javascript - 在 javascript/jquery 中传递可选参数

Question

JQuery .load() 定义如下:

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

data 和 complete() 参数是可选的。正确的？但是，我们如何调用它:

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

jquery 如何忽略第二个参数并知道我们提供了第一个和第三个参数？如果 3d 参数不是函数怎么办？

Answer 1

参见 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