //...
};

//...


function addForm(formInstance) {
    if (!implements(formInstance, 'Composite', 'FormItem')) {
        throw new Error("Object does not implement a required interface.");
    }
    //...
}

// The implements function, which checks to see if an object declares that it 
// implements the required interfaces.


function implements(object) {
    for (var i = 1; i < arguments.length; i++) { // Looping through all arguments 
        // after the first one.
        var interfaceName = arguments[i];
        var interfaceFound = false;

        for (var j = 0; j < object.implementsInterfaces.length; j++) {
            if (object.implementsInterfaces[j] == interfaceName) {
                interfaceFound = true;
                break;
            }
        }

        if (!interfaceFound) {
            return false; // An interface was not found.
        }
    }
    return true; // All interfaces were found.
}