Because the response from the API is PHP output, the usual `Content-Length` header is absent.
This [custom header technique](https://stackoverflow.com/questions/15097712/how-can-i-use-deflated-gzipped-content-with-an-xhr-onprogress-function/32799706#32799706) allows the client to know the total length of the data being received, in order to display a progress indicator.
Here's a code example with `XMLHttpRequest`:
```
xhr.addEventListener("progress", (e) => {
if (e.lengthComputable) {
onDownloadProgress({
loaded: e.loaded,
total: e.total,
});
} else {
const uncompressedContentLength = xhr.getResponseHeader(
"X-Uncompressed-Content-Length",
);
if (uncompressedContentLength) {
onDownloadProgress({
loaded: e.loaded,
total: Number(uncompressedContentLength),
});
}
}
});
```
Notes:
- `Fetch` can be used as well (only reason I use `XMLHttpRequest` is because `fetch` doesn't allow to track the progress of uploaded data (when creating a paste); whereas `XMLHttpRequest` does).
- `e.loaded` can be different between browsers; Firefox reports the length of the compressed data, Chrome reports the length of uncompressed data (see https://github.com/whatwg/xhr/issues/388). A workaround for this is to manually set our progress indicator to 100% when the request finishes.
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