Adds undo capabilities into your React.js component.

undo.js

var UndoMixin = {
  getInitialState: function() {
    return {
      undo: []
    };
  },

  handleUndo: function() {
    if (this.state.undo.length === 0) {
      return;
    }
    var nextUndo = _.initial(this.state.undo);
    var nextState = _.last(this.state.undo);
    nextState.undo = nextUndo;
    this.setState(nextState);
  },

  handleUndoMixinKeyDown: function(ev) {
    if (!(ev.which === 90 && (ev.ctrlKey || ev.altKey))) {
      return;
    }
    this.handleUndo();
  },

  componentDidMount: function() {
    document.addEventListener('keydown', this.handleUndoMixinKeyDown);
  },

  componentWillUnmount: function() {
    document.removeEventListener('keydown', this.handleUndoMixinKeyDown);
  },

  setStateWithUndo: function(state, callback) {
    var undo = this.state.undo.slice(0);
    var undoItem = _.clone(this.state);
    delete undoItem.undo;
    undo.push(undoItem);
    var nextState = _.extend({undo: undo}, state);
    this.setState(nextState, callback);
  }
};