JavaScript event naming conventions

Bootstrap defines two events for every action. One is triggered upon starting the action and named using the present tense (e.g. show). Another is triggered after the action has completed – this one is named with the past participle form (shown).

Everything is explained under http://getbootstrap.com/javascript/#js-events

EDIT

I guess this approach makes sense when animations come into play.

I would go with present tense.

Most, if not all, JavaScript frameworks seem to follow the convention set forth by the JavaScript DOM event API; i.e. to use present tense for event names. When I think about it, this seems most natural to me, despite the fact that events are handled after they are triggered. After all, the event is triggered on event name. Whatever action is performed which is cause to trigger the event, it happens in the current iteration of the event loop. In other words: as far as the event loop is concerned, the event and the action which triggers the event happen simultaneously.

Some frameworks, such as YUI3, offer an after hook as well as an on hook for custom events. This distinction is put to use in YUI's attribute library (amonst others):

Listeners registered using the on method, are notified before the stored state of the attribute has been updated. [...]

Since these listeners are invoked before any state change has occurred, they have the ability to prevent the change in state from occurring [...]

Listeners registered using the after method, are notified after the stored state of the attribute has been updated.

Source: http://yuilibrary.com/yui/docs/attribute/index.html#on-vs-after

I'm not personally aware of any frameworks that use past tense for event names, but then I'm not familiar with all of them.