从AJAX中的onreadystatechange事件返回值 [英] Returning values from the event onreadystatechange in AJAX

问题描述

我正在尝试在下面的代码中为变量val分配一个值:

I am trying to assign a value to the variable val in the code below:

var cmdValue = "/cmd/fetch?x=";
var val;
var returned_data;

function what(){
    val = update('#TAG#');
}


function update(tag) {
    var req1 = newXMLHttpRequest();
    req1.open("GET",cmdValue + tag, true);
    req1.send("");

    return req1.onreadystatechange= function () {
        if (req1.readyState == 4 && req1.status == 200) {
            returned_data = req1.responseText;
            return returned_data;
        }else{

        }
    };
}

我在Firebug中跟踪变量，结果证明val被分配了该功能.有没有办法让代码运行，然后将值分配给变量val?

I was tracking the variables in Firebug and it turns out that val gets assigned the function. Is there a way to get the code to run through and then assign the value to the variable val?

推荐答案

在异步编程中，您不要 return数据，因为您不知道何时该数据将变得可用-它是异步.

In asynchronous programming you do not return data because you don't know when that data is going to become available - it's asynchronous.

进行异步编程的方法是使用事件和/或回调.

The way to do asynchronous programming is using events and/or callbacks.

示例:

var cmdValue = "/cmd/fetch?x=";
var val;
var returned_data;
var performSomeAction = function(returned_data) {
    val = returned_data;
}

function what(){
    update('#TAG#',performSomeAction);
}



function update(tag,callback) {
    var req1 = newXMLHttpRequest();
    req1.open("GET",cmdValue + tag, true);
    req1.send("");

    req1.onreadystatechange= function () {
        if (req1.readyState == 4 && req1.status == 200) {
            returned_data = req1.responseText;
            //fire your callback function
            callback.apply(this,[returned_data]);
        }else{

        }
    };
}

这个问题是关于SO的最常见问题之一，至少在涉及javascript标签时-请在问自己的问题之前先搜索类似的问题.

This question is one of the most commonly asked questions on SO, at least when it comes to the javascript tag - please search for similar questions before asking your own.

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