Keyboard events: keydown filtering example can't block IME-composed input (Korean/Japanese/Chinese)

[Violet-Bora-Lee]

Article

Keyboard: keydown and keyup — the "Default actions" section, phone-number filter example:

<input onkeydown="return checkPhoneKey(event.key)" placeholder="Phone, please" type="tel">

Problem

The example (and the surrounding text) implies that returning false from keydown reliably prevents unwanted characters from being entered. That holds for direct (Latin) keyboard input, but not for languages typed through an IME (Input Method Editor), such as Korean, Japanese and Chinese.

With an IME active:

• keydown fires with event.key == "Process" (and keyCode 229), not the actual character.
• More importantly, calling preventDefault() / returning false on that keydown does not cancel the text insertion, because the character is produced by the IME composition, not by the key's default action.

As a result, Korean/Japanese/Chinese characters pass straight through the filter and appear in the field. This was reported by a Korean reader of the Korean translation (ko.javascript.info), and is easy to reproduce by switching to a Korean IME and typing into the demo.

Suggestion

Add a short note after the example, along these lines:

Please note: this filter only works for direct keyboard input. Text composed via an IME (used for languages such as Korean, Japanese or Chinese) is inserted by the input method, not by the key's default action, so it can't be blocked in keydown. That's one more reason why, for tracking or validating what actually gets into the field, the input event is a better choice.

This ties in nicely with the introduction of the article, which already recommends the input event for tracking input from any source.

Happy to send a PR if the note sounds good.

[iliakan]

Hello @Violet-Bora-Lee!

Good point — that would indeed be the right change.