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The XMLHttpRequest.upload property returns an XMLHttpRequestUpload object, representing the upload process. It is an opaque object, but being an XMLHttpRequestEventTarget, event listeners can be set on it to track its process.
Event listeners that can be triggered on an upload object are the following:
Event listeners
Data type of response property
onloadstart
The fetch starts
onprogress
Data transfer is going on
onabort
The fetch operation was aborted
onerror
The fetch failed
onload
The fetch succeeded
ontimeout
The fetch operation didn't complete by the timeout the author specified
onloadend
The fetch operation completed (either success or failure)