index.js

var has = Object.hasOwnProperty

module.exports = function(model){

  model.undoCommand = model.undoCommand || defaultSetter;
  model.redoCommand = model.redoCommand || defaultSetter;

  model.on('change', function(instance,attr,val,prev){
    if (isClass){
      handleChange(model,instance,attr,val,prev);
    } else {
      handleChange(model,model,instance,attr,val);
    }
  })

  model.on('save', function(instance){
    if (isClass){ 
      handleSave(instance);
    } else { 
      handleSave(model);
    }
  })

  var isClass = ('function' == typeof model);
  var target = (isClass ? model.prototype : model);

  target.undo = function(){
    withCommands(this, function(){
      this._cmds.undo();
    });
    return this;
  }

  target.redo = function(){
    withCommands(this, function(){
      this._cmds.redo();
    });
    return this;
  }

  target.undoAll = function(){
    withCommands(this, function(){
      this._cmds.undoAll();
    });
    return this;
  }

  target.redoAll = function(){
    withCommands(this, function(){
      this._cmds.redoAll();
    });
    return this;
  }

  return model;
}

// private

function handleChange(model,instance,attr,val,prev){
  if (!instance._undoing){
    var cmd = { 
      undo: function(){ 
              model.undoCommand(instance,attr,prev); 
              if ('undefined' == typeof prev && has.call(instance,'dirty')){
                delete instance.dirty[attr];
              }
              if (instance.emit) instance.emit('undo',attr,prev);
            },
      redo: function(){ 
              model.redoCommand(instance,attr,val); 
              if (instance.emit) instance.emit('redo',attr,val);
            }
    };
    (instance._cmds = instance._cmds || new CmdStack).push(cmd);
  }
}

function handleSave(instance){
  if (instance._cmds) instance._cmds.reset();
}

function withCommands(instance,fn){
  if (!instance._cmds) return;
  instance._undoing = true;
  fn.call(instance);
  instance._undoing = false;
}

function defaultSetter(instance,attr,val){
  return instance[attr](val);
}


function CmdStack(){
  this.reset();
}

CmdStack.prototype.reset = function(){
  this.resetDone();
  this.resetUndone();
}

CmdStack.prototype.resetUndone = function(){
  this.undone = [];
}

CmdStack.prototype.resetDone = function(){
  this.done = [];
}

CmdStack.prototype.push = function(cmd){
  this.done.push(cmd);
  this.resetUndone();
}

CmdStack.prototype.undoAll = function(){
  while (this.done.length) this.undo();
}

CmdStack.prototype.redoAll = function(){
  while (this.undone.length) this.redo();
}

CmdStack.prototype.undo = function(){
  var last = this.done.pop();
  if (last) {
    last.undo();
    this.undone.push( last );
  }
}

CmdStack.prototype.redo = function(){
  var last = this.undone.pop();
  if (last) {
    last.redo();
    this.done.push( last );
  }
}