Question

I have a problem with my javascript code. I have this prototype and I want to have it in a way where you can add ‘middleware’ or how you’d call it. (like in ExpressJS, app.get('/user/:id/edit', loadUser, andRestrictToSelf, function(...))

var Server = function(...){
    ...
};
Server.prototype.log = function(data){
    console.log(data);
};
console.log(typeof(Server.prototype.log)); //function

So I changed it to a way where I add the functions by calling a function, so i can later implement something like _add('fnName', loadUser, function(...)).

var Server = function(...){
    ...
};
_add = function(fnName, fn){
    Server.prototype.fnName = fn;
    console.log(typeof(Server.prototype.fnName)); //always function
};
_add('log', function(data){
    console.log(data);
});
console.log(typeof(Server.prototype.log)); //undefined

However this doesn’t work. The prototype isn’t changed.

I don’t want to do it like in ExpressJS (you add to an instance, not the prototype) because a Server object will be created for every user (in combination with socket.io), so adding functions to objects will be more overhead than adding to the prototype whose functions are available in all instances.

Answer 1

You need to use Server.prototype[fnName] instead, because Server.prototype.fnName defines a function called fnName on the Server’s prototype.

So in your example:

console.log(typeof(Server.prototype.log));   // undefined
console.log(typeof(Server.prototype.fnName)); // function

Code:

var Server = function(...){
  ...
};

_add = function(fnName, fn){
  Server.prototype[fnName] = fn;
  console.log(typeof(Server.prototype[fnName])); //always function
};

_add('log', function(data){
  console.log(data);
});

console.log(typeof(Server.prototype.log)); // function

See this snippet.