post request body is undefined if the post request is from a static file

Hi, I am not sure whether this is an issue, details as follows, the following code always fails the authentication (i.e., 'Verification function is called' is never shown in console):

const express = require('express')
const path = require('path')
const passport = require('passport')
const Strategy = require('passport-local').Strategy
const port = 30000

passport.use(new Strategy(
  function(username, password, cb) {
        console.log('Verification function is called');
        return cb(null, {username, id: '1'});
  }
));

var app = express();
app.use(passport.initialize());
app.use(passport.session());
app.get('/', (req, res) => res.send('Hello world!'));
app.get('/login.html', (req, res) => res.sendFile(path.join(__dirname, '/login.html')));
app.post('/login', passport.authenticate('local', { successRedirect: '/', failureRedirect: '/login.html' }));

app.listen(port, () => console.log(`Example app listening on port ${port}`));

here is the static file 'login.html':

<html>
        <head>
                <title>login</title>
        </head>
        <body>
                <form action="/login" method="post">
                            <div>
                                            <label>Username:</label>
                                            <input type="text" name="username"/>
                            </div>
                            <div>
                                            <label>Password:</label>
                                            <input type="password" name="password"/>
                            </div>
                            <div>
                                            <input type="submit" value="Log In"/>
                            </div>
                </form>
        </body>
</html>

After debug, I found the problem is in Strategy.prototype.authenticate, passport-local/lib/strategy.js, specially, this line:

return this.fail({ message: options.badRequestMessage || 'Missing credentials' }, 400);

And I add a log before this return such as:

console.log('here!!! body:' + req.body + ' query: ' + req.query + ' usernameField: ' + this._usernameField + ' passwordField:' + this._passwordField);

and here is the result:

here!!! body:undefined query: [object Object] usernameField: username passwordField:password

But actually, from fiddler, the request body is not null:

POST http://xxx:30000/login HTTP/1.1
Host: xxx:30000
Connection: keep-alive
Content-Length: 25
Cache-Control: max-age=0
Origin: http://xxx:30000
Upgrade-Insecure-Requests: 1
Content-Type: application/x-www-form-urlencoded
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.132 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Referer: http://xxx:30000/login.html
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9,ar;q=0.8,zh-CN;q=0.7,zh;q=0.6
Cookie: connect.sid=s%3AU9HkxMCRQJutHorlDOveMi91T8CngKVs.MNuCBvMhY7KAb%2Fmvn0oMbga8GFfYWrMeUeL8Bu1RMiw

username=adf&password=adf

Looks the request body is never populated to the passport library. Could you please take a look? Thanks!

passport / www.passportjs.org

post request body is undefined if the post request is from a static file #92