on demand script loader

loader.js

export const loadScript = (id, url, test) => new Promise(resolve => {
    let s = document.getElementById(id);

    if (!s) {
        s = document.createElement('script');
        s.setAttribute('id', id);
        s.defer = true;
        s.src = url;
        document.body.appendChild(s);
    }

    const doTest = () => {
        test(s) ? resolve() : setTimeout(doTest, 250);
    };

    doTest();
});

// how to use
loadScript('pagseguro', 'https://stc.sandbox.pagseguro.uol.com.br/pagseguro/api/v2/checkout/pagseguro.directpayment.js', () => !!window.PagSeguroDirectPayment)
            .then(() => {
                // do something
            });