即使.readystate == 4，XMLHttpRequest.responseXML也是NULL

Question

我正在使用javascript从XML文件加载数据。在检查就绪状态和状态的if语句之后，文件未被加载。就绪状态返回4并且状态返回200，因此最后一个条件（responseXML）不应为null，但由于某种原因，它保持为null并且未加载XML文件。

from flask import request, jsonify, render_template
import sys

@app.route("/function_route", methods=["GET", "POST"])
def my_function():
    if request.method == "POST":
        data = {}    // empty dict to store data
        data['title'] = request.json['title']
        data['release_date'] = request.json['movie_release_date']

       // do whatever you want with the data here e.g look up in database or something
       // if you want to print to console

        print(data, file=sys.stderr)

        // then return something back to frontend on success

        // this returns back received data and you should see it in browser console
        // because of the console.log() in the script.
        return jsonify(data)
    else:
        return render_template('the_page_i_was_on.html')

这是javascript文件中与加载XML有关的代码。打开控制台会显示日志，告诉我代码没有通过检查asyncRequest的if语句。