iD/modules/util/session_mutex.js

// @flow
// A per-domain session mutex backed by a cookie and dead man's
// switch. If the session crashes, the mutex will auto-release
// after 5 seconds.

// This is a type alias (https://flow.org/en/docs/types/aliases/) which allows flow to understand the object returned by utilSessionMutex in other files.
type utilSessionMutexType = {
    lock: () => boolean,
    unlock: () => void,
    locked: () => boolean
};

// This accepts a string and returns an object that complies with utilSessionMutexType
export function utilSessionMutex(name: string): utilSessionMutexType {
    var mutex = {};
    var intervalID: ?IntervalID; // This indicates a Maybe type - intervalId can be null so we need to use "?IntervalID", not "IntervalID"

    function renew() {
        var expires = new Date();
        expires.setSeconds(expires.getSeconds() + 5);
        document.cookie = name + '=1; expires=' + expires.toUTCString();
    }

    mutex.lock = function (): boolean {
        if (intervalID) return true;
        var cookie = document.cookie.replace(new RegExp('(?:(?:^|.*;)\\s*' + name + '\\s*\\=\\s*([^;]*).*$)|^.*$'), '$1');
        if (cookie) return false;
        renew();
        intervalID = window.setInterval(renew, 4000);
        return true;
    };

    mutex.unlock = function (): void {
        if (!intervalID) return;
        document.cookie = name + '=; expires=Thu, 01 Jan 1970 00:00:00 GMT';
        clearInterval(intervalID);
        intervalID = null;
    };

    mutex.locked = function (): boolean {
        return !!intervalID;
    };

    return mutex;
}