Build API server for checking the operation of front implementation with python3 and Flask

>$ python3 --version
Python 3.7.7

$ python3 -m pip install Flask
Collecting Flask
  Downloading Flask-1.1.2-py2.py3-none-any.whl (94 kB)
     |████████████████████████████████| 94 kB 792 kB/s 
Collecting Jinja2>=2.10.1
  Downloading Jinja2-2.11.2-py2.py3-none-any.whl (125 kB)
     |████████████████████████████████| 125 kB 6.5 MB/s 
Collecting Werkzeug>=0.15
  Downloading Werkzeug-1.0.1-py2.py3-none-any.whl (298 kB)
     |████████████████████████████████| 298 kB 3.8 MB/s 
Collecting itsdangerous>=0.24
  Downloading itsdangerous-1.1.0-py2.py3-none-any.whl (16 kB)
Collecting click>=5.1
  Downloading click-7.1.2-py2.py3-none-any.whl (82 kB)
     |████████████████████████████████| 82 kB 778 kB/s 
Collecting MarkupSafe>=0.23
  Downloading MarkupSafe-1.1.1-cp37-cp37m-macosx_10_6_intel.whl (18 kB)
Installing collected packages: MarkupSafe, Jinja2, Werkzeug, itsdangerous, click, Flask
Successfully installed Flask-1.1.2 Jinja2-2.11.2 MarkupSafe-1.1.1 Werkzeug-1.0.1 click-7.1.2 itsdangerous-1.1.0

>$ python3
>>> import flask
>>> flask.__version__
'1.1.2'

from flask import Flask
from flask import jsonify

app = Flask(__name__)

@app.route('/')
def hello():
  return jsonify({"message": "Hello"})

@app.route('/good')
def good():
  return jsonify({"message": "Good"})

if __name__ == "__main__":
    app.run(debug=True)

$ python3 hello.py

    //let url = 'http://127.0.0.1:5000/';
    let url = 'http://127.0.0.1:5000/good';
    let req = new XMLHttpRequest();
    req.open('GET', url);
    req.onreadystatechange = function() {
      if (req.readyState == 4 && req.status == 200) {
        console.log("XMLHttpRequest");
        //let data = eval('(' + req.responseText + ')');
        console.log(req.responseText);
        alert(JSON.parse(req.responseText).message);
      }
    };
    req.send(null); 

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