Question

I’m currently trying to teach myself more about javascript, and I find myself stumbling over the syntax for passing a method as an argument to another method call. Say you have two functions like these:

    function FirstFunction()
    {
        DoesSomething();
    }


    function SecondFunction(func)
    {
        func();
    }

In actually passing the FirstFunction to the SecondFunction, I seem to see a wild variety of variations on doing so:

    SecondFunction(FirstFunction);

or

    SecondFunction("FirstFunction()");

or sometimes, if FirstFunction was defined as follows:

    var thisisafunction = function FirstFunction()
    {
        DoesSomething();
    }

    SecondFunction(thisisafunction);

I’m guessing there’s no “one right way” to do this, so when is it appropriate to use each way of passing a function? Is it better to use one way over another in a certain situation?

Answer 1

Passing a string is always bad. It only works with functions like setTimeout which automatically eval() strings and it’s bad since it runs in the global context.

If you just want to pass a plain function without creating a closure or passing arguments, passing the function without () is fine:

myFunc(someCallbackFunc);

If you need to pass arguments or want a closure so you can access some non-global variables inside that function, use an anonymous function:

myFunc(function() {
    someCallbackFunc('meow');
});

Of course you can also assign the function to a variable first:

var cb = function() {
    someCallbackFunc('meow');
};
myFunc(cb);

While var cb = function someName() {}; is valid, I’d recommend against giving your function expressions names – it’s not supported by all browsers. The only advantage is having a function name in debuggers anyway…