node.js TypeError: path must be absolute or specify root to res.sendFile [failed to parse JSON]

The error is pretty clear, you need to specify an absolute (instead of relative) path and/or set root in the config object for res.sendFile(). Examples:

// assuming index.html is in the same directory as this script

res.sendFile(__dirname + '/index.html');

or specify a root (which is used as the base path for the first argument to res.sendFile():

res.sendFile('index.html', { root: __dirname });

Specifying the root path is more useful when you're passing a user-generated file path which could potentially contain malformed/malicious parts like .. (e.g. ../../../../../../etc/passwd). Setting the root path prevents such malicious paths from being used to access files outside of that base path.

Try adding root path.

app.get('/', function(req, res) {
    res.sendFile('index.html', { root: __dirname });
});

in .mjs files we for now don't have __dirname

hence

res.sendFile('index.html', { root: '.' })