The XMLHttpRequest.responseText property returns a DOMString that contains the response to the request as text, or null if the request was unsuccessful or has not yet been sent. The responseText property will have the partial response as it arrives even before the request is complete. If responseType is set to anything other than the empty string or "text", accessing responseText will throw InvalidStateError exception.
XMLHttpRequest supports both synchronous and asynchronous communications. In general, however, asynchronous requests should be preferred to synchronous requests for performance reasons.
The W3C XMLHttpRequest specification adds HTML parsing support to XMLHttpRequest, which originally supported only XML parsing. This feature allows Web apps to obtain an HTML resource as a parsed DOM using XMLHttpRequest.
The XMLHttpRequest.response property returns the response's body. It can be of the type ArrayBuffer, Blob, Document, JavaScript object, or a DOMString, depending of the value of XMLHttpRequest.responseType property. Value of response is null if the request is not complete or was not successful. However, if the value of responseType was set to "text" or the empty string, response can contain the partial text response while the request is still in the loading state.
The XMLHttpRequest.responseType property is an enumerated value that returns the type of the response. It also lets the author change the response type to one "arraybuffer", "blob", "document", "json", or "text". If an empty string is set as the value of responseType, it is assumed as type "text".
The XMLHttpRequest.responseURL property returns the serialized URL of the response or the empty string if the URL is null. If the URL is returned, URL fragment if present in the URL will be stripped away. The value of responseURL will be the final URL obtained after any redirects.
The XMLHttpRequest.responseXML property returns a Document containing the response to the request, or null if the request was unsuccessful, has not yet been sent, or cannot be parsed as XML or HTML. The response is parsed as if it were a text/xml stream. When the responseType is set to "document" and the request has been made asynchronously, the response is parsed as a text/html stream. responseXML is null for data: URLs.
The responseType property of the XMLHttpRequest object can be set to change the expected response type from the server. Possible values are the empty string (default), "arraybuffer", "blob", "document", "json", and "text". The response property will contain the entity body according to responseType, as an ArrayBuffer, Blob, Document, JSON, or string. This is null if the request is not complete or was not successful.
The XMLHttpRequest.status property returns the numerical status code of the response of the XMLHttpRequest. status will be an unsigned short. Before the request is complete, the value of status will be 0. It is worth noting that browsers report a status of 0 in case of XMLHttpRequest errors too.
The XMLHttpRequest.statusText property returns a DOMString containing the response's status message as returned by the HTTP server. Unlike XMLHTTPRequest.status which indicates a numerical status code, this property contains the text of the response status, such as "OK" or "Not Found". If the request's readyState is in UNSENT or OPENED state, the value of statusText will be an empty string.
The XMLHttpRequest.timeout property is an unsigned long representing the number of milliseconds a request can take before automatically being terminated. The default value is 0, which means there is no timeout. Timeout shouldn't be used for synchronous XMLHttpRequests requests used in a document environment or it will throw an InvalidAccessError exception. When a timeout happens, a timeout event is fired.
The FormData interface provides a way to easily construct a set of key/value pairs representing form fields and their values, which can then be easily sent using the XMLHttpRequest.send() method. It uses the same format a form would use if the encoding type were set to "multipart/form-data".