giantcrazyfunction.js

const getPositions = chatLog => {
    return chatLog.map((message, i) => {
        const previous = chatLog[i - 1];
        const next = chatLog[i + 1];

        const messageIsJoinOrExit = message.messageType === "JOIN_OR_EXIT";
        let previousIsJoinOrExit;
        let nextIsJoinOrExit;
        let previousIsFromSameUserAsMessage;
        let nextIsfromSameUserAsMessage;
        let previousAndNextAreFromSameUser;
        let previousAndNextAreJoinOrExit;
        let position;

        if (previous) {
            previousIsJoinOrExit = previous.messageType === "JOIN_OR_EXIT";
            previousIsFromSameUserAsMessage = previous.userId === message.userId;
        }

        if (next) {
            nextIsJoinOrExit = next.messageType === "JOIN_OR_EXIT";
            nextIsFromSameUserAsMessage = next.userId === message.userId;
        }

        if (previous && next) {
            previousAndNextAreFromSameUser = previous.userId === next.userId;
            previousAndNextAreJoinOrExit = previousIsJoinOrExit && nextIsJoinOrExit;
        }

        if (!messageIsJoinOrExit) {
            if (previous && next) {
                if (previousIsJoinOrExit && nextIsJoinOrExit) {
                    // message is ALONE
                    return "ALONE";
                } else if (!previousIsJoinOrExit && nextIsJoinOrExit) {
                    // message could be LAST or ALONE
                    return previousIsFromSameUserAsMessage ? "LAST" : "ALONE";
                } else if (previousIsJoinOrExit && !nextIsJoinOrExit) {
                    // message could be FIRST or ALONE
                    return nextIsFromSameUserAsMessage ? "FIRST" : "ALONE";
                } else if (!previousIsJoinOrExit && !nextIsJoinOrExit) {
                    // message could be FIRST, LAST, ALONE or BETWEEN
                    if (previousAndNextAreFromSameUser) {
                        return previousIsFromSameUserAsMessage ? "BETWEEN" : "ALONE";
                    } else {
                        return !previousIsFromSameUserAsMessage ? "LAST" : "FIRST";
                    }
                }
            }

            if (!previous && next) {
                // message could be FIRST or ALONE
                return nextIsJoinOrExit || !nextIsFromSameUserAsMessage
                    ? "ALONE"
                    : "FIRST";
            }

            if (previous && !next) {
                // message could be LAST or ALONE
                return previousIsJoinOrExit || !previousIsFromSameUserAsMessage
                    ? "ALONE"
                    : "FIRST";
            }

            if (!previous && !next) {
                // message is ALONE
                return "ALONE";
            }
        } else {
            // message is JOIN_OR_EXIT
            return "CENTER";
        }
    }).reverse();
};