billboard/billboard.js

function genRandomText(length) {
  var text = "";
  var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 .?-!(),\"'";

  for (var i = 0; i < length; i++)
    text += possible.charAt(Math.floor(Math.random() * possible.length));

  return text;
}

function genRandomInt(min, max) {
  // inclusive min & max
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

function shiftArrayToRight(arr, places) {
  for (var i = 0; i < places; i++) {
    arr.unshift(arr.pop());
  }
}

function textFillsScreen() {
  var root = document.compatMode == 'BackCompat' ? document.body : document.documentElement;
  return root.scrollHeight > root.clientHeight;
}

function fillScreenWithWhitespace() {
  var text = document.getElementById("text");
  var charArray = ["&nbsp"];
  var whitespace = "&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;";
  while(!textFillsScreen()) {
    text.innerHTML += whitespace;
    charArray.push.apply(charArray, ["&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;",
                                     "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;"]);
  }
  for (var i = 0; i < 26; i++) {
    text.innerHTML += whitespace;
    charArray.push.apply(charArray, ["&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;",
                                     "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;", "&nbsp;"]);
  }
  return charArray;
}

window.onload = function () {
  var textElem = document.getElementById('text');
  var charArray = fillScreenWithWhitespace();
  window.setInterval(function () {
    for (var i = 0; i < genRandomInt(1, 4); i++) {
      charArray[genRandomInt(0, charArray.length - 1)] = genRandomText(1);
      textElem.innerHTML = charArray.join("");
    }
    shiftArrayToRight(charArray, 2);
  }, 25)
}