How to write text to a text file by Photoshop JavaScript?

This works for me, saves text with the same name as original document, but with extension txt:

function saveTxt(txt)
{
    var Name = app.activeDocument.name.replace(/\.[^\.]+$/, '');
    var Ext = decodeURI(app.activeDocument.name).replace(/^.*\./,'');
    if (Ext.toLowerCase() != 'psd')
        return;

    var Path = app.activeDocument.path;
    var saveFile = File(Path + "/" + Name +".txt");

    if(saveFile.exists)
        saveFile.remove();

    saveFile.encoding = "UTF8";
    saveFile.open("e", "TEXT", "????");
    saveFile.writeln(txt);
    saveFile.close();
}

I don't know how it works, photoshop scripting is a huge mess, I just kept mixing together a few scripts that I found until it worked.

Also, if anyone needs this, here is a script that also saves active document as png image:

function savePng()
{
    var Name = app.activeDocument.name.replace(/\.[^\.]+$/, '');
    var Ext = decodeURI(app.activeDocument.name).replace(/^.*\./,'');
    if (Ext.toLowerCase() != 'psd')
        return;

    var Path = app.activeDocument.path;
    var saveFile = File(Path + "/" + Name +".png");

    if(saveFile.exists)
        saveFile.remove();

    var o = new ExportOptionsSaveForWeb();
        o.format = SaveDocumentType.PNG;
        o.PNG8 = false;
        o.transparency = true;
        o.interlaced = false;
        o.includeProfile = false;
    activeDocument.exportDocument(saveFile, ExportType.SAVEFORWEB, o);
}

I have read the docs and combined best parts of psycho brm's and corrin_m's answer.
MY ANSWER ALSO CHECKS FOR ERRORS.

It is not necessary to delete file if it exists because opening with "w" will overwrite existing file it.

/* =======================================================
 * Saves file as text. Overwrites old file if exists.
 * Returns empty string if no errors, otherwise error message.
 * =======================================================*/
function saveAsTextFile(filePath, content) {
    var saveFile = new File(filePath);

    saveFile.encoding = "UTF8";
    saveFile.open("w");
    if (saveFile.error != "")
        return saveFile.error;

    saveFile.write(content);
    if (saveFile.error != "")
        return saveFile.error;

    saveFile.close();
    if (saveFile.error != "")
        return saveFile.error;

    return "";
}

This is how I am using the function in my scripts

error = saveAsTextFile(filePath, content);
if (error === "") {
  alert(filePath + " saved OK.");
}
else {
  alert("Error saving " + filePath + "\n" + error);
}

BTW I am keeping this in separate file called common-code.jsx and I can include it with following line (single line comments are intentional).

// @include 'common-code.jsx'

File system access is documented in Adobe's JavaScript Tools Guide (PDF).

Download the PDF file and check out the "File System Access" section.