You can register a callback with Stack Exchange Chat, which gets called prior to the change happening in the DOM (i.e. if you want to find a new message in the DOM, you need to use a `setTimeout()` to act after the DOM change). Callbacks can be registered with `CHAT.addEventHandlerHook(callback)`, which takes a function as its single argument. Note that the function [must not return a truthy value][1] (or be `async`); if it does, while the callback will execute on every event, the DOM will not be updated and new messages will not show up.

The callback function is passed arguments that look like:

<!-- language: lang-json -->

    arguments: {
        0: {
            event_type: 1,
            time_stamp: 1552493100,
            content: 'Test message with a tag <a href="//stackoverflow.com/questions/tagged/some-tag"><span class="ob-post-tag" style="background-color: #E0EAF1; color: #3E6D8E; border-color: #3E6D8E; border-style: solid;">some-tag</span></a>.',
            id: 92913194,
            user_id: 3773011,
            user_name: "Makyen",
            room_id: 68414,
            room_name: "SOCVR Testing Facility",
            message_id: 45625173
        },
        1: false,
        2: 92913190
    }

The first argument is a chat "event" Object. The type of event is indicated by the numeric value of `event_type`. There are 34 types which are possible for `event_type`.

<!-- language: lang-js -->

    const chatEventTypes = {
        1: 'MessagePosted',
        2: 'MessageEdited',
        3: 'UserEntered',
        4: 'UserLeft',
        5: 'RoomNameChanged',
        6: 'MessageStarred',
        7: 'DebugMessage',
        8: 'UserMentioned',
        9: 'MessageFlagged',
        10: 'MessageDeleted',
        11: 'FileAdded',
        12: 'ModeratorFlag',
        13: 'UserSettingsChanged',
        14: 'GlobalNotification',
        15: 'AccessLevelChanged',
        16: 'UserNotification',
        17: 'Invitation',
        18: 'MessageReply',
        19: 'MessageMovedOut',
        20: 'MessageMovedIn',
        21: 'TimeBreak',
        22: 'FeedTicker',
        29: 'UserSuspended',
        30: 'UserMerged',
        34: 'UserNameOrAvatarChanged',
    };


The callback will receive events for all the rooms the user is in, so you will want to filter them by matching the `room_id` to the current room.

The list of event types was copied from the SE's minimized chat JavaScript.

  [1]: https://chat.stackoverflow.com/transcript/message/54614089