Question

I am trying to work out how to implement an asynchronous callback in node given that I don’t know what the arguments could be.

I’ll state the sync version of what I’m trying to do so as to make this clearer.

function isAuthorized(userID){
    // check for user Permissions here
    return isAuthorized;
}

Using this any function can call it and, from the value it returns, determine if the user is authorized. However, the function that I use to get the userID (and check for permissions) are both asyncronous functions and require callbacks. However I have no idea how to get this implemented when I don’t know the arguments needed in the callback.

The function that calls this could want to send an number of arguments to the callback (normally it’d just wait for a return in sync) and that’s what’s confusing me.

function isAuthorized(socket, callback, args){
    socket.get('userID', function (err, userID) {
        //check userID for permission
        callback(args);
    });
}

I realize I could convert the arguments into an array and send that array, but I wanted to know if there is a more general way to do this that doesn’t force me to make all my callbacks convert from an array of arguments.

Hope you can help,
Pluckerpluck

Answer 1

You can always create a function to pull those arguments into an array for you, and then reuse it in each of your async functions:

function asyncWithCallbackArgs(obj, callback) {
  var args = Array.prototype.slice.call(arguments, 2);
  callback.apply(obj, args);
}

This will then enable you to do things like this:

function callback (c, d, e, f) {
  console.log([c, d, e, f]);
}

asyncWithCallbackArgs({}, callback,'c','d','e','f');​

There’s a fiddle here to play with.