Assign next tabId to every new tab and save in session storage

tabId.js

(function() {
    // Does this tab have a tabId
    let tabId = sessionStorage.getItem("tabId");
    if (tabId === null) {
        // no tabId for this tab
        // Is lastTabId stored in local storage
        let lastTabId = localStorage.getItem("lastTabId");
        if (lastTabId === null) {
            // no lastTabId saved in local storage
            lastTabId = -1;
        }
        // Generate next tabId
        lastTabId = 1 + parseInt(lastTabId);
        // Save as lastTabId
        localStorage.setItem("lastTabId", lastTabId);
        // assign
        tabId = lastTabId;
        // store tabId
        sessionStorage.setItem("tabId", tabId);
    }
})();