XMLHttpRequest has 5 readyStates, and I only use 1 of them (the last one, 4).
What are the others for, and what practical applications can I use them in?
The full list of readyState values is:
State Description
0 The request is not initialized
1 The request has been set up
2 The request has been sent
3 The request is in process
4 The request is complete
(from https://www.w3schools.com/js/js_ajax_http_response.asp)
In practice you almost never use any of them except for 4.
Some XMLHttpRequest implementations may let you see partially received responses in responseText when readyState==3, but this isn't universally supported and shouldn't be relied upon.
