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Hi, what about using isNaN instead of keys and keys.indexOf?

(function() {
    'use strict';

    (window.opera ? document.body : document).addEventListener('keydown', function(e) {
        if (isNaN(e.key) || e.isComposing || e.ctrlKey || e.altKey) return;
        e.cancelBubble = true;
        e.stopImmediatePropagation();
        return false;
    }, !window.opera);
})();

I realised it also needs a e.key === ' ' condition

if (e.key === ' ' || isNaN(e.key) || e.isComposing || e.ctrlKey || e.altKey) return;`

The source code is based on a very old stackoverflow post

• No need to detect window.opera any more.
• return false in a event listener callback is a totally wrong concept. No effect on it.
• Setting cancelBubble is already depreciated.

Obviously this script is just created by newbie.

(function () {
    const keys = new Set('0123456789'.split(''));
    // keys.add(' '); // You can add other keys. Check in https://www.toptal.com/developers/keycode
    document.addEventListener('keydown', function (e) {
        if (keys.has(e.key) || e.isComposing || e.ctrlKey || e.altKey) return;
        e.stopPropagation();
        e.stopImmediatePropagation();
    }, true);
})();

(function () {
    const keys = new Set('0123456789'.split(''));
    // keys.add(' '); // You can add other keys. Check in https://www.toptal.com/developers/keycode
    document.addEventListener('keydown', function (e) {
        if (keys.has(e.key) || e.isComposing || e.ctrlKey || e.altKey) return;
        e.stopPropagation();
        e.stopImmediatePropagation();
    }, true);
})();

Why not const keys = '0123456789' and keys.includes(e.key) instead?

(function () {
const keys = new Set('0123456789'.split(''));
// keys.add(' '); // You can add other keys. Check in https://www.toptal.com/developers/keycode
document.addEventListener('keydown', function (e) {
if (keys.has(e.key) || e.isComposing || e.ctrlKey || e.altKey) return;
e.stopPropagation();
e.stopImmediatePropagation();
}, true);
})();
Why not const keys = '0123456789' and keys.includes(e.key) instead?

Yes it can.

But if you want to handle other keys as well, Set is a better choice.

If you want to test more keys, the keys string would be long and searching would be time consuming.

Set is O(1) which is fast.

For "0123456789" case, they should have no difference.