The first `await` is waiting for the response-headers to arrive, so you know the status code and can decide what to do next. The second `await` is waiting for the full body to arrive (and get parsed as JSON).

It's designed that way to support doing things other than buffering the whole body; you might choose to stream it, close the connection early etc. But it comes at the cost of awkward double-awaiting for the common case (always load the whole body and then decide what happens next).

So you can say:

    let r = await fetch(...);
    if(!r.ok) ...
    let len = response.headers.get("Content-Length");
    if(!len || new Number(len) > 1000 * 1000)
        throw new Error("Eek!");

It isn't, the following works fine...

    var data = await fetch(url).then(r => r.json());

Understanding Promises/A (thenables) and async/await can sometimes be difficult or confusing, especially when mixing the two like above.

IMU because you don't necessarily want the response body. The first promise resolves after the headers are received, the .json() promise resolves only after the full body is received (and JSON.parse'd, but that's sync anyway).