backbone-previous-sample.js

var Model = Backbone.Model.extend({
  validate: function(attr) {
    var prev = this.previous('key');
    console.log(prev);
  }
});

var model = new Model({ key: 1 });
model.set('key', 2, { validate: true }); //=> undefined
model.set('key', 3, { validate: true }); //=> 1