为什么用JavaScript中的arguments [arguments.length-1]表示回调函数？

Question

I had copied a piece of code from somehwere which basically was used to extend the JQuery's hide method and extend it to call a callback

But am not clear with how the method works ..

       js.executeScript("window.webdrivercallback = function(){};" +

            "var _oldhide = $.fn.hide;" +
            "$.fn.hide = function(speed, callback) {" +
            "    var retThis = _oldhide.apply(this,arguments);" +
            "    window.webdrivercallback.apply();" +
            "    return retThis;" +
            "};"
    );

additionally in selenium webdriver code we have to add a piece of code something like:

js.executeAsyncScript("window.webdrivercallback = arguments[arguments.length - 1];");

I am unable to understand how can we create a javascript function webdrivercallback by just passing arguments like arguments[arguments.length-1] .What is the significance of this statement? What is arguments and from where are values being passed to the arguments field as I couldn't find a single place from where values were passed to arguments field. Could someone help me understand this piece of code.

Answer 1

In this case, the last argument is a function, arguments is an array (well, very similar to an array) that contains all the parameters sent to the function. So, arguments[arguments.length-1] will always be the last parameter sent to the function.

For example, consider you have:

function someFunction(param1, param2, param3){
    //arguments would have here: [param1, param2, param3]
    //so arguments[arguments.length-1] is the last param: param3
    var callback = arguments[arguments.length-1];   //This is what looks strange to you
    alert(typeof callback); //alerts 'function' if last param is well.. a function;
    callback();  //Execute the callback as a proof!   
}

so, let's call it:

someFunction('a', 'b', function(){
    alert("callback executed!!");
});

As you can see, it alerts "function", because the last parameter was a function. So, it can be assigned as a callback without problems.

Also, don't forget that we can call a function with more or less parameters than it was defined, so arguments[arguments.length-1] is a trick for always getting the last one.

Hope it's clear.

Cheers, from La Paz, Bolivia