Question

The JQuery .load() definition follows:

.load( url [, data] [, complete(responseText, textStatus, XMLHttpRequest)] );

The data and complete() arguments are optional. Right? But then, how we can call this:

$('#result').load('ajax/test.html', function(){
    alert('Load was performed.');
}

How jquery ignores the 2nd argument and knows that we provided the 1st and the 3d argument? And what if the 3d argument is not a function?

Answer 1

If load is called with two arguments or more, jQuery checks to see if the second argument is a function or an object. If it’s a function it’s called when the ajax call is completed. Otherwise it’s used as the params passed with the ajax call.

Relevant parts from the source:

if ( params ) {
    // If it's a function
        if ( jQuery.isFunction( params ) ) {
        // We assume that it's the callback
        callback = params;
        params = null;
    // Otherwise, build a param string
    } else if ( typeof params === "object" ) {
        params = jQuery.param( params, jQuery.ajaxSettings.traditional );
        type = "POST";
    }
}

Where params is the second argument to load.

isFunction is the result of the following functions:

isFunction: function( obj ) {
    return jQuery.type(obj) === "function";
},
type: function( obj ) {
    return obj == null ?
        String( obj ) :
        class2type[ toString.call(obj) ] || "object";
},

where class2type is an assosiative array containing, among other things, this element:

class2type[ "[object Function]" ] = "function";