How to use javascript fetch within fetch and populate array without having to use a timeout because of asynchronous calls

Question

I'm using the fetch function to populate an array. I'm using (some of the) data form the first fetch to start another fetch. My problem is that I have to use a timeout otherwise my array stays empty (probably due to asynchronous calls).

let output = [];
fetch('url1')
.then(response => response.json())
.then((response) => {
    let postsArray = response["bla"]["blabla"]["blablabla"]["endpoint"];
    if (postsArray.length > 0) {
        for (let i = 0; i < postsArray.length; i++) {
            let postID = postsArray[i]["bla"]["endpoint"];
            fetch('url/' + postID + '/blabla')
            .then(response => response.json())
            .then((response) => {
                let userInfo = response["bla"]["blabla"]["endpoint"];
                let myItem = {};
                myItem["bla"] = userInfo["bla"];
                myItem["blabla"] = userInfo["blabla"];
                output.push(myItem);
            });
        }
    }
});
//which I don't want to use
setTimeout(function(){
    alert(JSON.stringify(output));
}, 3000);

Is it possible to get rid of the setTimeout()? I don't want to use an await or something (due to cross-browser issues).

Any thoughts on this?