Speculative idea for `await` in JS

await.js

// Let `doAjax`, `fadeIn`, `fadeOut`, and `delay` be promise-returning functions.

// In all the following examples, `example` is meant to return a promise that is fulfilled when
// all operations are completed, or rejected if any of the steps fail.

// ES5
function example() {
    return doAjax("data.json").then(function (data) {
        document.getElementById("data").innerText = data;

        var status = document.getElementById("status");
        return fadeIn(status).then(function () {
            return delay(2000);
        }).then(function () {
            return fadeOut(status);
        });
    });
}

// ES6
const example = Q.async(function* () {
    document.getElementById("data").innerText = yield doAjax("data.json");

    const status = document.getElementById("status");
    yield fadeIn(status);
    yield delay(2000);

    return fadeOut(status); // not sure this is right? maybe just `yield fadeOut(status); return;`?
});

// ES-some-time-after-6

function^ example() {
    document.getElementById("data").innerText = await doAjax("data.json");

    const status = document.getElementById("status");
    await fadeIn(status);
    await delay(2000);

    await fadeOut(status);
}

I prefer yield fadeOut(status); return; or even return yield fadeOut(status) if you wanted to return the result of the fadeOut. Although I suppose if we always recursively un-nest promises it doesn't make much difference anyway.

How does it work if you have >1param ?

For any future visitors, @lukehoban took up this torch and has made it a TC39 proposal: https://github.com/lukehoban/ecmascript-asyncawait