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Detect if the caps lock is on

Written byPhuoc Nguyen
03 Mar, 2020
Level 2 — Intermediate
Assume that we want to let users know if the caps lock is on while they are entering the value for an input (a password field, for example):
<input type="text" id="textbox" />

<div id="message" />
The element with `id` of `message` will be used to show the message.

1. Use the getModifierState() function

The getModifierState() function returns the state of given modifier key. It can let us know if the `CapsLock` key is pressed by calling `getModifierState('CapsLock')` from the event object.
const textboxEle = document.getElementById('textbox');
const messageEle = document.getElementById('message');

textboxEle.addEventListener('keydown', function (e) {
const capsLockOn = e.getModifierState('CapsLock');

// Update the content of message
messageEle.innerHTML = capsLockOn ? 'Caps lock is ON' : '';

// Show or hide the message based on the CapsLock state
messageEle.style.display = capsLockOn ? 'block' : 'none';
This approach doesn't support the case that users press the Shift key.

2. Support the Shift key

The caps lock is treated as ON if users press
  • an uppercase letter without Shift
  • or an lowercase letter with Shift
const textboxEle = document.getElementById('textbox');
const messageEle = document.getElementById('message');

textboxEle.addEventListener('keypress', function (e) {
const isMac = /Mac/.test(navigator.platform);

const keyCode = e.keyCode || e.which;

// Is the _Shift_ key pressed?
const shiftKey = e.shiftKey || keyCode === 16;

// Get the pressed character
const s = String.fromCharCode(keyCode);
const capsLockOn =
(s.toUpperCase() === s && s.toLowerCase() !== s && !(shiftKey && isMac)) ||
(s.toUpperCase() !== s && s.toLowerCase() === s && shiftKey);

messageEle.innerHTML = capsLockOn ? 'Caps lock is ON' : '';
messageEle.style.display = capsLockOn ? 'block' : 'none';

See also

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