const login = require('facebook-chat-api');

const peshkata = 'GROUP ID HERE';
let email = 'FB USERNAME HERE';
let password = 'FB PASSWORD HERE';

login({ email, password }, (err, api) => {
    if (err) return console.error(err);

    // Function which adds a user to the group.
    let reAdd = (user, threadId, time) => {
        return api.addUserToGroup(user, threadId, (err) => {
            if (err) {
                // If the request errored try again in an increment from now to a second.
                setTimeout(() => reAdd(user, threadId, time + 1000), time + 1000);
                return console.error(err);
            }
        });
    };

    // Listen to events and not only messages. This is to capture leave events.
    api.setOptions({ listenEvents: true });

    // Set listen callback.
    api.listen((err, message) => {
        // Check for errors.
        if (err) return console.error(err);

        // Check for Secret Peshka.
        if (message.threadID !== peshkata) return;

        // Check for leave event.
        if (message.type === 'event' && message.logMessageType === 'log:unsubscribe') {
            // Send message.
            api.sendMessage('Ne.', message.threadID);

            // Check if we have the id of the person who left - type safety.
            if (message.logMessageData && message.logMessageData.leftParticipantFbId) {
                // Queue a re-add.
                setTimeout(() => reAdd(message.logMessageData.leftParticipantFbId, message.threadID, 1000), 1000);
                return;
            }

            // If we don't have the id of the person who left - this case is impossible.
            return api.sendMessage('wtf', message.threadID);
        }
    });
});