HTML iframe URL change listener for tracking when a new iframe page starts to load

iframechange.js

function iframeURLChange(iframe, callback) {
    var lastDispatched = null;

    var dispatchChange = function () {
        var newHref = iframe.contentWindow.location.href;

        if (newHref !== lastDispatched) {
            callback(newHref);
            lastDispatched = newHref;
        }
    };

    var unloadHandler = function () {
        // Timeout needed because the URL changes immediately after
        // the `unload` event is dispatched.
        setTimeout(dispatchChange, 0);
    };

    function attachUnload() {
        // Remove the unloadHandler in case it was already attached.
        // Otherwise, there will be two handlers, which is unnecessary.
        iframe.contentWindow.removeEventListener("unload", unloadHandler);
        iframe.contentWindow.addEventListener("unload", unloadHandler);
    }

    iframe.addEventListener("load", function () {
        attachUnload();

        // Just in case the change wasn't dispatched during the unload event...
        dispatchChange();
    });

    attachUnload();
}

// Usage:
iframeURLChange(document.getElementById("mainframe"), function (newURL) {
    console.log("URL changed:", newURL);
});