XMLHttpRequest

XMLHttpRequest is a JavaScript object that was designed by Microsoft and adopted by Mozilla, Apple, and Google. It's now being standardized in the W3C. It provides an easy way to retrieve data from a URL without having to do a full page refresh. A Web page can update just a part of the page without disrupting what the user is doing.  XMLHttpRequest is used heavily in AJAX programming.

Despite its name, XMLHttpRequest can be used to retrieve any type of data, not just XML, and it supports protocols other than HTTP (including file and ftp).

To create an instance of XMLHttpRequest, simply do this:

var myRequest = new XMLHttpRequest();

For details about how to use XMLHttpRequest, see Using XMLHttpRequest.

Method overview

XMLHttpRequest(JSObject objParameters);
void abort();
DOMString getAllResponseHeaders();
DOMString? getResponseHeader(DOMString header);
void open(DOMString method, DOMString url, optional boolean async, optional DOMString? user, optional DOMString? password);
void overrideMimeType(DOMString mime);
void send();
void send(ArrayBuffer data);
void send(ArrayBufferView data);
void send(Blob data);
void send(Document data);
void send(DOMString? data);
void send(FormData data);
void setRequestHeader(DOMString header, DOMString value);
Non-standard methods
[noscript] void init(in nsIPrincipal principal, in nsIScriptContext scriptContext, in nsPIDOMWindow ownerWindow);
[noscript] void openRequest(in AUTF8String method, in AUTF8String url, in boolean async, in AString user, in AString password);
void sendAsBinary(in DOMString body); Deprecated since Gecko 31

Properties

Attribute Type Description

onreadystatechange

Function?

A JavaScript function object that is called whenever the readyState attribute changes. The callback is called from the user interface thread.

Warning: This should not be used with synchronous requests and must not be used from native code.
readyState unsigned short

The state of the request:

Value State Description
0 UNSENT open()has not been called yet.
1 OPENED send()has not been called yet.
2 HEADERS_RECEIVED send() has been called, and headers and status are available.
3 LOADING Downloading; responseText holds partial data.
4 DONE The operation is complete.
response varies

The response entity body according to responseType, as an ArrayBuffer, Blob, Document, JavaScript object (for "json"), or string. This is null if the request is not complete or was not successful.

responseText Read only DOMString The response to the request as text, or null if the request was unsuccessful or has not yet been sent.
responseType XMLHttpRequestResponseType

Can be set to change the response type.

Value Data type of response property
"" (empty string) String (this is the default)
"arraybuffer" ArrayBuffer
"blob" Blob
"document" Document
"json" JavaScript object, parsed from a JSON string returned by the server
"text" String
"moz-blob" Used by Firefox to allow retrieving partial Blob data from progress events. This lets your progress event handler start processing data while it's still being received.
"moz-chunked-text"

Similar to "text", but is streaming. This means that the value in response is only available during dispatch of the "progress" event and only contains the data received since the last "progress" event.

When response is accessed during a "progress" event it contains a string with the data. Otherwise it returns null.

This mode currently only works in Firefox.

"moz-chunked-arraybuffer"

Similar to "arraybuffer", but is streaming. This means that the value in response is only available during dispatch of the "progress" event and only contains the data received since the last "progress" event.

When response is accessed during a "progress" event it contains a string with the data. Otherwise it returns null.

This mode currently only works in Firefox.

Note: Starting with Gecko 11.0 (Firefox 11.0 / Thunderbird 11.0 / SeaMonkey 2.8), as well as WebKit build 528, these browsers no longer let you use the responseType attribute when performing synchronous requests. Attempting to do so throws an NS_ERROR_DOM_INVALID_ACCESS_ERR exception. This change has been proposed to the W3C for standardization.
responseXML Read only Document?

The response to the request as a DOM Document object, 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.

Note: If the server doesn't apply the text/xml Content-Type header, you can use overrideMimeType()to force XMLHttpRequest to parse it as XML anyway.
status Read only unsigned short The status of the response to the request. This is the HTTP result code (for example, status is 200 for a successful request).
statusText Read only DOMString The response string returned by the HTTP server. Unlike status, this includes the entire text of the response message ("200 OK", for example).
timeout unsigned long

The number of milliseconds a request can take before automatically being terminated. A value of 0 (which is the default) means there is no timeout.

Note: You may not use a timeout for synchronous requests with an owning window.
ontimeout Function

A JavaScript function object that is called whenever the request times out.

upload XMLHttpRequestUpload The upload process can be tracked by adding an event listener to upload.
withCredentials boolean

Indicates whether or not cross-site Access-Control requests should be made using credentials such as cookies or authorization headers. The default is false.

Note: This never affects same-site requests.
Note: Starting with Gecko 11.0 (Firefox 11.0 / Thunderbird 11.0 / SeaMonkey 2.8), Gecko no longer lets you use the withCredentials attribute when performing synchronous requests. Attempting to do so throws an NS_ERROR_DOM_INVALID_ACCESS_ERR exception.

Non-standard properties

Attribute Type Description
channel Read only nsIChannel The channel used by the object when performing the request. This is null if the channel hasn't been created yet. In the case of a multi-part request, this is the initial channel, not the different parts in the multi-part request. Requires elevated privileges to access.
mozAnon Read only boolean

If true, the request will be sent without cookie and authentication headers.

mozSystem Read only boolean

If true, the same origin policy will not be enforced on the request.

mozBackgroundRequest boolean

Indicates whether or not the object represents a background service request. If true, no load group is associated with the request, and security dialogs are prevented from being shown to the user. Requires elevated privileges to access.

In cases in which a security dialog (such as authentication or a bad certificate notification) would normally be shown, the request simply fails instead.

Note: This property must be set before calling open().
mozResponseArrayBuffer Obsolete since Gecko 6 Read only ArrayBuffer The response to the request, as a JavaScript typed array. This is NULL if the request was not successful, or if it hasn't been sent yet.
multipart Obsolete since Gecko 22 boolean

This Gecko-only feature was removed in Firefox/Gecko 22. Please use Server-Sent Events, Web Sockets, or responseText from progress events instead.

Indicates whether or not the response is expected to be a stream of possibly multiple XML documents. If set to true, the content type of the initial response must be multipart/x-mixed-replace or an error will occur. All requests must be asynchronous.

This enables support for server push; for each XML document that's written to this request, a new XML DOM document is created and the onload handler is called between documents.

Note: When this is set, the onload handler and other event handlers are not reset after the first XMLdocument is loaded, and the onload handler is called after each part of the response is received.

Constructor

XMLHttpRequest()

The constructor initiates an XMLHttpRequest. It must be called before any other method calls.

Gecko/Firefox 16 adds a non-standard parameter to the constructor that can enable anonymous mode (see Bug 692677). Setting the mozAnon flag to true effectively resembles the AnonXMLHttpRequest() constructor described in the XMLHttpRequest specification which has not been implemented in any browser yet (as of September 2012).

XMLHttpRequest (
  JSObject objParameters
);
Parameters (non-standard)
objParameters
There are two flags you can set:
mozAnon
Boolean: Setting this flag to true will cause the browser not to expose the origin and user credentials when fetching resources. Most important, this means that cookies will not be sent unless explicitly added using setRequestHeader.
mozSystem
Boolean: Setting this flag to true allows making cross-site connections without requiring the server to opt-in using CORS. Requires setting mozAnon: true, i.e. this can't be combined with sending cookies or other user credentials. This only works in privileged (reviewed) apps; it does not work on arbitrary webpages loaded in Firefox.

Methods

abort()

Aborts the request if it has already been sent.

getAllResponseHeaders()

DOMString getAllResponseHeaders();

Returns all the response headers as a string, or null if no response has been received. Note: For multipart requests, this returns the headers from the current part of the request, not from the original channel.

getResponseHeader()

DOMString? getResponseHeader(DOMString header);

Returns the string containing the text of the specified header, or null if either the response has not yet been received or the header doesn't exist in the response.

open()

Initializes a request. This method is to be used from JavaScript code; to initialize a request from native code, use openRequest()instead.

Note: Calling this method for an already active request (one for which open()or openRequest()has already been called) is the equivalent of calling abort().
void open(
   DOMString method,
   DOMString url,
   optional boolean async,
   optional DOMString user,
   optional DOMString password
);
Parameters
method
The HTTP method to use, such as "GET", "POST", "PUT", "DELETE", etc. Ignored for non-HTTP(S) URLs.
url
The URL to send the request to.
async
An optional boolean parameter, defaulting to true, indicating whether or not to perform the operation asynchronously. If this value is false, the send()method does not return until the response is received. If true, notification of a completed transaction is provided using event listeners. This must be true if the multipart attribute is true, or an exception will be thrown.
Note: Starting with Gecko 30.0 (Firefox 30.0 / Thunderbird 30.0 / SeaMonkey 2.27), synchronous requests on the main thread have been deprecated due to the negative effects to the user experience.
user
The optional user name to use for authentication purposes; by default, this is an empty string.
password
The optional password to use for authentication purposes; by default, this is an empty string.

overrideMimeType()

Overrides the MIME type returned by the server. This may be used, for example, to force a stream to be treated and parsed as text/xml, even if the server does not report it as such. This method must be called before send().

void overrideMimeType(DOMString mimetype);

send()

Sends the request. If the request is asynchronous (which is the default), this method returns as soon as the request is sent. If the request is synchronous, this method doesn't return until the response has arrived.

Note: Any event listeners you wish to set must be set before calling send().
Note: Be aware to stop using a plain ArrayBuffer as parameter. This is not part of the XMLHttpRequest specification any longer. Use an ArrayBufferView instead (see compatibility table for version information).
void send();
void send(ArrayBuffer data);
void send(ArrayBufferView data);
void send(Blob data);
void send(Document data);
void send(DOMString? data);
void send(FormData data);
Notes

If the data is a Document, it is serialized before being sent. When sending a Document, versions of Firefox prior to version 3 always send the request using UTF-8 encoding; Firefox 3 properly sends the document using the encoding specified by body.xmlEncoding, or UTF-8 if no encoding is specified.

If it's an nsIInputStream, it must be compatible with nsIUploadChannel's setUploadStream()method. In that case, a Content-Length header is added to the request, with its value obtained using nsIInputStream's available()method. Any headers included at the top of the stream are treated as part of the message body. The stream's MIMEtype should be specified by setting the Content-Type header using the setRequestHeader() method prior to calling send().

The best way to send binary content (like in files upload) is using an ArrayBufferView or Blobs in conjuncton with the send() method. However, if you want to send a stringifiable raw data, use the sendAsBinary() method instead, or the StringView Non native typed arrays superclass.

setRequestHeader()

Sets the value of an HTTP request header. You must call setRequestHeader() after open(), but before send(). If this method is called several times with the same header, the values are merged into one single request header.

void setRequestHeader(
   DOMString header,
   DOMString value
);
Parameters
header
The name of the header whose value is to be set.
value
The value to set as the body of the header.

Non-standard methods

init()

Initializes the object for use from C++ code.

Warning: This method must not be called from JavaScript.
[noscript] void init(
   in nsIPrincipal principal,
   in nsIScriptContext scriptContext,
   in nsPIDOMWindow ownerWindow
);
Parameters
principal
The principal to use for the request; must not be null.
scriptContext
The script context to use for the request; must not be null.
ownerWindow
The window associated with the request; may be null.

openRequest()

Initializes a request. This method is to be used from native code; to initialize a request from JavaScript code, use open()instead. See the documentation for open().

Requires Gecko 1.9 (Firefox 3)

sendAsBinary()

Deprecated since Gecko 31

A variant of the send() method that sends binary data.

Note: This non-standard method is considered deprecated as of Gecko 31 (Firefox 31 / Thunderbird 31 / SeaMonkey 2.28), and will be removed soon. The standard send(Blob data) method can be used instead.
void sendAsBinary(
   in DOMString body
);

This method, used in conjunction with the readAsBinaryString method of the FileReader API, makes it possible to read and upload any type of file and to stringify the raw data.

Parameters
body
The request body as a DOMstring. This data is converted to a string of single-byte characters by truncation (removing the high-order byte of each character).
sendAsBinary() polyfill

Since sendAsBinary() is an experimental feature, here is a polyfill for browsers that don't support the sendAsBinary() method but support typed arrays.

/*\
|*|
|*|  :: XMLHttpRequest.prototype.sendAsBinary() Polyfill ::
|*|
|*|  https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest#sendAsBinary()
|*|
\*/

if (!XMLHttpRequest.prototype.sendAsBinary) {
  XMLHttpRequest.prototype.sendAsBinary = function (sData) {
    var nBytes = sData.length, ui8Data = new Uint8Array(nBytes);
    for (var nIdx = 0; nIdx < nBytes; nIdx++) {
      ui8Data[nIdx] = sData.charCodeAt(nIdx) & 0xff;
    }
    /* send as ArrayBufferView...: */
    this.send(ui8Data);
    /* ...or as ArrayBuffer (legacy)...: this.send(ui8Data.buffer); */
  };
}
Note: It's possible to build this polyfill putting two types of data as argument for send(): an ArrayBuffer (ui8Data.buffer – the commented code) or an ArrayBufferView (ui8Data, which is a typed array of 8-bit unsigned integers – uncommented code). However, on Google Chrome, when you try to send an ArrayBuffer, the following warning message will appear: ArrayBuffer is deprecated in XMLHttpRequest.send(). Use ArrayBufferView instead. Another possible approach to send binary data is the StringView Non native typed arrays superclass in conjunction with the send() method.

Notes

  • By default, Firefox 3 limits the number of XMLHttpRequest connections per server to 6 (previous versions limit this to 2 per server). Some interactive web sites may keep an XMLHttpRequest connection open, so opening multiple sessions to such sites may result in the browser hanging in such a way that the window no longer repaints and controls don't respond. This value can be changed by editing the network.http.max-persistent-connections-per-server preference in about:config.
  • From Gecko 7.0 headers set by setRequestHeader() are sent with the request when following a redirect. Previously these headers would not be sent.
  • XMLHttpRequest is implemented in Gecko using the nsIXMLHttpRequest, nsIXMLHttpRequestEventTarget, and nsIJSXMLHttpRequest interfaces.
  • When a request reaches its timeout value,   a "timeout" event is raised.

Events

onreadystatechange as a property of the XMLHttpRequest instance is supported in all browsers.

Since then, a number of additional event handlers were implemented in various browsers (onload, onerror, onprogress, etc.). These are supported in Firefox. In particular, see nsIXMLHttpRequestEventTarget and Using XMLHttpRequest.

More recent browsers, including Firefox, also support listening to the XMLHttpRequest events via standard addEventListener APIs in addition to setting on* properties to a handler function.

Permissions

When using System XHR via the mozSystem property, for example for Firefox OS apps, you need to be sure to add the systemXHR permission into your manifest file. System XHR can be used in privileged or certified apps.

"permissions": {
    "systemXHR":{}
}

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support (XHR1) 1 1.0 5 (via ActiveXObject)
7 (XMLHttpRequest)
(Yes) 1.2
send(ArrayBuffer) 9 9 10 11.60 ?
send(ArrayBufferView) 22 20 ? ? ?
send(Blob) 7 3.6 10 12 ?
send(FormData) 6 4 10 12 ?
sendAsBinary(DOMString) Not supported – use the polyfill 1.9 Not supported Not supported Not supported
response 10 6 10 11.60 ?
responseType = 'arraybuffer' 10 6 10 11.60 ?
responseType = 'blob' 19 6 10 12 ?
responseType = 'document' 18 11 10 Not supported 6.1
responseType = 'json' Not supported 10 Not supported 12 Not supported
Progress Events 7 3.5 10 12 ?
withCredentials 3 3.5 10 12 4
timeout 29 12.0 8 12 Not supported
responseType = 'moz-blob' Not supported 12.0 Not supported Not supported Not supported
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support ? 0.16 (Yes) ? ? ?

Gecko notes

Gecko 11.0 (Firefox 11.0 / Thunderbird 11.0 / SeaMonkey 2.8) removed support for using the responseType and withCredentials attributes when performing synchronous requests. Attempting to do so throws an NS_ERROR_DOM_INVALID_ACCESS_ERR exception. This change has been proposed to the W3C for standardization.

Gecko 12.0 (Firefox 12.0 / Thunderbird 12.0 / SeaMonkey 2.9) and later support using XMLHttpRequest to read from data: URLs.

Gecko 20.0 (Firefox 20.0 / Thunderbird 20.0 / SeaMonkey 2.17) adds the support of sending an ArrayBufferView - sending a plain ArrayBuffer is not part of the XMLHttpRequest specification any longer and should be treated as deprecated.

See also