go2js Interface pesudo implementation

gistfile1.js

function Interface(decl){
    this.decl = decl;
    this.impl = {}; // map funcname -> type
}

Interface.prototype = {
    check : function(typedef){
        if(typedef.uuid === undefined){
            typedef.uuid = GenerateGloballyUniqueId();
        }

        if(this.impl[typedef.uuid] !== undefined)
            return this.impl[typedef.uuid];
        
        var matches = true,
            proto = typedef.prototype;

        for(var fn in this.decl){
            var typefn = proto[fn];
            if(typefn === undefined){
                // doesn't contain fn
                matches = false;
                break;
            }

            var signature = this.decl[fn];
            if(!sameSignature(typefn.signature, signature){
                matches = false;
                break;
            }
        }

        this.impl[typedef.uuid] = matches;
    },
    assert : function(elem){
        if(this.check(elem.constructor))
            return elem
        else
            throw "Does Not Implement";
    }
}

SomeInterface = new Interface({
    someFn : {arg : [Boolean], ret: [Boolean]}
})

function someStruct(val){
    this.value = val;
}
someStruct.uuid = "main/someStruct";

someStruct.prototype.someFn = function(val) {
    return val || (this.value == 5);
}
someStruct.prototype.someFn.signature = {
    name : "main.someStruct/someFn", // useful for debugging
    arg : [Boolean], ret: [Boolean]};

// in the code it would look
var value = new someStruct(5);
// the type assertion
var xxx = SomeInterface.assert(value);