markdown_keyboard_shortcuts.user.js

// ==UserScript==
// @name        SO_like_keyboards_shortcuts
// @namespace   sputnick
// @include     https://github.com/*
// @include     https://gist.github.com/*
// @include     https://*.slack.com/*
// @version     1
// @grant       none
// ==/UserScript==

var id, name;
if (document.location.href.search(/github\.com/) > 0 ) {
    id = 'new_comment_field';
    name = 'github';
}
else if (document.location.href.search(/slack\.com/) > 0 ) {
    id = 'message-input';
    name = 'slack';
}

(function() {
    document.addEventListener('keydown', function(e){
        //window.alert(e.which);

        // ctrl+b: bold
        if (e.ctrlKey && e.which == 66) {
            override(e);
            if (name === 'slack') { wrapText(id, '*', '*'); }
            else if (name === 'github') { wrapText(id, '**', '**'); }
            return false;
        }
        // ctrl+i: emphasis
        if (e.ctrlKey && e.which == 73) {
            override(e);
            wrapText(id, '_', '_');
            return false;
        }
        // ctrl+k: code
        if (e.ctrlKey && e.which == 75) {
            override(e);
            wrapText(id, '`', '`');
            return false;
        }
        // ctrl+l: link
        if (name === 'github' && e.ctrlKey && e.which == 76) {
            override(e);
            var p = window.prompt("Link paste bellow");
            wrapText(id, '[', ']('+p+')');
            return false;
        }
    }, true);
})();

function wrapText(elementID, openTag, closeTag) {
    var textArea = $('#' + elementID);
    var len = textArea.val().length;
    var start = textArea[0].selectionStart;
    var end = textArea[0].selectionEnd;
    var selectedText = textArea.val().substring(start, end);
    if (openTag == '`' && selectedText.search(/\n/) > 0 ) {
        openTag = '```\n';
        closeTag = '\n```';
    }
    var replacement = openTag + selectedText + closeTag;
    textArea.val(textArea.val().substring(0, start) + replacement + textArea.val().substring(end, len));
}

function override(e) {
    if (document.activeElement.tagName == "TEXTAREA") {
        e.stopImmediatePropagation();
        e.cancelBubble = true;
        e.preventDefault();
    }
}