xhr readystate
_______________________________________________________________________________________
| Value | State | Description | Active/Abortable |
|_______|___________________|_______________________________________|___________________|
| 0 | UNSENT | Client has been created. | NO |
| | | open() not called yet. | |
|-------|-------------------|---------------------------------------|-------------------|
| 1 | OPENED | open() has been called. | YES |
|-------|-------------------|---------------------------------------|-------------------|
| 2 | HEADERS_RECEIVED | send() has been called, and headers | YES |
| | | and status are available. | |
|-------|-------------------|---------------------------------------|-------------------|
| 3 | LOADING | Downloading; responseText holds | YES |
| | | partial data. | |
|-------|-------------------|---------------------------------------|-------------------|
| 4 | DONE | The operation is complete. | NO |
|_______|___________________|_______________________________________|___________________|