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Synopsis

Synopsis for an XMLHttpRequest object:
Properties
onreadystatechange is used to set the callback function called when the response from the server is ready.
readyState returns the status of the request. It changes from 0 to 4:
0: request not initialized
1: server connection established
2: request received
3: processing request
4: request finished and the response is ready
status returns the status of the request:
200: OK
404: Page not found
responseText contains the response data in the form of a string.
responseXML contains the response data in the form of XML data (6.6.1).
Methods
open($s1,$s2,$b) sets the connection type to $s1 and requests the server to return the output of $s2.

$s1 can be “GET” or “POST”. If “GET” is used, the query string is to be attached to the url in $s2, and the result may be cached. “POST” is more robust and secure, with no size limitations. If “POST” is used, the query string must be specified in send(), and setRequestHeader() must be called.

$s2 can be a “.php”,”.asp” or any data file. $b specifies whether the execution is asynchronous. If it is set to true, Javascript will execute other scripts while waiting for the server response. The callback function must also be set with onreadystatechange. If it is set to false, the execution flow will block until the server response is ready.
setRequestHeader($s1,$s2) needs to be called if the “POST” method is used. $s1 specifies the header name. $s2 specifies the header value.
send([$s]) sends the request after it has been set. $s contains the query string when the “POST” method is used.


A very detailed synopsis of XMLHttpRequest can be found at:

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest