Readline: Function keys (e.g. F1) return "undefined" key name

[elie-g]

• Version: v14.11.0
• Platform: Linux 5.4.0-42-generic // Ubuntu 20.04 // x64
• Subsystem: readline.emitKeypressEvents (exact place in code: here)

What steps will reproduce the bug?

1. Execute the code below in a terminal.
2. Press Alt+F3 (see Additionnal information).

const { StringDecoder } = require('string_decoder')
const { emitKeypressEvents } = require('readline')

const stdin = process.openStdin();
emitKeypressEvents(stdin);
if (stdin.isTTY) {
  console.log('Standard input is a TTY');
  stdin.setRawMode(true);
}

const strDecoder = new StringDecoder('utf8');

const escapeEsc = (str) => str?.replace(/\x1b/g, 'ESC');

stdin.on('keypress', (str, key) => {
  // key.name === "undefined" <<<<<<<<<<<<<<<<<<<<<<<<<<<<<< HERE
  console.log('KEYPRESS => data: «%o», key: %o', escapeEsc(str), key);
});

stdin.on('data', (data) => {
  const str = strDecoder.write(data);
  console.log('DATA => decoded: «%s», raw: %o', escapeEsc(str), data);
});

How often does it reproduce? Is there a required condition?

Every time!

What is the expected behavior?

key.name should contain the name of the function key (e.g. "f1").
See Additional information.

What do you see instead?

key.name contains "undefined" (as a string / not undefined).
See Additional information.

Output after pressing Alt + F3

Additional information

Tested key combinations (expected/current result)

Key combo	Expected key.name	Current key.name	Matches expectation?
Alt + F1	"f1"	???	???
Alt + F2	"f2"	???	???
Alt + F3	"f3"	"undefined"
Alt + F4	"f4"	???	???
Alt + F5	"f5"	"f5"
Alt + F6	"f6"	???	???
Alt + F7	"f7"	"f7"
Alt + F8	"f8"	???	???
Alt + F9	"f9"	"f9"
Alt + F10	"f10"	"f10"
Alt + F11	"f11"	???	???
Alt + F12	"f12"	"f12"
Alt + Shift + F1	"f1"	"undefined"
Alt + Shift + F2	"f2"	???	???
Alt + Shift + F3	"f3"	"undefined"
Alt + Shift + F4	"f4"	"undefined"
Alt + Shift + F5	"f5"	"f5"
Alt + Shift + F6	"f6"	???	???
Alt + Shift + F7	"f7"	"f7"
Alt + Shift + F8	"f8"	"f8"
Alt + Shift + F9	"f9"	"f9"
Alt + Shift + F10	"f10"	???	???
Alt + Shift + F11	"f11"	???	???
Alt + Shift + F12	"f12"	"f12"
Ctrl + F1	"f1"	"undefined"
Ctrl + F2	"f2"	???	???
Ctrl + F3	"f3"	"undefined"
Ctrl + F4	"f4"	"undefined"
Ctrl + F5	"f5"	"f5"
Ctrl + F6	"f6"	"f6"
Ctrl + F7	"f7"	"f7"
Ctrl + F8	"f8"	"f8"
Ctrl + F9	"f9"	"f9"
Ctrl + F10	"f10"	"f10"
Ctrl + F11	"11"	???	???
Ctrl + F12	"f12"	"f12"
Ctrl + Shift + F1	"f1"	"undefined"
Ctrl + Shift + F2	"f2"	???	???
Ctrl + Shift + F3	"f3"	"undefined"
Ctrl + Shift + F4	"f4"	"undefined"
Ctrl + Shift + F5	"f5"	"f5"
Ctrl + Shift + F6	"f6"	"f6"
Ctrl + Shift + F7	"f7"	"f7"
Ctrl + Shift + F8	"f8"	"f8"
Ctrl + Shift + F9	"f9"	"f9"
Ctrl + Shift + F10	"f10"	???	???
Ctrl + Shift + F11	"11"	???	???
Ctrl + Shift + F12	"f12"	"f12"

The key combinations where the 3^rd and 4^th columns contain ??? could not be tested cause they were already bound to something else like Alt + F4 which closes the application.