JavaScript Redo & Undo for Textarea Simple Example

/*
 @ Created By Mamdouh Freelancer
 @ Feel free to contact me any time
 # https://fb.me/Mamdouh.Freelancer
 # https://stackoverflow.com/users/7040620/mamdouh-freelancer
*/
    function RedoUndo(a) {
      if (!a) console.error("Add textarea using new RedoUndo(textareaElement)");
      var curr = 0;
      this.redoButton = function(b) {
        if (!b) console.error("Add Redo Button using .redoButton(buttonElement)");
        b.addEventListener("click", redo, false);
      }
      this.undoButton = function(b) {
        if (!b) console.error("Add Undo Button using .undoButton(buttonElement)");
        b.addEventListener("click", undo, false);
      }
      var inputs = [""];
      if (a.className.indexOf("RedoUndo") > -1) a.className += " RedoUndo";
      a.addEventListener("keyup", push2arr, false);
      a.addEventListener("change", push2arr, false);

      function redo() {
        if (curr < inputs.length - 1 && curr >= 0) {
          curr++;
          a.value = inputs[curr];
        }
      }

      function undo() {
        if (curr < inputs.length && curr > 0) {
          curr--;
          a.value = inputs[curr];
        }
      }

      function push2arr() {
        if (curr < inputs.length - 1) {
          inputs.splice(curr + 1, inputs.length);
        }
        console.log(curr + " - " + inputs.length);
        if (inputs[inputs.length - 1] != this.value) {
          curr++;
          inputs.push(this.value);
        }
      }
    }
    //usage
    var trgt = document.getElementById("trgt");
    var redo = document.getElementById("red");
    var undo = document.getElementById("und");
    var ru = new RedoUndo(trgt);
    ru.redoButton(redo);
    ru.undoButton(und);