XMLHttpRequest

1. Introduction

This section is non-normative.

The XMLHttpRequest object is an API for fetching resources.

The name XMLHttpRequest is historical and has no bearing on its functionality.

function processData(data) {
  // taking care of data
}

function handler() {
  if(this.status == 200 &&
    this.responseXML != null &&
    this.responseXML.getElementById('test').textContent) {
    // success!
    processData(this.responseXML.getElementById('test').textContent);
  } else {
    // something went wrong
    …
  }
}

var client = new XMLHttpRequest();
client.onload = handler;
client.open("GET", "unicorn.xml");
client.send();

function log(message) {
  var client = new XMLHttpRequest();
  client.open("POST", "/log");
  client.setRequestHeader("Content-Type", "text/plain;charset=UTF-8");
  client.send(message);
}

function fetchStatus(address) {
  var client = new XMLHttpRequest();
  client.onload = function() {
    // in case of network errors this might not give reliable results
    returnStatus(this.status);
  }
  client.open("HEAD", address);
  client.send();
}

1.1. Specification history

The XMLHttpRequest object was initially defined as part of the WHATWG’s HTML effort. (Based on Microsoft’s implementation many years prior.) It moved to the W3C in 2006. Extensions (e.g., progress events and cross-origin requests) to XMLHttpRequest were developed in a separate draft (XMLHttpRequest Level 2) until end of 2011, at which point the two drafts were merged and XMLHttpRequest became a single entity again from a standards perspective. End of 2012 it moved back to the WHATWG.

Discussion that led to the current draft can be found in the following mailing list archives:

• [email protected]
• [email protected]
• [email protected]
• [email protected]

2. Terminology

This specification depends on the Infra Standard. [INFRA]

This specification uses terminology from DOM, DOM Parsing and Serialization, Encoding, Fetch, File API, HTML, URL, Web IDL, and XML.

[DOM] [DOM-PARSING] [ENCODING] [FETCH] [FILEAPI] [HTML] [URL] [WEBIDL] [XML] [XML-NAMES]

3. Interface XMLHttpRequest

[Exposed=(Window,DedicatedWorker,SharedWorker)]
interface XMLHttpRequestEventTarget : EventTarget {
  // event handlers
  attribute EventHandler onloadstart;
  attribute EventHandler onprogress;
  attribute EventHandler onabort;
  attribute EventHandler onerror;
  attribute EventHandler onload;
  attribute EventHandler ontimeout;
  attribute EventHandler onloadend;
};

[Exposed=(Window,DedicatedWorker,SharedWorker)]
interface XMLHttpRequestUpload : XMLHttpRequestEventTarget {
};

enum XMLHttpRequestResponseType {
  "",
  "arraybuffer",
  "blob",
  "document",
  "json",
  "text"
};

[Exposed=(Window,DedicatedWorker,SharedWorker)]
interface XMLHttpRequest : XMLHttpRequestEventTarget {
  constructor();

  // event handler
  attribute EventHandler onreadystatechange;

  // states
  const unsigned short UNSENT = 0;
  const unsigned short OPENED = 1;
  const unsigned short HEADERS_RECEIVED = 2;
  const unsigned short LOADING = 3;
  const unsigned short DONE = 4;
  readonly attribute unsigned short readyState;

  // request
  undefined open(ByteString method, USVString url);
  undefined open(ByteString method, USVString url, boolean async, optional USVString? username = null, optional USVString? password = null);
  undefined setRequestHeader(ByteString name, ByteString value);
           attribute unsigned long timeout;
           attribute boolean withCredentials;
  [SameObject] readonly attribute XMLHttpRequestUpload upload;
  undefined send(optional (Document or XMLHttpRequestBodyInit)? body = null);
  undefined abort();

  // response
  readonly attribute USVString responseURL;
  readonly attribute unsigned short status;
  readonly attribute ByteString statusText;
  ByteString? getResponseHeader(ByteString name);
  ByteString getAllResponseHeaders();
  undefined overrideMimeType(DOMString mime);
           attribute XMLHttpRequestResponseType responseType;
  readonly attribute any response;
  readonly attribute USVString responseText;
  [Exposed=Window] readonly attribute Document? responseXML;
};

An XMLHttpRequest object has an associated:

upload object

An XMLHttpRequestUpload object.

state

One of unsent, opened, headers received, loading, and done; initially unsent.

send() flag

A flag, initially unset.

timeout

An unsigned integer, initially 0.

cross-origin credentials

A boolean, initially false.

request method

A method.

request URL

A URL.

author request headers

A header list, initially empty.

request body

Initially null.

synchronous flag

A flag, initially unset.

upload complete flag

A flag, initially unset.

upload listener flag

A flag, initially unset.

timed out flag

A flag, initially unset.

response

A response, initially a network error.

received bytes

A byte sequence, initially the empty byte sequence.

response type

One of the empty string, "arraybuffer", "blob", "document", "json", and "text"; initially the empty string.

response object

An object, failure, or null, initially null.

fetch controller

A fetch controller, initially a new fetch controller. The send() method sets it to a useful fetch controller, but for simplicity it always holds a fetch controller.

override MIME type

A MIME type or null, initially null. Can get a value when overrideMimeType() is invoked.

3.1. Constructors

client = new XMLHttpRequest()

Returns a new XMLHttpRequest object.

The new XMLHttpRequest() constructor steps are:

1. Set this’s upload object to a new XMLHttpRequestUpload object.

3.2. Garbage collection

An XMLHttpRequest object must not be garbage collected if its state is either opened with the send() flag set, headers received, or loading, and it has one or more event listeners registered whose type is one of readystatechange, progress, abort, error, load, timeout, and loadend.

If an XMLHttpRequest object is garbage collected while its connection is still open, the user agent must terminate the XMLHttpRequest object’s fetch controller.

3.3. Event handlers

The following are the event handlers (and their corresponding event handler event types) that must be supported on objects implementing an interface that inherits from XMLHttpRequestEventTarget as attributes:

The following is the event handler (and its corresponding event handler event type) that must be supported as attribute solely by the XMLHttpRequest object:

3.4. States

client . readyState

Returns client’s state.

The readyState getter steps are to return the value from the table below in the cell of the second column, from the row where the value in the cell in the first column is this’s state:

3.5. Request

Registering one or more event listeners on an XMLHttpRequestUpload object will result in a CORS-preflight request. (That is because registering an event listener causes the upload listener flag to be set, which in turn causes the use-CORS-preflight flag to be set.)

3.5.1. The open() method

client . open(method, url [, async = true [, username = null [, password = null]]])

Sets the request method, request URL, and synchronous flag.

Throws a "SyntaxError" DOMException if either method is not a valid method or url cannot be parsed.

Throws a "SecurityError" DOMException if method is a case-insensitive match for `CONNECT`, `TRACE`, or `TRACK`.

Throws an "InvalidAccessError" DOMException if async is false, the current global object is a Window object, and the timeout attribute is not zero or the responseType attribute is not the empty string.

Synchronous XMLHttpRequest outside of workers is in the process of being removed from the web platform as it has detrimental effects to the end user’s experience. (This is a long process that takes many years.) Developers must not pass false for the async argument when the current global object is a Window object. User agents are strongly encouraged to warn about such usage in developer tools and may experiment with throwing an "InvalidAccessError" DOMException when it occurs.

The open(method, url) and open(method, url, async, username, password) method steps are:

1. If this’s relevant global object is a Window object and its associated Document is not fully active, then throw an "InvalidStateError" DOMException.

2. If method is not a method, then throw a "SyntaxError" DOMException.

3. If method is a forbidden method, then throw a "SecurityError" DOMException.

4. Normalize method.

5. Let parsedURL be the result of encoding-parsing a URL url, relative to this’s relevant settings object.

6. If parsedURL is failure, then throw a "SyntaxError" DOMException.

7. If the async argument is omitted, set async to true, and set username and password to null.

   Unfortunately legacy content prevents treating the async argument being undefined identical from it being omitted.

8. If parsedURL’s host is non-null, then:

   1. If the username argument is not null, set the username given parsedURL and username.

   2. If the password argument is not null, set the password given parsedURL and password.

9. If async is false, the current global object is a Window object, and either this’s timeout is not 0 or this’s response type is not the empty string, then throw an "InvalidAccessError" DOMException.

10. Terminate this’s fetch controller.

    A fetch can be ongoing at this point.

11. Set variables associated with the object as follows:

    • Unset this’s send() flag.

    • Unset this’s upload listener flag.

    • Set this’s request method to method.

    • Set this’s request URL to parsedURL.

    • Set this’s synchronous flag if async is false; otherwise unset this’s synchronous flag.

    • Empty this’s author request headers.

    • Set this’s response to a network error.

    • Set this’s received bytes to the empty byte sequence.

    • Set this’s response object to null.

    Override MIME type is not overridden here as the overrideMimeType() method can be invoked before the open() method.

12. If this’s state is not opened, then:

    1. Set this’s state to opened.

    2. Fire an event named readystatechange at this.

The reason there are two open() methods defined is due to a limitation of the editing software used to write the XMLHttpRequest Standard.

3.5.2. The setRequestHeader() method

client . setRequestHeader(name, value)

Appends a value to an existing request header or adds a new request header.

Throws an "InvalidStateError" DOMException if either state is not opened or the send() flag is set.

Throws a "SyntaxError" DOMException if name is not a header name or if value is not a header value.

The setRequestHeader(name, value) method must run these steps:

1. If this’s state is not opened, then throw an "InvalidStateError" DOMException.

2. If this’s send() flag is set, then throw an "InvalidStateError" DOMException.

3. Normalize value.

4. If name is not a header name or value is not a header value, then throw a "SyntaxError" DOMException.

   An empty byte sequence represents an empty header value.

5. If (name, value) is a forbidden request-header, then return.

6. Combine (name, value) in this’s author request headers.

// The following script:
var client = new XMLHttpRequest();
client.open('GET', 'demo.cgi');
client.setRequestHeader('X-Test', 'one');
client.setRequestHeader('X-Test', 'two');
client.send();

// …results in the following header being sent:
// X-Test: one, two

3.5.3. The timeout getter and setter

client . timeout

Can be set to a time in milliseconds. When set to a non-zero value will cause fetching to terminate after the given time has passed. When the time has passed, the request has not yet completed, and this’s synchronous flag is unset, a timeout event will then be dispatched, or a "TimeoutError" DOMException will be thrown otherwise (for the send() method).

When set: throws an "InvalidAccessError" DOMException if the synchronous flag is set and the current global object is a Window object.

The timeout getter steps are to return this’s timeout.

The timeout setter steps are:

1. If the current global object is a Window object and this’s synchronous flag is set, then throw an "InvalidAccessError" DOMException.

2. Set this’s timeout to the given value.

This implies that the timeout attribute can be set while fetching is in progress. If that occurs it will still be measured relative to the start of fetching.

3.5.4. The withCredentials getter and setter

client . withCredentials

True when credentials are to be included in a cross-origin request. False when they are to be excluded in a cross-origin request and when cookies are to be ignored in its response. Initially false.

When set: throws an "InvalidStateError" DOMException if state is not unsent or opened, or if the send() flag is set.

The withCredentials getter steps are to return this’s cross-origin credentials.

The withCredentials setter steps are:

1. If this’s state is not unsent or opened, then throw an "InvalidStateError" DOMException.

2. If this’s send() flag is set, then throw an "InvalidStateError" DOMException.

3. Set this’s cross-origin credentials to the given value.

3.5.5. The upload getter

client . upload

Returns the associated XMLHttpRequestUpload object. It can be used to gather transmission information when data is transferred to a server.

The upload getter steps are to return this’s upload object.

3.5.6. The send() method

client . send([body = null])

Initiates the request. The body argument provides the request body, if any, and is ignored if the request method is GET or HEAD.

Throws an "InvalidStateError" DOMException if either state is not opened or the send() flag is set.

The send(body) method steps are:

1. If this’s state is not opened, then throw an "InvalidStateError" DOMException.

2. If this’s send() flag is set, then throw an "InvalidStateError" DOMException.

3. If this’s request method is `GET` or `HEAD`, then set body to null.

4. If body is not null, then:

   1. Let extractedContentType be null.

   2. If body is a Document, then set this’s request body to body, serialized, converted, and UTF-8 encoded.

   3. Otherwise:

      1. Let bodyWithType be the result of safely extracting body.

      2. Set this’s request body to bodyWithType’s body.

      3. Set extractedContentType to bodyWithType’s type.

   4. Let originalAuthorContentType be the result of getting `Content-Type` from this’s author request headers.

   5. If originalAuthorContentType is non-null, then:

      1. If body is a Document or a USVString, then:

         1. Let contentTypeRecord be the result of parsing originalAuthorContentType.

         2. If contentTypeRecord is not failure, contentTypeRecord’s parameters["charset"] exists, and parameters["charset"] is not an ASCII case-insensitive match for "UTF-8", then:

            1. Set contentTypeRecord’s parameters["charset"] to "UTF-8".

            2. Let newContentTypeSerialized be the result of serializing contentTypeRecord.

            3. Set (`Content-Type`, newContentTypeSerialized) in this’s author request headers.

   6. Otherwise:

      1. If body is an HTML document, then set (`Content-Type`, `text/html;charset=UTF-8`) in this’s author request headers.

      2. Otherwise, if body is an XML document, set (`Content-Type`, `application/xml;charset=UTF-8`) in this’s author request headers.

      3. Otherwise, if extractedContentType is not null, set (`Content-Type`, extractedContentType) in this’s author request headers.

5. If one or more event listeners are registered on this’s upload object, then set this’s upload listener flag.

6. Let req be a new request, initialized as follows:

   method

   This’s request method.

   URL

   This’s request URL.

   header list

   This’s author request headers.

   unsafe-request flag

   Set.

   body

   This’s request body.

   client

   This’s relevant settings object.

   mode

   "cors".

   use-CORS-preflight flag

   Set if this’s upload listener flag is set.

   credentials mode

   If this’s cross-origin credentials is true, then "include"; otherwise "same-origin".

   use-URL-credentials flag

   Set if this’s request URL includes credentials.

   initiator type

   "xmlhttprequest".

7. Unset this’s upload complete flag.

8. Unset this’s timed out flag.

9. If req’s body is null, then set this’s upload complete flag.

10. Set this’s send() flag.

11. If this’s synchronous flag is unset, then:

    1. Fire a progress event named loadstart at this with 0 and 0.

    2. Let requestBodyTransmitted be 0.

    3. Let requestBodyLength be req’s body’s length, if req’s body is non-null; otherwise 0.

    4. Assert: requestBodyLength is an integer.

    5. If this’s upload complete flag is unset and this’s upload listener flag is set, then fire a progress event named loadstart at this’s upload object with requestBodyTransmitted and requestBodyLength.

    6. If this’s state is not opened or this’s send() flag is unset, then return.

    7. Let processRequestBodyChunkLength, given a bytesLength, be these steps:

       1. Increase requestBodyTransmitted by bytesLength.

       2. If not roughly 50ms have passed since these steps were last invoked, then return.

       3. If this’s upload listener flag is set, then fire a progress event named progress at this’s upload object with requestBodyTransmitted and requestBodyLength.

       These steps are only invoked when new bytes are transmitted.

    8. Let processRequestEndOfBody be these steps:

       1. Set this’s upload complete flag.

       2. If this’s upload listener flag is unset, then return.

       3. Fire a progress event named progress at this’s upload object with requestBodyTransmitted and requestBodyLength.

       4. Fire a progress event named load at this’s upload object with requestBodyTransmitted and requestBodyLength.

       5. Fire a progress event named loadend at this’s upload object with requestBodyTransmitted and requestBodyLength.

    9. Let processResponse, given a response, be these steps:

       1. Set this’s response to response.

       2. Handle errors for this.

       3. If this’s response is a network error, then return.

       4. Set this’s state to headers received.

       5. Fire an event named readystatechange at this.

       6. If this’s state is not headers received, then return.

       7. If this’s response’s body is null, then run handle response end-of-body for this and return.

       8. Let length be the result of extracting a length from this’s response’s header list.

       9. If length is not an integer, then set it to 0.

       10. Let processBodyChunk given bytes be these steps:

           1. Append bytes to this’s received bytes.

           2. If not roughly 50ms have passed since these steps were last invoked, then return.

           3. If this’s state is headers received, then set this’s state to loading.

           4. Fire an event named readystatechange at this.

              Web compatibility is the reason readystatechange fires more often than this’s state changes.

           5. Fire a progress event named progress at this with this’s received bytes’s length and length.

       11. Let processEndOfBody be this step: run handle response end-of-body for this.

       12. Let processBodyError be these steps:

           1. Set this’s response to a network error.

           2. Run handle errors for this.

       13. Incrementally read this’s response’s body, given processBodyChunk, processEndOfBody, processBodyError, and this’s relevant global object.

    10. Set this’s fetch controller to the result of fetching req with processRequestBodyChunkLength set to processRequestBodyChunkLength, processRequestEndOfBody set to processRequestEndOfBody, and processResponse set to processResponse.

    11. Let now be the present time.

    12. Run these steps in parallel:

        1. Wait until either req’s done flag is set or this’s timeout is not 0 and this’s timeout milliseconds have passed since now.

        2. If req’s done flag is unset, then set this’s timed out flag and terminate this’s fetch controller.

12. Otherwise, if this’s synchronous flag is set:

    1. Let processedResponse be false.

    2. Let processResponseConsumeBody, given a response and nullOrFailureOrBytes, be these steps:

       1. If nullOrFailureOrBytes is not failure, then set this’s response to response.

       2. If nullOrFailureOrBytes is a byte sequence, then append nullOrFailureOrBytes to this’s received bytes.

       3. Set processedResponse to true.

    3. Set this’s fetch controller to the result of fetching req with processResponseConsumeBody set to processResponseConsumeBody and useParallelQueue set to true.

    4. Let now be the present time.

    5. Pause until either processedResponse is true or this’s timeout is not 0 and this’s timeout milliseconds have passed since now.

    6. If processedResponse is false, then set this’s timed out flag and terminate this’s fetch controller.

    7. Report timing for this’s fetch controller given the current global object.

    8. Run handle response end-of-body for this.

To handle response end-of-body for an XMLHttpRequest object xhr, run these steps:

1. Handle errors for xhr.

2. If xhr’s response is a network error, then return.

3. Let transmitted be xhr’s received bytes’s length.

4. Let length be the result of extracting a length from this’s response’s header list.

5. If length is not an integer, then set it to 0.

6. If xhr’s synchronous flag is unset, then fire a progress event named progress at xhr with transmitted and length.

7. Set xhr’s state to done.

8. Unset xhr’s send() flag.

9. Fire an event named readystatechange at xhr.

10. Fire a progress event named load at xhr with transmitted and length.

11. Fire a progress event named loadend at xhr with transmitted and length.

To handle errors for an XMLHttpRequest object xhr, run these steps:

1. If xhr’s send() flag is unset, then return.

2. If xhr’s timed out flag is set, then run the request error steps for xhr, timeout, and "TimeoutError" DOMException.

3. Otherwise, if xhr’s response’s aborted flag is set, run the request error steps for xhr, abort, and "AbortError" DOMException.

4. Otherwise, if xhr’s response is a network error, then run the request error steps for xhr, error, and "NetworkError" DOMException.

The request error steps for an XMLHttpRequest object xhr, event, and optionally exception are:

1. Set xhr’s state to done.

2. Unset xhr’s send() flag.

3. Set xhr’s response to a network error.

4. If xhr’s synchronous flag is set, then throw exception.

5. Fire an event named readystatechange at xhr.

   At this point it is clear that xhr’s synchronous flag is unset.

6. If xhr’s upload complete flag is unset, then:

   1. Set xhr’s upload complete flag.

   2. If xhr’s upload listener flag is set, then:

      1. Fire a progress event named event at xhr’s upload object with 0 and 0.

      2. Fire a progress event named loadend at xhr’s upload object with 0 and 0.

7. Fire a progress event named event at xhr with 0 and 0.

8. Fire a progress event named loadend at xhr with 0 and 0.

3.5.7. The abort() method

client . abort()

Cancels any network activity.

The abort() method steps are:

1. Abort this’s fetch controller.

2. If this’s state is opened with this’s send() flag set, headers received, or loading, then run the request error steps for this and abort.

3. If this’s state is done, then set this’s state to unsent and this’s response to a network error.

   No readystatechange event is dispatched.

3.6. Response

3.6.1. The responseURL getter

The responseURL getter steps are to return the empty string if this’s response’s URL is null; otherwise its serialization with the exclude fragment flag set.

3.6.2. The status getter

The status getter steps are to return this’s response’s status.

3.6.3. The statusText getter

The statusText getter steps are to return this’s response’s status message.

3.6.4. The getResponseHeader() method

The getResponseHeader(name) method steps are to return the result of getting name from this’s response’s header list.

The Fetch Standard filters this’s response’s header list. [FETCH]

var client = new XMLHttpRequest();
client.open("GET", "unicorns-are-awesome.txt", true);
client.send();
client.onreadystatechange = function() {
  if(this.readyState == this.HEADERS_RECEIVED) {
    print(client.getResponseHeader("Content-Type"));
  }
}

text/plain; charset=UTF-8

3.6.5. The getAllResponseHeaders() method

A byte sequence a is legacy-uppercased-byte less than a byte sequence b if the following steps return true:

1. Let A be a, byte-uppercased.

2. Let B be b, byte-uppercased.

3. Return A is byte less than B.

The getAllResponseHeaders() method steps are:

1. Let output be an empty byte sequence.

2. Let initialHeaders be the result of running sort and combine with this’s response’s header list.

3. Let headers be the result of sorting initialHeaders in ascending order, with a being less than b if a’s name is legacy-uppercased-byte less than b’s name.

   Unfortunately, this is needed for compatibility with deployed content.

4. For each header in headers, append header’s name, followed by a 0x3A 0x20 byte pair, followed by header’s value, followed by a 0x0D 0x0A byte pair, to output.

5. Return output.

The Fetch Standard filters this’s response’s header list. [FETCH]

var client = new XMLHttpRequest();
client.open("GET", "narwhals-too.txt", true);
client.send();
client.onreadystatechange = function() {
  if(this.readyState == this.HEADERS_RECEIVED) {
    print(this.getAllResponseHeaders());
  }
}

connection: Keep-Alive
content-type: text/plain; charset=utf-8
date: Sun, 24 Oct 2004 04:58:38 GMT
keep-alive: timeout=15, max=99
server: Apache/1.3.31 (Unix)
transfer-encoding: chunked

3.6.6. Response body

To get a response MIME type for an XMLHttpRequest object xhr, run these steps:

1. Let mimeType be the result of extracting a MIME type from xhr’s response’s header list.

2. If mimeType is failure, then set mimeType to text/xml.

3. Return mimeType.

To get a final MIME type for an XMLHttpRequest object xhr, run these steps:

1. If xhr’s override MIME type is null, return the result of get a response MIME type for xhr.

2. Return xhr’s override MIME type.

To get a final encoding for an XMLHttpRequest object xhr, run these steps:

1. Let label be null.

2. Let responseMIME be the result of get a response MIME type for xhr.

3. If responseMIME’s parameters["charset"] exists, then set label to it.

4. If xhr’s override MIME type’s parameters["charset"] exists, then set label to it.

5. If label is null, then return null.

6. Let encoding be the result of getting an encoding from label.

7. If encoding is failure, then return null.

8. Return encoding.

The above steps intentionally do not use the get a final MIME type as it would not be web compatible.

To set a document response for an XMLHttpRequest object xhr, run these steps:

1. If xhr’s response’s body is null, then return.

2. Let finalMIME be the result of get a final MIME type for xhr.

3. If finalMIME is not an HTML MIME type or an XML MIME type, then return.

4. If xhr’s response type is the empty string and finalMIME is an HTML MIME type, then return.

   This is restricted to xhr’s response type being "document" in order to prevent breaking legacy content.

5. If finalMIME is an HTML MIME type, then:

   1. Let charset be the result of get a final encoding for xhr.

   2. If charset is null, prescan the first 1024 bytes of xhr’s received bytes and if that does not terminate unsuccessfully then let charset be the return value.

   3. If charset is null, then set charset to UTF-8.

   4. Let document be a document that represents the result parsing xhr’s received bytes following the rules set forth in the HTML Standard for an HTML parser with scripting disabled and a known definite encoding charset. [HTML]

   5. Flag document as an HTML document.

6. Otherwise, let document be a document that represents the result of running the XML parser with XML scripting support disabled on xhr’s received bytes. If that fails (unsupported character encoding, namespace well-formedness error, etc.), then return null. [HTML]

   Resources referenced will not be loaded and no associated XSLT will be applied.

7. If charset is null, then set charset to UTF-8.

8. Set document’s encoding to charset.

9. Set document’s content type to finalMIME.

10. Set document’s URL to xhr’s response’s URL.

11. Set document’s origin to xhr’s relevant settings object’s origin.

12. Set xhr’s response object to document.

To get a text response for an XMLHttpRequest object xhr, run these steps:

1. If xhr’s response’s body is null, then return the empty string.

2. Let charset be the result of get a final encoding for xhr.

3. If xhr’s response type is the empty string, charset is null, and the result of get a final MIME type for xhr is an XML MIME type, then use the rules set forth in the XML specifications to determine the encoding. Let charset be the determined encoding. [XML] [XML-NAMES]

   This is restricted to xhr’s response type being the empty string to keep the non-legacy response type value "text" simple.

4. If charset is null, then set charset to UTF-8.

5. Return the result of running decode on xhr’s received bytes using fallback encoding charset.

Authors are strongly encouraged to always encode their resources using UTF-8.

3.6.7. The overrideMimeType() method

client . overrideMimeType(mime)

Acts as if the `Content-Type` header value for a response is mime. (It does not change the header.)

Throws an "InvalidStateError" DOMException if state is loading or done.

The overrideMimeType(mime) method steps are:

1. If this’s state is loading or done, then throw an "InvalidStateError" DOMException.

2. Set this’s override MIME type to the result of parsing mime.

3. If this’s override MIME type is failure, then set this’s override MIME type to application/octet-stream.

3.6.8. The responseType getter and setter

client . responseType [ = value ]

Returns the response type.

Can be set to change the response type. Values are: the empty string (default), "arraybuffer", "blob", "document", "json", and "text".

When set: setting to "document" is ignored if the current global object is not a Window object.

When set: throws an "InvalidStateError" DOMException if state is loading or done.

When set: throws an "InvalidAccessError" DOMException if the synchronous flag is set and the current global object is a Window object.

The responseType getter steps are to return this’s response type.

The responseType setter steps are:

1. If the current global object is not a Window object and the given value is "document", then return.

2. If this’s state is loading or done, then throw an "InvalidStateError" DOMException.

3. If the current global object is a Window object and this’s synchronous flag is set, then throw an "InvalidAccessError" DOMException.

4. Set this’s response type to the given value.

3.6.9. The response getter

client . response

Returns the response body.

The response getter steps are:

1. If this’s response type is the empty string or "text", then:

   1. If this’s state is not loading or done, then return the empty string.

   2. Return the result of getting a text response for this.

2. If this’s state is not done, then return null.

3. If this’s response object is failure, then return null.

4. If this’s response object is non-null, then return it.

5. If this’s response type is "arraybuffer", then set this’s response object to a new ArrayBuffer object representing this’s received bytes. If this throws an exception, then set this’s response object to failure and return null.

   Allocating an ArrayBuffer object is not guaranteed to succeed. [ECMASCRIPT]

6. Otherwise, if this’s response type is "blob", set this’s response object to a new Blob object representing this’s received bytes with type set to the result of get a final MIME type for this.

7. Otherwise, if this’s response type is "document", set a document response for this.

8. Otherwise:

   1. Assert: this’s response type is "json".

   2. If this’s response’s body is null, then return null.

   3. Let jsonObject be the result of running parse JSON from bytes on this’s received bytes. If that threw an exception, then return null.

   4. Set this’s response object to jsonObject.

9. Return this’s response object.

3.6.10. The responseText getter

client . responseText

Returns response as text.

Throws an "InvalidStateError" DOMException if responseType is not the empty string or "text".

The responseText getter steps are:

1. If this’s response type is not the empty string or "text", then throw an "InvalidStateError" DOMException.

2. If this’s state is not loading or done, then return the empty string.

3. Return the result of getting a text response for this.

3.6.11. The responseXML getter

client . responseXML

Returns the response as document.

Throws an "InvalidStateError" DOMException if responseType is not the empty string or "document".

The responseXML getter steps are:

1. If this’s response type is not the empty string or "document", then throw an "InvalidStateError" DOMException.

2. If this’s state is not done, then return null.

3. Assert: this’s response object is not failure.

4. If this’s response object is non-null, then return it.

5. Set a document response for this.

6. Return this’s response object.

3.7. Events summary

This section is non-normative.

The following events are dispatched on XMLHttpRequest or XMLHttpRequestUpload objects:

4. Interface FormData

typedef (File or USVString) FormDataEntryValue;

[Exposed=(Window,Worker)]
interface FormData {
  constructor(optional HTMLFormElement form, optional HTMLElement? submitter = null);

  undefined append(USVString name, USVString value);
  undefined append(USVString name, Blob blobValue, optional USVString filename);
  undefined delete(USVString name);
  FormDataEntryValue? get(USVString name);
  sequence<FormDataEntryValue> getAll(USVString name);
  boolean has(USVString name);
  undefined set(USVString name, USVString value);
  undefined set(USVString name, Blob blobValue, optional USVString filename);
  iterable<USVString, FormDataEntryValue>;
};

Each FormData object has an associated entry list (an entry list). It is initially empty.

This section used to define entry, an entry’s name and value, and the create an entry algorithm. These definitions have been moved to the HTML Standard. [HTML]

The new FormData(form, submitter) constructor steps are:

1. If form is given, then:

   1. If submitter is non-null, then:

      1. If submitter is not a submit button, then throw a TypeError.

      2. If submitter’s form owner is not form, then throw a "NotFoundError" DOMException.

   2. Let list be the result of constructing the entry list for form and submitter.

   3. If list is null, then throw an "InvalidStateError" DOMException.

   4. Set this’s entry list to list.

The append(name, value) and append(name, blobValue, filename) method steps are:

1. Let value be value if given; otherwise blobValue.

2. Let entry be the result of creating an entry with name, value, and filename if given.

3. Append entry to this’s entry list.

The reason there is an argument named value as well as blobValue is due to a limitation of the editing software used to write the XMLHttpRequest Standard.

The delete(name) method steps are to remove all entries whose name is name from this’s entry list.

The get(name) method steps are:

1. If there is no entry whose name is name in this’s entry list, then return null.

2. Return the value of the first entry whose name is name from this’s entry list.

The getAll(name) method steps are:

1. If there is no entry whose name is name in this’s entry list, then return the empty list.

2. Return the values of all entries whose name is name, in order, from this’s entry list.

The has(name) method steps are to return true if there is an entry whose name is name in this’s entry list; otherwise false.

The set(name, value) and set(name, blobValue, filename) method steps are:

1. Let value be value if given; otherwise blobValue.

2. Let entry be the result of creating an entry with name, value, and filename if given.

3. If there are entries in this’s entry list whose name is name, then replace the first such entry with entry and remove the others.

4. Otherwise, append entry to this’s entry list.

The reason there is an argument named value as well as blobValue is due to a limitation of the editing software used to write the XMLHttpRequest Standard.

The value pairs to iterate over are this’s entry list’s entries with the key being the name and the value being the value.

5. Interface ProgressEvent

[Exposed=(Window,Worker)]
interface ProgressEvent : Event {
  constructor(DOMString type, optional ProgressEventInit eventInitDict = {});

  readonly attribute boolean lengthComputable;
  readonly attribute unsigned long long loaded;
  readonly attribute unsigned long long total;
};

dictionary ProgressEventInit : EventInit {
  boolean lengthComputable = false;
  unsigned long long loaded = 0;
  unsigned long long total = 0;
};

Events using the ProgressEvent interface indicate some kind of progression.

The lengthComputable, loaded, and total getter steps are to return the value they were initialized to.

5.1. Firing events using the ProgressEvent interface

To fire a progress event named e at target, given transmitted and length, means to fire an event named e at target, using ProgressEvent, with the loaded attribute initialized to transmitted, and if length is not 0, with the lengthComputable attribute initialized to true and the total attribute initialized to length.

5.2. Suggested names for events using the ProgressEvent interface

This section is non-normative.

The suggested type attribute values for use with events using the ProgressEvent interface are summarized in the table below. Specification editors are free to tune the details to their specific scenarios, though are strongly encouraged to discuss their usage with the WHATWG community to ensure input from people familiar with the subject.

The error, abort, timeout, and load event types are mutually exclusive.

Throughout the web platform the error, abort, timeout and load event types have their bubbles and cancelable attributes initialized to false, so it is suggested that for consistency all events using the ProgressEvent interface do the same.

5.3. Security considerations

For cross-origin requests some kind of opt-in, e.g., the CORS protocol defined in the Fetch Standard, has to be used before events using the ProgressEvent interface are dispatched as information (e.g., size) would be revealed that cannot be obtained otherwise. [FETCH]

5.4. Example

<!DOCTYPE html>
<title>Waiting for Magical Unicorns</title>
<progress id=p></progress>
<script>
  var progressBar = document.getElementById("p"),
      client = new XMLHttpRequest()
  client.open("GET", "magical-unicorns")
  client.onprogress = function(pe) {
    if(pe.lengthComputable) {
      progressBar.max = pe.total
      progressBar.value = pe.loaded
    }
  }
  client.onloadend = function(pe) {
    progressBar.value = pe.loaded
  }
  client.send()
</script>

Acknowledgments

Thanks to Addison Phillips, Adrian Bateman, Ahmed Kamel, Alan Thomas, Alex Hopmann, Alex Vincent, Alexey Proskuryakov, Ali Alabbas, Andrea Marchesini, Asbjørn Ulsberg, Bertrand Guay-Paquet, Björn Höhrmann, Boris Zbarsky, Caitlin Potter, Cameron McCormack, 白丞祐 (Cheng-You Bai), Chris Marrin, Christophe Jolif, Charles McCathieNevile, Chirag S Kumar, Dan Winship, David Andersson, David Flanagan, David Håsäther, David Levin, Dean Jackson, Denis Sureau, Domenic Denicola, Dominik Röttsches, Doug Schepers, Douglas Livingstone, Elliott Sprehn, Elliotte Harold, Eric Lawrence, Eric Uhrhane, Erik Arvidsson, Erik Dahlström, Feras Moussa, Gideon Cohn, Glenn Adams, Gorm Haug Eriksen, Gregory Terzian, Håkon Wium Lie, Hallvord R. M. Steen, Henri Sivonen, Hiroshige Hayashizaki, Huub Schaeks, Ian Clelland, Ian Davis, Ian Hickson, Ivan Herman, Jake Archibald, Jared Jacobs, Jarred Nicholls, Jeff Walden, Jens Lindström, Jim Deegan, Jim Ley, Joe Farro, Jonas Sicking, Julian Reschke, 송정기 (Jungkee Song), 呂康豪 (Kang-Hao Lu), Karl Dubost, Keith Yeung, 田村健人 (Kent TAMURA), Lachlan Hunt, Maciej Stachowiak, Magnus Kristiansen, Manish Goregaokar, Marc Hadley, Marcos Caceres, Mark Baker, Mark Birbeck, Mark Nottingham, Mark S. Miller, Martin Hassman, Mike Pennisi, Mohamed Zergaoui, Ms2ger, Noam Rosenthal, Odin Hørthe Omdal, Olli Pettay, Pawel Glowacki, Peter Michaux, Philip Jägenstedt, Philip Taylor, Rashika Jaggi, Robin Berjon, Rune F. Halvorsen, Ruud Steltenpool, Ryo Onodera, Sam Sneddon, Sergiu Dumitriu, Shivakumar Jagalur Matt, Sigbjørn Finne, Simon Pieters, Stewart Brodie, Sunava Dutta, Takeshi Kurosawa, Takeshi Yoshino, Thomas Roessler, Thomas Wisniewski, Tom Magliery, Travis Leithead, triple-underscore, Yaron Tausky, Yehuda Katz, Youenn Fablet, and Zhenbin Xu for their contributions to this standard.

Special thanks to the Microsoft employees who first implemented the XMLHttpRequest interface, which was first widely deployed by the Windows Internet Explorer browser.

Special thanks to Ian Hickson for drafting an initial version of this specification in the HTML Standard (then Web Applications 1.0). [HTML]

Special thanks to the W3C SVG WG for drafting the original ProgressEvent class as part of the SVG Micro DOM.

This standard is written by Anne van Kesteren (Apple, [email protected]).

Intellectual property rights

Copyright © WHATWG (Apple, Google, Mozilla, Microsoft). This work is licensed under a Creative Commons Attribution 4.0 International License. To the extent portions of it are incorporated into source code, such portions in the source code are licensed under the BSD 3-Clause License instead.

This is the Living Standard. Those interested in the patent-review version should view the Living Standard Review Draft.