4chan Ø-script

// ==UserScript==
// @name        4chan-ø
// @namespace   4chan-ø
// @version     1.3
// @grant       none
// @match       http*://boards.4chan.org/int*
// @include     http*://boards.4chan.org/int*
// ==/UserScript==

var isFirefox = navigator.userAgent.toLowerCase().indexOf('firefox') > -1;

window.onload = function() {
    window.onkeyup = function(e) {
        if (e.target.tagName.toLowerCase() === 'textarea') {
            if (isFirefox) {
                if (e.key.toLowerCase() !== 'ø') {
                    return;
                }
            } else {
                if (e.keyCode !== 186 && e.keyCode !== 192) {
                    return;
                }
            }

            var elem = e.target;

            var pos = elem.selectionStart;

            elem.value = elem.value
                .replace(/Ø/g, 'Ǿ')
                .replace(/ø/g, 'ǿ');

            elem.setSelectionRange(pos, pos);
        }
    };
};