Question

Very often, I find myself using a callback function and I don’t have its documentation handy, and it would be nice to see all of the arguments that are meant to be passed to that callback function.

// callback is a function that I don't know the args for...
// and lets say it was defined to be used like: callback(name, number, value)
something.doSomething( callback );

How can I determine what args its passing into that?

Note: looking at the source code can be unhelpful when the code itself is obfuscated and minified (as many js frameworks are)

Answer 1

To get the list of arguments without breaking functionality, overwrite the callback function in this way:

var original = callback;
callback = function() {
    // Do something with arguments:
    console.log(arguments);
    return original.apply(this, arguments);
};

1. The context, this is preserved.
2. All arguments are correctly passed.
3. The return value is correctly passed.

NOTE: This method works in most cases. Though there are edge cases where this method will fail, including:

• Read-only properties (e.g. defined using Object.defineProperty with writable:false)
• Properties that are defined using getters/setters, when the getter/setter is not symmetric.
• Host objects and plugin APIs: E.g. Flash and ActiveX.