How to make javascript fetch synchronous?

If you came here because you dropped "how to make javascript fetch synchronous" into a search engine:

That doesn't make much sense. Performing network operations is not something which requires CPU work, thus blocking it during a fetch(...) makes little sense. Instead, properly work with asynchrony as shown in the duplicate linked above.

Original answer for the question:

You want your fetch function to return sth:

function fetchOHLC(yUrl){
    return fetch(yUrl)
    .then(response => response.json())
    .then(function(response) {
            alert(JSON.stringify(response.query));

        var t = response.created;
        var o = response.open;
        var h = response.high;
        var l = response.low;
        var c = response.close;

    return {t,o,h,l,c};

    })
    .catch(function(error) {
        console.log(error);
    });    
}

Now fetchData contains a promise, which can be easily used:

var fetchData = fetchOHLC(yUrl);
fetchData.then(alert); //not empty ! its {t,o,h,l,c}

If you want some fancy ES7, you could rewrite the whole thing like this:

async function fetchOHLC(yUrl) {
  try {
    const res = await ( await fetch(yUrl) ).json();
    alert(JSON.stringify(r.query));
    return {t:r.created,o:r.open,h:r.high,l:r.low,c:r.close};
  } catch(e) { console.log(e); }
}

(async function () {
  const fetchData = await fetchOHLC(yUrl);
  alert(fetchData);
})()