Constant FETCH_JS 

pub const FETCH_JS: &str = r#"
(async function(argsJson) {
    const args = JSON.parse(argsJson);
    const timeoutMs = Number(args.timeoutMs || 0);
    const controller = timeoutMs > 0 && typeof AbortController !== 'undefined'
        ? new AbortController()
        : null;
    const timer = controller
        ? setTimeout(() => controller.abort(), timeoutMs)
        : null;
    try {
        const r = await fetch(args.url, {
            method: args.method || 'GET',
            headers: args.headers || {},
            body: args.body,
            credentials: 'include',
            signal: controller ? controller.signal : undefined,
        });
        const text = await r.text();
        const headers = {};
        r.headers.forEach((v,k) => { headers[k] = v; });
        return JSON.stringify({ ok: true, status: r.status, statusText: r.statusText, headers, body: text });
    } catch (e) {
        const aborted = controller && controller.signal.aborted;
        return JSON.stringify({
            ok: false,
            error: aborted ? `fetch timed out after ${timeoutMs}ms` : String(e),
            errorName: e && e.name ? e.name : null,
        });
    } finally {
        if (timer) clearTimeout(timer);
    }
})
"#;

Performs a fetch from the page context. Receives JSON-string args.