etherpad-lite/doc/api/hooks_overview.md

Hooks

All hooks are called with two arguments:

1. name - the name of the hook being called
2. context - an object with some relevant information about the context of the call

Return values

A hook should always return a list or undefined. Returning undefined is equivalent to returning an empty list. All the returned lists are appended to each other, so if the return values where [1, 2], undefined, [3, 4,], undefined and [5], the value returned by callHook would be [1, 2, 3, 4, 5].

This is, because it should never matter if you have one plugin or several plugins doing some work - a single plugin should be able to make callHook return the same value a set of plugins are able to return collectively. So, any plugin can return a list of values, of any length, not just one value.