JavaScript for changing disabled to active button

form_active.js

(function(){
  document.addEventListener('DOMContentLoaded', function(event) {
    new ActiveForm().onEvent(event.target);
  }, false);
})();

function ActiveForm() {
  this.submit = document.getElementById('submit');
}

ActiveForm.prototype.init = function(target) {
  this.onEvent(target);
};
ActiveForm.prototype.onEvent = function(target) {
  var self = this;
  target.addEventListener('keydown', function(e) {
    self.active(e.target);
  },false);
  target.addEventListener('keyup', function(e) {
    self.active(e.target);
  },false);
  target.addEventListener('keypress', function(e) {
    self.active(e.target);
  },false);
  target.addEventListener('change', function(e) {
    self.active(e.target);
  },false);
};
ActiveForm.prototype.active = function(that) {
  if (that.value.length > 0) {
    this.removeAttr();
  } else {
    this.AddAttr();
  }
};
ActiveForm.prototype.removeAttr = function() {
  this.submit.removeAttribute('disabled');
};
ActiveForm.prototype.AddAttr = function() {
  this.submit.setAttribute('disabled', 'disabled');
};