Voir https://xhr.spec.whatwg.org/

=== readyState ===

^UNSENT^0|The object has been constructed.|
^OPENED^1|The open() method has been successfully invoked. During this state request headers can be set using setRequestHeader() and the request can be made using the send() method.|
^HEADERS_RECEIVED^2|All redirects (if any) have been followed and all HTTP headers of the final response have been received. Several response members of the object are now available.|
^LOADING^3|The response entity body is being received.|
^DONE^4|The data transfer has been completed or something went wrong during the transfer (e.g. infinite redirects).|


=== Status ===

- If the state is UNSENT or OPENED, return 0.
- If the error flag is set, return 0.
- Return the HTTP status code.

=== Events ===

note : ajouter on devant chaque nom d'event pour usage, ex pour readystatechange :
<code>xhr.onreadystatechange = function() {
    	// blablabla
};</code>
(xhr étant un objet XMLHttpRequest)
^Event name^Interface^Dispatched when…^
^readystatechange|Event|The readyState attribute changes value, except when it changes to UNSENT.|
^loadstart|ProgressEvent|The request starts.|
^progress|ProgressEvent|Transmitting data.|
^abort|ProgressEvent|The request has been aborted. For instance, by invoking the abort() method.|
^error|ProgressEvent|The request has failed.|
^load|ProgressEvent|The request has successfully completed.|
^timeout|ProgressEvent|The author specified timeout has passed before the request completed.|
^loadend|ProgressEvent|The request has completed (either in success or failure).|