JavaScript Snippet to detect when tab is closed

detectabs.js

// Get or Create a new tab id (accessible only on the tab itself)
    var tabID = sessionStorage.tabID ? sessionStorage.tabID : sessionStorage.tabID = Math.random();
    // Get all connected tabs from localStorage (shared between all tabs)
    var connectedTabs = localStorage.getItem("ConnectedTabs");

    // Validate if tabID was created
    if (tabID) {
        if (!connectedTabs) {
            // If there's no tabs connected yet, create an array with itself tabID
            var tabs = [tabID];
        } else {
            // Parse the tabs array
            connectedTabs = JSON.parse(connectedTabs);

            // Check if this tab is already added in connections array
            var alreadyAdded = connectedTabs.indexOf(tabID);

            // Push the new tabID to array of tabs only if it was not added yet
            if (alreadyAdded < 0) {
                connectedTabs.push(tabID);
            }

            var tabs = connectedTabs;
        }

        // Save the JSON
        localStorage.setItem("ConnectedTabs", JSON.stringify(tabs));
    }

    // Create a new listener to trigger when tab is closed
    window.addEventListener("beforeunload", function (e) {
        // Get tabs in storage
        var connectedTabs = localStorage.getItem("ConnectedTabs");
        
        // If it's everything okay in tabs retrieve...
        if (connectedTabs) {
            // Parse the tabs array again
            connectedTabs = JSON.parse(connectedTabs);

            // If there's only one tab connected remove the storage position
            if (connectedTabs.length === 1) {
                localStorage.removeItem("ConnectedTabs");
            } else { // But if have more than one tab connected...
                // Find the position of our tab by her tabID in storage
                var currentTab = connectedTabs.indexOf(tabID);

                // Remove the tab from our storage list
                connectedTabs.splice(currentTab, 1);
                // Save our updated list keeping only connected tabs
                localStorage.setItem("ConnectedTabs", JSON.stringify(connectedTabs));
            }
        }
    });