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?
See the source code.
load: function( url, params, callback ) {
if (typeof url !== "string" && _load) {
// My comment: the function.apply ia an
// alternative way to call a javascript function,
// where the number of parameter in `arguments`
// couldn't be determined
// the `arguments` is a special javascript variable.
// It's an array containing the parameters within
// the context of a function
return _load.apply(this, arguments);
// Don't do a request if no elements are being requested
} else if ( !this.length ) {
return this;
}
var off = url.indexOf( " " );
if ( off >= 0 ) {
var selector = url.slice( off, url.length );
url = url.slice( 0, off );
}
// Default to a GET request
var type = "GET";
// If the second parameter was provided
if ( params ) {
// If it's a function
if ( jQuery.isFunction( params ) ) {
// We assume that it's the callback
callback = params;
params = undefined;
// Otherwise, build a param string
} else if ( typeof params === "object" ) {
params = jQuery.param( params, jQuery.ajaxSettings.traditional );
type = "POST";
}
}
.... other code
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";
Let's see the source code, so you will see if the second parameter is a function, then pass the second parameter params to the third parameter callback and set the second parameter params to undefined.
// If the second parameter was provided
if ( params ) {
// If it's a function
if ( jQuery.isFunction( params ) ) {
// We assume that it's the callback
callback = params;
params = undefined;
// Otherwise, build a param string
} else if ( typeof params === "object" ) {
params = jQuery.param( params, jQuery.ajaxSettings.traditional );
type = "POST";
}
}
dataparameter should not be a function, and inspects the type of the argument. If it is a function, then it assumes that it must be the completion handler, because that is the only parameter that should be a function.