XHR responseText 是空字符串

2022-01-15 00:00:00 google-app-engine webapp2 xmlhttprequest javascript

HTML页面:

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>xhr</title>
</head>
<body>
    <script>
        var xhr_test = new XMLHttpRequest();
        xhr_test.open("GET","xhrtest",true);
        xhr_test.send();
        alert(xhr_test.responseText);
    </script>
</body>
</html>

main.py 文件:

The main.py file:

import webapp2
from handlers import cookies,pages
application = webapp2.WSGIApplication([
        ('/xhr',pages.XHR),
        ('/xhrtest', cookies.XHRTest)
        ],
            debug=True)

请求处理程序:

class XHRTest(webapp2.RequestHandler):
    def get(self):
        self.response.write('0')

和，

class XHR(webapp2.RequestHandler):
    def get(self):
        f = open('static/html/xhr.html','r')
        self.response.write(f.read())

现在，当我点击 URL localhost:8080/xhrtest 时，浏览器会立即将响应 0 显示为页面内容.

Now, when I hit upon the url localhost:8080/xhrtest the browser promptly shows the response 0 as the page's content.

点击url localhost:8080/xhr间接点击/xhrtest，在alert框中弹出一个空字符串(responseText是一个空字符串)但是检查在network标签下chrome的response标签，可以看到请求的响应是0.

Hitting the url localhost:8080/xhr which indirectly hits /xhrtest, pops up an empty string in the alert box (the responseText is an empty string) but checking chrome's response tab under the network tab, I can see that the request's response is 0.

那么为什么 xhr_test.responseText 不能显示相同的响应?

So why is xhr_test.responseText not able to display the same response?

推荐答案

send 调用是异步的(您已将 'async' 参数设置为 true)，这意味着您的警报会在请求完成之前立即发生.您应该向 xhr.onreadystatechange 添加一个事件侦听器并在其中使用响应.

The call to send is asynchronous (you've set the 'async' parameter to true), which means that your alert is happening immediately, before the request finishes. You should add an event-listener to xhr.onreadystatechange and use the response within that.

将true"更改为false"可以使这个简单的示例工作，但在一般情况下不是一个好主意.

Changing the 'true' to 'false' would make this simple example work, but is not a good idea in the general case.

Ajax 上的 MDN 页面解释了如何使用 XMLHttpRequest.

The MDN page on Ajax explains how XMLHttpRequest should be used.

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