Web-based text editor (#2)
You can optimize some queries.

var selected_file = document.getElementById("selected_file")


Unless your HTML changes considerably (replacing nodes periodically), every call to getElementById() will return the same node. You can access the node more efficiently if you cache the node in a local variable, and use the same local variable throughout your code, instead of looking it up multiple times.

Same with:

// Help
var help = document.getElementById("help_content"),
    overlay = document.getElementById("overlay");


As long as all these elements exist when your code initializes, you can lookup and store them in variables once, instead of each time Help() (etc) is called. In your code, the performance increase will be very minor, but it can add up in other situations.



In your keydown block:

if (key === 79) {
...
if (key === 83) {
...

// function rename()
(filename.lastIndexOf(".txt") === -1)


You can shave a few bytes by using == instead of ===. Since you're comparing primitives, not objects, == and === function equivalently.

Using === (instead of ==) is a good practice when you're not sure, but it is unnecessary in the places you are using it.



It looks like you didn't update your last check in the keydown callback

if (e.keyCode === 9 || e.which === 9) { // tab


Can be rewritten as:

if (key == 9) { // tab




function confirmNav() { // Confirm navigation
  if (isModified) {
      var a = window.confirm("You have unsaved changes that will be lost.");
      if (a) {
          isModified = false;
      ...




While nothing is technically wrong here, I'd either like to see the confirm inlined into the if() or a more descriptive variable name. a is meaningless.

 if (window.confirm("You have unsaved changes that will be lost.")) {
 // or 
 var leavePage = window.confirm("You have unsaved changes that will be lost.");
    if (leavePage) {




Your code only has three long lines. The longest, at 136 chars:

var s = textarea.selectionStart
textarea.value = textarea.value.substring(0, textarea.selectionStart) + "\t" + textarea.value.substring(textarea.selectionEnd);
textarea.selectionEnd = s + 1;


I compacted it be using the local you already declared, and by adding a second. Now 97 chars:

var sStart = textarea.selectionStart,
    txt = textarea.value;
textarea.value = txt.substring(0, sStart) + "\t" + txt.substring(textarea.selectionEnd);


I don't think s was a bad variable name (s for selection or start is meaningful), but sStart is more clear.



You wrote some lovely, readable, and coherent JavaScript.

Here is everything:

var appname = " - simple notepad",
    textarea = document.getElementById("textarea"),
    help = document.getElementById("help_content"),
    overlay = document.getElementById("overlay"),
    selected_file = document.getElementById("selected_file"),
    prnt_helper = document.getElementById("prnt_helper"),
    untitled = "untitled.txt" + appname,
    filename = "*.txt",
    isModified;

document.title = untitled;

textarea.onpaste = textarea.onkeypress = function() {
    isModified = true;
};

function confirmNav() { // Confirm navigation
    if (isModified) {
        var leavePage = window.confirm("You have unsaved changes that will be lost.");
        if (leavePage) {
            isModified = false;
        }
    }
}

function New() { // New
    confirmNav();
    if (!isModified) {
        textarea.value = "";
        document.title = untitled;
    }
    textarea.focus();
}

function Open() { // Open
    confirmNav();
    if (!isModified) {
        selected_file.click();
    }
}

function loadFileAsText() { // load file as text
    var file = selected_file.files[0],
        fileReader = new FileReader();
    fileReader.onloadend = function(e) {
        if (e.target.readyState == FileReader.DONE) {
            filename = file.name;
            document.title = filename + appname;
            textarea.value = e.target.result;
            textarea.focus();
        }
    };
    fileReader.readAsText(file);
}

function rename() { // Rename
    filename = window.prompt("Name this note:", filename);
    if (filename) {
        filename = (filename.lastIndexOf(".txt") == -1) ? filename + ".txt" : filename;
        document.title = filename + appname;
    }
}

function Save() { // Save
    rename();
    if (filename) {
        var blob = new Blob([textarea.value], {
            type: "text/plain;charset=utf-8"
        });
        window.saveAs(blob, filename);
        isModified = false;
    }
    textarea.focus();
}

function Print() { // Print
    prnt_helper.innerHTML = textarea.value;
    window.print();
    prnt_helper.innerHTML = "";
    textarea.focus();
}

function Help() { // Help
    function closeHelpAndOverlay() {
        help.setAttribute("aria-hidden", "true");
        overlay.setAttribute("aria-hidden", "true");
        textarea.focus();
    }

    if (help["aria-hidden"] == "true") {
        closeHelpAndOverlay();
    } else {
        help.setAttribute("aria-hidden", "false");
        overlay.setAttribute("aria-hidden", "false");
        textarea.blur();
        overlay.onclick = function() {
            closeHelpAndOverlay();
        };
        // document.onkeydown = function(e) { // esc to close help
        //     if (e.keyCode == 27 || e.which == 27) {
        //         closeHelpAndOverlay();
        //     }
        // };
    }
}

// Confirm close
window.onbeforeunload = function() {
    if (isModified) {
        return "You have unsaved changes that will be lost.";
    }
};

// Keyboard shortcuts
document.onkeydown = function(e) {
    var key = e.keyCode || e.which;
    if (e.ctrlKey) {
        if (e.altKey) {
            if (key == 78) {
                // Ctrl+Alt+N
                New();
            }
        }
        if (key == 79) {
            // Ctrl+O
            e.preventDefault();
            Open();
        }
        if (key == 83) {
            // Ctrl+S
            e.preventDefault();
            Save();
        }
        if (key == 80) {
            // Ctrl+P
            e.preventDefault();
            Print();
        }
        if (key == 191) {
            // Ctrl+/
            e.preventDefault();
            Help();
        }
    }
    if (key == 9) { // tab
        e.preventDefault();
        var sStart = textarea.selectionStart,
            txt = textarea.value;
        textarea.value = txt.substring(0, sStart) + "\t" + txt.substring(textarea.selectionEnd);
        textarea.selectionEnd = sStart + 1;
    }
};