XHR responseText is empty string

Html Page:

<!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>

The main.py file:

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

The Request handlers:

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

and,

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

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

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.

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

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.

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

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

来源：https://stackoverflow.com/questions/19549269/xhr-responsetext-is-empty-string

标签

javascript

google-app-engine

xmlhttprequest

webapp2