var buffer = {
  content: [],
  add: function(el) {
    this.counter = this.current = this.content.push(el)-1;
  },
  undo: function() {
    if(this.current>0) this.current-= 1;
    return this.content[this.current];
  },
  redo: function() {
    if(this.current<this.counter) this.current+= 1;
    return this.content[this.current];
  },
  counter: 0,
  current: 0
}

window.onload = (function() {
  document.getElementById("content").onkeydown = function(e) {
    if(this.value!=buffer.content[buffer.current]) buffer.add(this.value);
    if (e.keyCode == 90 && e.ctrlKey) { //ctrl+z
      e.preventDefault();
      this.value = buffer.undo();
    }
    if (e.keyCode == 89 && e.ctrlKey) { //ctrl+y
      e.preventDefault();
      this.value = buffer.redo();
    }
  }
});