Question

147

How to get all arguments of a callback function

rated 0 times [ 148] [ 1] / answers: 1 / hits: 27590 / 12 Years ago, thu, march 22, 2012, 12:00:00

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)

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kinsley

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answered 12 Years ago adrianobeds · Accepted Answer

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);

};

The context, this is preserved.

All arguments are correctly passed.

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.