We should formalize the control flow here by factoring the event handling logic into discrete methods.

Each conditional body could be defined as a handler, i.e:

```javascript
StopwatchPanel.prototype.onStart = function() {
  this.interval = window.setInterval(this.update.bind(this), 50);
  this.nodes.reset.removeAttribute('disabled');
  this.toggle(this.nodes.pause, this.nodes.start, this.nodes.resume);
  this.toggle(this.nodes.lap, this.nodes.reset);
};
```

And we could delegate to the appropriate method from `StopwatchPanel#handleEvent` like so:

```javascript
var handlerName = 'on' + button.action[0].toUpperCase() + button.action.slice(1);
this[handlerName](event);
```