Question

document.onkeydown.toString() returns onkeydown function but I only want to get this function’s name.

Is it possible?

var exHandler;
function getCurrentHandler(){
   currentHandler = document.onkeydownhandler.toString() - i want to get name of this to call setPreviousHandler with exHandler

document.onkeydown = newFunction;

setPreviousHandler(exHandler);
}

function setPreviousHandler(targetHandler){
   document.onkeydown = targetHandler;
}

Answer 1

It depends on the way the function was assigned.
Take look at this:

function test() { alert('test'); }
window.onkeydown = test;
window.onkeydown.toString(); 

output is function test() { alert('test'); } and you can parse this string to get its name as string:

var fnsting = window.onkeydown.toString(); 
// use regexp to extract name
var name = fnsting.match( /function\s*(.*?)\s*\(/ )[1]; // name is 'test'

(it will allways returns name from function definition (when you assing this function to other variable .toString() always returns the same string)

but following example:

window.onkeydown = function () { alert('test'); };
window.onkeydown.toString(); 

will print function () { alert('test'); } because this function is anonymous, so you can’t get its name.

Moreover when you create function like this:

var test = function () { alert('test'); };
window.onkeydown = test;
window.onkeydown.toString(); 

and output will be function () { alert('test'); } because on creation function did not have name (it was function expression assigned to variable) so you also can not extract it’s name.

But you can make simple comparision:

function foo() { alert('foo'); }
var bar = function () { alert('bar'); };

window.onkeydown = foo;

// try to find function
if (window.onkeydown === foo) {
    alert('this is foo');
} else if (window.onkeydown === bar) {
    alert('this is bar');
}

// or like this:
var handler = window.onkeydown;
if (handler === foo) {
    alert('this is foo');
} else if (handler === bar) {
    alert('this is bar');
}

But there is also issue with this, because when you assign anonymous function like this:

window.onkeydown = function () { alert('test'); };

You don’t have any other reference to this function (it is not assigned to other variable) so you can’t compare it with other function (comparision always returns false).