Javascript/JQuery arguments by type

Question

I'm not quite sure if the title is correct because I'm not sure how to describe my question, but basically I'm wondering how jQuery can handle functions that take things like ("Some String", true, function(){}) and ("Some String", function() {}). IE, it seems like it's an overloaded function, I'd expect the function to be something like

function DoSomething(theStr, theBool, theFunc) {
    //...
}

but that doesn't explain how it can handle the 2 argument call, at least to me. Anyone able to explain, or is it just a lot of if/else.

Nick Craver · Accepted Answer · 2010-11-29 17:08:28Z

3

jQuery does type checking or arguments internally, shifting parameters (like a callback, typically at the end) forward if there are no arguments in-between. Let's take for example $.get() where the data argument is optional, the signature looks like this:

jQuery.get(url, [data], [callback(data, textStatus, XMLHttpRequest)], [dataType])

And the check looks like:

    if ( jQuery.isFunction( data ) ) {
        type = type || callback;
        callback = data;
        data = null;
    }

jQuery.isFuncton() is just a repeatedly used shortcut for jQuery.type(obj) === "function" which is really using Object.toString(), but this used to be done with typeof() directly.

You can see the full source from jQuery 1.4.4 here.

edited Nov 29, 2010 at 17:08

answered Nov 29, 2010 at 17:03

Nick Craver

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Comments

Jani Hartikainen · Accepted Answer · 2010-11-29 16:57:34Z

3

How about just opening the file containing the culprit code?

My guess is that it's either doing type-checking on the variables (using typeof or such), or it's using arguments.length to determine how many arguments were passed, and using that to choose from a list of predefined allowed parameters.

answered Nov 29, 2010 at 16:57

Jani Hartikainen

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Matt · Accepted Answer · 2010-11-29 17:14:36Z

1

JavaScript does not support method overloading through the use of different method signatures.

You can, in fact, pass 0 arguments to methods that list one or more, or pass more arguments than are listed.

All arguments passed to a method can be accessible through the keyword 'arguments' which behaves like an array.

That being said, you can then check for the presence of arguments directly or via the 'arguments' array.

function ABC(arg1, arg2, arg3){ 
    if(typeof(arg2) === 'undefined') { }
    if(typeof(arg3) === 'function'){ }

    // arguments[0] == arg1, etc
}

ABC(1) ; // arg2 and arg3 are undefined
ABC(1,2,3,4); // arg1, arg2, and arg3 are defined, plus arguments[3] === 4

Knowing the above, you can therefore figure out how many arguments were provided, and use typeof() calls to determine what type.

edited Nov 29, 2010 at 17:14

answered Nov 29, 2010 at 17:05

Matt

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3 Comments

Tim Down Over a year ago

I assume you mean typeof arg2 == 'undefined' rather than typeof(arg2 === 'undefined'), which will always give you "boolean"

Matt Over a year ago

Actually, I meant typeof(arg2) === 'undefined'. Thanks for pointing out the error!

Tim Down Over a year ago

No problem. Your code is now correct, but I just want to point out that there's no need for the parentheses: typeof is an operator, not a function.

Álvaro González · Accepted Answer · 2010-11-29 17:18:07Z

In JavaScript, the argument list in a function definition doesn't force you to call the function with exactly those arguments:

// Given this definition....
function foo(one, two){
}

// ... all these calls are valid:
foo();
foo(1);
foo(1, 2);
foo(1, 2, 3);

And, of course, JavaScript is loosely typed as well:

foo(1, 2);
foo("Hello", "World");
foo(new Date, {foo: "bar"});

Using these two concepts, the language allows you to overload methods to your will:

function foo(){
    var info = "Argument list:\n";
    for(var i=0, len=arguments.length; i<len; i++){
        info += "- Argument #" + (i+1) + " is a " + typeof(arguments[i]) + "\n";
    }
    alert(info);
}

foo(1, "1", {}, [], function(){});

Gives:

Argument list:
- Argument #1 is a number
- Argument #2 is a string
- Argument #3 is a object
- Argument #4 is a object
- Argument #5 is a function

Allan Nienhuis · Accepted Answer · 2010-11-29 17:02:09Z

0

+1 for Jani's answer. Just check the type of each parameter, and adjust your logic accordingly.

answered Nov 29, 2010 at 17:02

Allan Nienhuis

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