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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 (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:

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.

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.