css/fetching/support/echo-helper.js

function get_resource_echo_url(uid, url) {
    return `/css/fetching/support/echo-headers.py?token=${uid}&location=${url}`
}

function wait_for_resource(url) {
    return new Promise(resolve => {
        const po = new PerformanceObserver(list => {
            const entries = list.getEntries();
            if (entries.find(e => e.name.includes(url)))
                resolve();
        })
        po.observe({type: "resource", buffered: true});
    });
}

async function get_headers(uid) {
    return await (await fetch(`/css/fetching/support/echo-headers.py?token=${uid}&location=echo`)).json()
}