I prefer async and await.

async-await.js

const myRequest = new Request(window.location.origin + "/test");
async function callRequest() {
    res_p = await fetch(myRequest);
    res_text = await res_p.text();
    return res_text
}

callRequest().then(val => {
    console.log(val);
});

promise-only.js

const myRequest = new Request(window.location.origin + "/test");
fetch(myRequest)
    .then(res => {
        // console.log(res.json());
        res.text()
            .then(val => {
                console.log(val);
            })
    });