function tw (instance) {
  // (A) SET DEFAULT OPTIONS
  if (instance.forward === undefined) { instance.forward = 100; }
  if (instance.backward === undefined) { instance.backward = 50; }
  if (instance.pause === undefined) { instance.pause = 1000; }
  if (instance.loop === undefined) { instance.loop = true; }
  if (instance.cursor === undefined) { instance.cursor = true; }
  if (typeof instance.text != "object") { instance.text = [instance.text]; }

  // (B) PROPERTIES & FLAGS
  instance.current = 0;      // current block of text
  instance.char = 0;         // current character
  instance.direction = true; // true forward, false backward
  instance.draw = true;      // continue to "type text"?

  // (C) ATTACH FAKE CURSOR
  if (instance.cursor) { instance.target.classList.add("cursor"); }

  // (D) TYPEWRITER EFFECT
  instance.typist = () => {
    // (D1) NEXT CHARACTER
    if (instance.direction) {
      instance.char++;
      instance.draw = instance.char <= instance.text[instance.current].length;
    } else {
      instance.char--;
      instance.draw = instance.char >= 0;
    }

    // (D2) DRAW HTML
    if (instance.draw) {
      instance.target.innerHTML = instance.char==0 ? "&nbsp;" : instance.text[instance.current].substring(0, instance.char);
    }

    // (D3) PAUSE & LOOP?
    else {
      // (D3-1) CLEAR TIMER + REVERSE DIRECTION
      clearInterval(instance.timer);
      instance.direction = !instance.direction;

      // (D3-2) NEXT BLOCK OF TEXT
      if (instance.direction) {
        instance.current++;
        if (instance.loop && instance.current == instance.text.length) {
          instance.current = 0;
        }

        if (instance.current <= instance.text.length) {
          instance.timer = setTimeout(() => {
            instance.timer = setInterval(instance.typist, instance.forward);
          }, instance.pause);
        }
      }

      // (D3-3) PAUSE THEN CLEAR TEXT
      else {
        instance.timer = setTimeout(() => {
          instance.timer = setInterval(instance.typist, instance.backward);
        }, instance.pause);
      }
    }
  };

  // (E) START
  instance.timer = setInterval(instance.typist, instance.forward);
}

// (F) ATTACH TYPEWRITER & THEME SWITCHER
window.onload = () => {
  // (F1) GET DEMO <DIV>
  var demo = document.getElementById("demo");

  // (F2) THEME SWITCHER
  document.getElementById("theme").onchange = function () {
    let cur = demo.classList.contains("cursor");
    demo.className = this.value;
    if (cur) { demo.classList.add("cursor"); }
  };

  // (F3) ATTACH TYPEWRITER
  tw({
    // (F3-1) REQUIRED
    target : demo,
    text : [
      "Wow. Much text. Very paragraph. Such contents.",
      "Foo! It works!",
      "Another random paragraph here."
    ],
   
    // (F3-2) OPTIONAL
    forward : 80,  // delay between each character, default 100 ms
    backward : 40, // delay between each character, default 50 ms
    pause : 1500,  // pause before next block of text, default 1 sec
    loop : true,   // loop text blocks, default true
    cursor : true  // add fake cursor, default true
  });
};