Question

I like to write my callback functions in a private namespace and then call them by reference. eg

var myCallback = function() {

};

// ... somewhere else in the class.
SomeOtherObject.doSomething(myCallback);

This makes the code neater and more extendable. However when I need to pass any parameters to the callback from the original function it becomes messy and I have to enclose the callback reference in a closure. eg

var myCallback = function(a_var, another_var) {

};

// ... somewhere else in the class.
a_var = "something";
SomeOtherObject.doSomething(function(another_var) {
    myCallback(a_var, another_var);
};

This results in the callback requiring two function calls instead of one, creating overhead. It also means I have an undocumented anonymous function for the closure – or I document both functions. (It’s not so much the missing function definition which is annoying as the parameter definitions.) If the callback is small – as good functions are – then it makes it seem not worthwhile to put it the private scope.

What is the best way to do this?

Answer 1

Partial application implementations allow you to pass variables in this way.

For example, using native ES5’s bind:

SomeOtherObject.doSomething( myCallback.bind(null, a_var) );

another_var will be passed to that function when called

It’s not supported everywhere though, so you can grab the one from underscore and use that instead.