XMLHttpRequest

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This Is a Commit Snapshot of the Standard

This document contains the contents of the standard as of the 6f14c60f5ae81381c12ff50f5f6898a1583b570d commit, and should only be used as a historical reference. This commit may not even have been merged into master.

Do not attempt to implement this version of the specification. Do not reference this version as authoritative in any way. Instead, see https://xhr.spec.whatwg.org/ for the living standard.

Abstract

The XMLHttpRequest Standard defines an API that provides scripted client functionality for transferring data between a client and a server.

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.

Some simple code to do something with data from an XML document fetched over the network:

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();

If you just want to log a message to the server:

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

Or if you want to check the status of a document on the server:

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:

2. Conformance

All diagrams, examples, and notes in this specification are non-normative, as are all sections explicitly marked non-normative. Everything else in this specification is normative.

The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this specification are to be interpreted as described in RFC2119. For readability, these words do not appear in all uppercase letters in this specification. [RFC2119]

2.1. Extensibility

User agents, Working Groups, and other interested parties are strongly encouraged to discuss new features with the WHATWG community.

3. Terminology

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

[DOM] [DOMPS] [ENCODING] [FETCH] [FILEAPI] [HTML] [HTTP] [URL] [WEBIDL] [XML] [XMLNS]

It uses the typographic conventions from HTML. [HTML]

4. 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"
};

[Constructor,
 Exposed=(Window,DedicatedWorker,SharedWorker)]
interface XMLHttpRequest : XMLHttpRequestEventTarget {
  // 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
  void open(ByteString method, USVString url);
  void open(ByteString method, USVString url, boolean async, optional USVString? username = null, optional USVString? password = null);
  void setRequestHeader(ByteString name, ByteString value);
           attribute unsigned long timeout;
           attribute boolean withCredentials;
  [SameObject] readonly attribute XMLHttpRequestUpload upload;
  void send(optional (Document or BodyInit)? body = null);
  void abort();

  // response
  readonly attribute USVString responseURL;
  readonly attribute unsigned short status;
  readonly attribute ByteString statusText;
  ByteString? getResponseHeader(ByteString name);
  ByteString getAllResponseHeaders();
  void 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 XMLHttpRequestUpload object.

An XMLHttpRequest object has an associated state, which is one of unsent, opened, headers received, loading, and done. Unless stated otherwise it is unsent.

An XMLHttpRequest object has an associated send() flag. Unless stated otherwise it is unset.

4.1. Constructors

client = new XMLHttpRequest()
Returns a new XMLHttpRequest object.

The XMLHttpRequest() constructor, when invoked, must return a new XMLHttpRequest object.

4.2. 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 ongoing fetch operated by the XMLHttpRequest object.

4.3. 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:

event handler event handler event type
onloadstart loadstart
onprogress progress
onabort abort
onerror error
onload load
ontimeout timeout
onloadend loadend

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

event handler event handler event type
onreadystatechange readystatechange

4.4. States

client . readyState

Returns client’s state.

The readyState attribute’s getter must 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 context object’s state:

unsent UNSENT (numeric value 0) The object has been constructed.
opened OPENED (numeric value 1) The open() method has been successfully invoked. During this state request headers can be set using setRequestHeader() and the fetch can be initiated using the send() method.
headers received HEADERS_RECEIVED (numeric value 2) All redirects (if any) have been followed and all HTTP headers of the response have been received.
loading LOADING (numeric value 3) The response’s body is being received.
done DONE (numeric value 4) The data transfer has been completed or something went wrong during the transfer (e.g. infinite redirects).

4.5. Request

Each XMLHttpRequest object has the following request-associated concepts: request method, request URL, author request headers, request body, synchronous flag, upload complete flag, upload listener flag, and timed out flag.

The author request headers is an initially empty header list.

The request body is initially null.

The synchronous flag, upload complete flag, upload listener flag and timed out flag are initially unset.

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.)


4.5.1. 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 exception if either method is not a valid HTTP method or url cannot be parsed.

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

Throws an InvalidAccessError exception if async is false, 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 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 exception when it occurs.

The open(method, url) and open(method, url, async, username, password) methods, when invoked, must run these steps:

  1. Let settingsObject be context object’s relevant settings object.

  2. If settingsObject has a responsible document and it is not fully active, then throw an "InvalidStateError" DOMException.

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

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

  5. Normalize method.

  6. Let parsedURL be the result of parsing url with settingsObject’s API base URL and settingsObject’s API URL character encoding.

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

  8. 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.

  9. If parsedURL’s host is non-null, run these substeps:

    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.

  10. If async is false, current global object is a Window object, and the timeout attribute value is not zero or the responseType attribute value is not the empty string, then throw an "InvalidAccessError" DOMException.

  11. Terminate the ongoing fetch operated by the XMLHttpRequest object.

    A fetch can be ongoing at this point.

  12. Set variables associated with the object as follows:

  13. If the state is not opened, run these substeps:

    1. Set state to opened.

    2. Fire an event named readystatechange.

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

4.5.2. The setRequestHeader() method

client . setRequestHeader(name, value)

Combines a header in author request headers.

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

Throws a SyntaxError exception if name is not a header name or if value is not a header value.

As indicated in the algorithm below certain headers cannot be set and are left up to the user agent. In addition there are certain other headers the user agent will take control of if they are not set by the author as indicated at the end of the send() method section.

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

  1. If state is not opened, throw an InvalidStateError exception.

  2. If the send() flag is set, throw an InvalidStateError exception.

  3. Normalize value.

  4. If name is not a name or value is not a value, throw a SyntaxError exception.

    An empty byte sequence represents an empty header value.

  5. Terminate these steps if name is a forbidden header name.

  6. Combine name/value in author request headers.

Some simple code demonstrating what happens when setting the same header twice:

// 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

4.5.3. The timeout attribute

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 the synchronous flag is unset, a timeout event will then be dispatched, or a TimeoutError exception will be thrown otherwise (for the send() method).

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

The timeout attribute must return its value. Initially its value must be zero.

Setting the timeout attribute must run these steps:

  1. If current global object is a Window object and the synchronous flag is set, throw an InvalidAccessError exception.

  2. Set its value to the new 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.

4.5.4. The withCredentials attribute

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 exception if state is not unsent or opened, or if the send() flag is set.

The withCredentials attribute must return its value. Initially its value must be false.

Setting the withCredentials attribute must run these steps:

  1. If state is not unsent or opened, throw an InvalidStateError exception.

  2. If the send() flag is set, throw an InvalidStateError exception.

  3. Set the withCredentials attribute’s value to the given value.

The withCredentials attribute has no effect when fetching same-origin resources.

4.5.5. The upload attribute

client . upload

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

The upload attribute must return the associated XMLHttpRequestUpload object.

As indicated earlier, each XMLHttpRequest object has an associated XMLHttpRequestUpload object.

4.5.6. The send() method

client . send([body = null])

Initiates the request. The optional argument provides the request body. The argument is ignored if request method is GET or HEAD.

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

The send(body) method must run these steps:

  1. If state is not opened, throw an InvalidStateError exception.

  2. If the send() flag is set, throw an InvalidStateError exception.

  3. If the request method is GET or HEAD, set body to null.

  4. If body is null, go to the next step.

    Otherwise, let encoding and mimeType be null, and then follow these rules, switching on body:

    Document

    Set encoding to `UTF-8`.

    Set mimeType to `text/html` if body is an HTML document, and to `application/xml` otherwise. Then append `;charset=UTF-8` to mimeType.

    Set request body to body, serialized, converted to Unicode, and utf-8 encoded.

    BodyInit

    If body is a string, set encoding to `UTF-8`.

    Set request body and mimeType to the result of extracting body.

    If mimeType is non-null and author request headers does not contain `Content-Type`, then append `Content-Type`/mimeType to author request headers.

    Otherwise, if the header whose name is a byte-case-insensitive match for `Content-Type` in author request headers has a value that is a valid MIME type, which has a `charset` parameter whose value is not a byte-case-insensitive match for encoding, and encoding is not null, then set all the `charset` parameters whose value is not a byte-case-insensitive match for encoding of that header’s value to encoding.

  5. If one or more event listeners are registered on the associated XMLHttpRequestUpload object, then set upload listener flag.

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

    method
    request method
    url
    request URL
    header list
    author request headers
    unsafe-request flag
    Set.
    body
    request body
    client
    context object’s relevant settings object
    synchronous flag
    Set if the synchronous flag is set.
    mode
    "cors"
    use-CORS-preflight flag
    Set if upload listener flag is set.
    credentials mode
    If the withCredentials attribute value is true, "include", and "same-origin" otherwise.
    use-URL-credentials flag
    Set if either request URL’s username is not the empty string or request URL’s password is non-null.
  7. Unset the upload complete flag.

  8. Unset the timed out flag.

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

  10. Set the send() flag.

  11. If the synchronous flag is unset, run these substeps:

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

    2. If the upload complete flag is unset and upload listener flag is set, then fire a progress event named loadstart on the XMLHttpRequestUpload object with 0 and req’s body’s total bytes.

    3. If state is not opened or the send() flag is unset, then return.

    4. Fetch req. Handle the tasks queued on the networking task source per below.

      Run these subsubsteps in parallel:

      1. Wait until either req’s done flag is set or

        1. the timeout attribute value number of milliseconds has passed since these subsubsteps started

        2. while timeout attribute value is not zero.

      2. If req’s done flag is unset, then set the timed out flag and terminate fetching.

      To process request body for request, run these subsubsteps:

      1. If not roughly 50ms have passed since these subsubsteps were last invoked, terminate these subsubsteps.

      2. If upload listener flag is set, then fire a progress event named progress on the XMLHttpRequestUpload object with request’s body’s transmitted bytes and request’s body’s total bytes.

      These subsubsteps are only invoked when new bytes are transmitted.

      To process request end-of-body for request, run these subsubsteps:

      1. Set the upload complete flag.

      2. If upload listener flag is unset, then terminate these subsubsteps.

      3. Let transmitted be request’s body’s transmitted bytes.

      4. Let length be request’s body’s total bytes.

      5. Fire a progress event named progress on the XMLHttpRequestUpload object with transmitted and length.

      6. Fire a progress event named load on the XMLHttpRequestUpload object with transmitted and length.

      7. Fire a progress event named loadend on the XMLHttpRequestUpload object with transmitted and length.

      To process response for response, run these subsubsteps:

      1. Set response to response.

      2. Handle errors for response.

      3. If response is a network error, return.

      4. Set state to headers received.

      5. Fire an event named readystatechange.

      6. If state is not headers received, then return.

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

      8. Let reader be the result of getting a reader from response’s body’s stream.

        This operation will not throw an exception.

      9. Let read be the result of reading a chunk from response’s body’s stream with reader.

        When read is fulfilled with an object whose done property is false and whose value property is a Uint8Array object, run these subsubsubsteps and then run the above subsubstep again:

        1. Append the value property to received bytes.

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

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

        4. Fire an event named readystatechange.

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

        5. Fire a progress event named progress with response’s body’s transmitted bytes and response’s body’s total bytes.

        These subsubsubsteps are only invoked when new bytes are transmitted.

        When read is fulfilled with an object whose done property is true, run handle response end-of-body for response.

        When read is rejected with an exception, run handle errors for response.

  12. Otherwise, if the synchronous flag is set, run these substeps:

    1. Let response be the result of fetching req.

      If the timeout attribute value is not zero, then set the timed out flag and terminate fetching if it has not returned within the amount of milliseconds from the timeout.

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

    3. Let reader be the result of getting a reader from response’s body’s stream.

      This operation will not throw an exception.

    4. Let promise be the result of reading all bytes from response’s body’s stream with reader.

    5. Wait for promise to be fulfilled or rejected.

    6. If promise is fulfilled with bytes, then append bytes to received bytes.

    7. Run handle response end-of-body for response.

To handle response end-of-body for response, run these steps:

  1. If the synchronous flag is set, set response to response.

  2. Handle errors for response.

  3. If response is a network error, return.

  4. If the synchronous flag is unset, update response’s body using response.

  5. Let transmitted be response’s body’s transmitted bytes.

  6. Let length be response’s body’s total bytes.

  7. Fire a progress event named progress with transmitted and length.

  8. Set state to done.

  9. Unset the send() flag.

  10. Fire an event named readystatechange.

  11. Fire a progress event named load with transmitted and length.

  12. Fire a progress event named loadend with transmitted and length.

To handle errors for response run these steps:

  1. If the send() flag is unset, return.

  2. If response is a network error, run the request error steps for event error and exception NetworkError.

  3. If the timed out flag is set, then run the request error steps for event timeout and exception TimeoutError.

  4. Otherwise, if response’s body’s stream is errored, then:

    1. Set state to done.

    2. Unset the send() flag.

    3. Set response to a network error.

  5. Otherwise, if response’s aborted flag is set, then run the request error steps for event abort and exception AbortError.

The request error steps for event event and optionally an exception exception are:

  1. Set state to done.

  2. Unset the send() flag.

  3. Set response to a network error.

  4. If the synchronous flag is set, throw an exception exception.

  5. Fire an event named readystatechange.

    At this point it is clear that the synchronous flag is unset.

  6. If the upload complete flag is unset, follow these substeps:

    1. Set the upload complete flag.

    2. If upload listener flag is unset, then terminate these substeps.

    3. Fire a progress event named event on the XMLHttpRequestUpload object with 0 and 0.

    4. Fire a progress event named loadend on the XMLHttpRequestUpload object with 0 and 0.

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

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

4.5.7. The abort() method

client . abort()
Cancels any network activity.

The abort() method, when invoked, must run these steps:

  1. Terminate the ongoing fetch with the aborted flag set.

  2. If state is either opened with the send() flag set, headers received, or loading, run the request error steps for event abort.

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

    No readystatechange event is dispatched.

4.6. Response

An XMLHttpRequest has an associated response. Unless stated otherwise it is a network error.

An XMLHttpRequest also has an associated received bytes (a byte sequence). Unless stated otherwise it is the empty byte sequence.

4.6.1. The responseURL attribute

The responseURL attribute must return the empty string if response’s url is null and its serialization with the exclude fragment flag set otherwise.

4.6.2. The status attribute

The status attribute must return the response’s status.

4.6.3. The statusText attribute

The statusText attribute must return the response’s status message.

4.6.4. The getResponseHeader() method

The getResponseHeader(name) method must run these steps:

  1. If response’s header list does not contain name, then return null.

  2. Return the combined value given name and response’s header list.

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

For the following script:

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

The print() function will get to process something like:

text/plain; charset=UTF-8

4.6.5. The getAllResponseHeaders() method

The getAllResponseHeaders() method, when invoked, must run these steps:

  1. Let output be an empty byte sequence.

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

  3. 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.

  4. Return output.

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

For the following script:

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());
  }
}

The print() function will get to process something like:

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

4.6.6. Response body

The response MIME type is the result of running these steps:

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

  2. If mimeType is the empty byte sequence, then set mimeType to `text/xml`.

  3. Return mimeType.

The override MIME type is initially null and can get a value if overrideMimeType() is invoked. Final MIME type is the override MIME type unless that is null in which case it is the response MIME type.

The response charset is the value of the charset parameter of the `Content-Type` header or null if there was no `charset` parameter or the header could not be parsed or was omitted. The override charset is initially null and can get a value if overrideMimeType() is invoked. Final charset is the override charset unless that is null in which case it is the response charset.


An XMLHttpRequest object has an associated response object (an object, failure, or null). Unless stated otherwise it is null.

An arraybuffer response is the return value of these steps:

  1. Set response object to a new ArrayBuffer object representing received bytes. If this throws an exception, then set response object to failure and return null.

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

  2. Return response object.

A blob response is the return value of these steps:

  1. Set response object to a new Blob object representing received bytes with type final MIME type.

  2. Return response object.

A document response is the return value of these steps:

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

  2. If final MIME type is not an HTML MIME type or an XML MIME type, then return null.

  3. If responseType is the empty string and final MIME type is an HTML MIME type, then return null.

    This is restricted to responseType being "document" in order to prevent breaking legacy content.

  4. If final MIME type is an HTML MIME type, then run these substeps:

    1. Let charset be the final charset.

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

    3. If charset is null, set charset to utf-8.

    4. Let document be a document that represents the result parsing 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.

  5. Otherwise, let document be a document that represents the result of running the XML parser with XML scripting support disabled on 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.

  6. If charset is null, set charset to utf-8.

  7. Set document’s encoding to charset.

  8. Set document’s content type to final MIME type.

  9. Set document’s URL to response’s url.

  10. Set document’s origin to context object’s relevant settings object’s origin.

  11. Set response object to document and return it.

A JSON response is the return value of these steps:

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

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

  3. Set response object to jsonObject and return it.

A text response is the return value of these steps:

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

  2. Let charset be the final charset.

  3. If responseType is the empty string, charset is null, and final MIME type 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] [XMLNS]

    This is restricted to responseType being the empty string to keep the non-legacy responseType value "text" simple.

  4. If charset is null, set charset to utf-8.

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

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

4.6.7. The overrideMimeType() method

client . overrideMimeType(mime)

Acts as if the `Content-Type` header for response is mime.

Throws an InvalidStateError exception if state is loading or done.

The overrideMimeType(mime) method must run these steps:

  1. If state is loading or done, throw an InvalidStateError exception.

  2. Set override MIME type to `application/octet-stream`.

  3. If mime is a parsable MIME type, then set override MIME type to its MIME type portion.

  4. If override MIME type has a `charset` parameter, then set override charset to its value.

4.6.8. The responseType attribute

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 current global object is not a Window object.

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

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

The responseType attribute must return its value. Initially its value must be the empty string.

Setting the responseType attribute must run these steps:

  1. If current global object is not a Window object and the given value is "document", terminate these steps.

  2. If state is loading or done, throw an InvalidStateError exception.

  3. If current global object is a Window object and the synchronous flag is set, throw an InvalidAccessError exception.

  4. Set the responseType attribute’s value to the given value.

4.6.9. The response attribute

client . response

Returns the response’s body.

The response attribute must return the result of running these steps:

If responseType is the empty string or "text"
  1. If state is not loading or done, return the empty string.

  2. Return the text response.

Otherwise
  1. If state is not done, return null.

  2. If response object is failure, then return null.

  3. If response object is non-null, then return it.

  4. If responseType is "arraybuffer"

    Return the arraybuffer response.

    If responseType is "blob"

    Return the blob response.

    If responseType is "document"

    Return the document response.

    If responseType is "json"

    Return the JSON response.

4.6.10. The responseText attribute

client . responseText

Returns the text response.

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

The responseText attribute must return the result of running these steps:

  1. If responseType is not the empty string or "text", throw an InvalidStateError exception.

  2. If state is not loading or done, return the empty string.

  3. Return the text response.

4.6.11. The responseXML attribute

client . responseXML

Returns the document response.

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

The responseXML attribute must return the result of running these steps:

  1. If responseType is not the empty string or "document", throw an InvalidStateError exception.

  2. If state is not done, return null.

  3. If response object is non-null, then return it.

  4. Return the document response.

The responseXML attribute has XML in its name for historical reasons. It also returns HTML resources as documents.

4.7. Events summary

This section is non-normative.

The following events are dispatched on XMLHttpRequest and/or XMLHttpRequestUpload objects:

Event name Interface Dispatched when…
readystatechange Event The readyState attribute changes value, except when it changes to UNSENT.
loadstart ProgressEvent The fetch initiates.
progress ProgressEvent Transmitting data.
abort ProgressEvent When the fetch has been aborted. For instance, by invoking the abort() method.
error ProgressEvent The fetch failed.
load ProgressEvent The fetch succeeded.
timeout ProgressEvent The author specified timeout has passed before the fetch completed.
loadend ProgressEvent The fetch completed (success or failure).

5. Interface FormData

typedef (File or USVString) FormDataEntryValue;

[Constructor(optional HTMLFormElement form),
 Exposed=(Window,Worker)]
interface FormData {
  void append(USVString name, USVString value);
  void append(USVString name, Blob blobValue, optional USVString filename);
  void delete(USVString name);
  FormDataEntryValue? get(USVString name);
  sequence<FormDataEntryValue> getAll(USVString name);
  boolean has(USVString name);
  void set(USVString name, USVString value);
  void set(USVString name, Blob blobValue, optional USVString filename);
  iterable<USVString, FormDataEntryValue>;
};

The FormData object represents an ordered list of entries. Each entry consists of a name and a value.

For the purposes of interaction with other algorithms, an entry’s type is "string" if its value is a string and "file" otherwise. If an entry’s type is "file", its filename is the empty string if value is not a File object, and otherwise its filename is the value of entry’s value’s name attribute.

To create an entry for name, value, and optionally a filename, run these steps:

  1. Let entry be a new entry.

  2. Set entry’s name to name.

  3. If value is a Blob object and not a File object, then set value to a new File object, representing the same bytes, whose name attribute value is "blob".

  4. If value is (now) a File object and filename is given, then set value to a new File object, representing the same bytes, whose name attribute value is filename.

  5. Set entry’s value to value.

  6. Return entry.


The FormData(form) constructor must run these steps:

  1. Let fd be a new FormData object.

  2. If form is given, set fd’s entries to the result of constructing the form data set for form.

  3. Return fd.

The append(name, value) and append(name, blobValue, filename) methods, when invoked, must run these steps:

  1. Let value be value if given, and blobValue otherwise.

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

  3. Append entry to context object’s list of entries.

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, when invoked, must remove all entries whose name is name.

The get(name) method, when invoked, must return the value of the first entry whose name is name, and null otherwise.

The getAll(name) method, when invoked, must return the values of all entries whose name is name, in list order, and the empty sequence otherwise.

The has(name) method, when invoked, must return true if there is an entry whose name is name, and false otherwise.

The set(name, value) and set(name, blobValue, filename) methods, when invoked, must run these steps:

  1. Let value be value if given, and blobValue otherwise.

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

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

  4. Otherwise, append entry to context object’s list of entries.

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 the entries with the key being the name and the value the value.

6. Interface ProgressEvent

[Constructor(DOMString type, optional ProgressEventInit eventInitDict),
 Exposed=(Window,DedicatedWorker,SharedWorker)]
interface ProgressEvent : Event {
  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 attributes must return the value they were initialized to.

6.1. 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.

6.2. 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.

type attribute value Description Times When
loadstart Progress has begun. Once. First.
progress In progress. Once or more. After loadstart has been dispatched.
error Progression failed. Zero or once (mutually exclusive). After the last progress has been dispatched.
abort Progression is terminated.
timeout Progression is terminated due to preset time expiring.
load Progression is successful.
loadend Progress has stopped. Once. After one of error, abort, timeout or load has been dispatched.

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.

6.3. 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]

6.4. Example

In this example XMLHttpRequest, combined with concepts defined in the sections before, and the HTML progress element are used together to display the process of fetching a resource.

<!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>

Fully working code would of course be more elaborate and deal with more scenarios, such as network errors or the end user terminating the request.

References

Normative References

[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[DOMPS]
Travis Leithead. DOM Parsing and Serialization. URL: https://w3c.github.io/DOM-Parsing/
[ECMASCRIPT]
ECMAScript Language Specification. URL: https://tc39.github.io/ecma262/
[ENCODING]
Anne van Kesteren. Encoding Standard. Living Standard. URL: https://encoding.spec.whatwg.org/
[FETCH]
Anne van Kesteren. Fetch Standard. Living Standard. URL: https://fetch.spec.whatwg.org/
[FILEAPI]
Marijn Kruisselbrink. File API. URL: https://w3c.github.io/FileAPI/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[HTTP]
R. Fielding, Ed.; J. Reschke, Ed.. Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing. June 2014. Proposed Standard. URL: https://tools.ietf.org/html/rfc7230
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
[MIMESNIFF]
Gordon P. Hemsley. MIME Sniffing Standard. Living Standard. URL: https://mimesniff.spec.whatwg.org/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[URL]
Anne van Kesteren. URL Standard. Living Standard. URL: https://url.spec.whatwg.org/
[WEBIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. URL: https://heycam.github.io/webidl/
[XML]
Tim Bray; et al. Extensible Markup Language (XML) 1.0 (Fifth Edition). 26 November 2008. REC. URL: https://www.w3.org/TR/xml/
[XMLNS]
Tim Bray; et al. Namespaces in XML 1.0 (Third Edition). 8 December 2009. REC. URL: https://www.w3.org/TR/xml-names/

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, Chris Marrin, Christophe Jolif, Charles McCathieNevile, 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, Geoffrey Sneddon, Gideon Cohn, Glenn Adams, Gorm Haug Eriksen, Håkon Wium Lie, Hallvord R. M. Steen, Henri Sivonen, Hiroshige Hayashizaki, Huub Schaeks, 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, Lachlan Hunt, Maciej Stachowiak, Magnus Kristiansen, Manish Goregaokar, Marc Hadley, Marcos Caceres, Mark Baker, Mark Birbeck, Mark Nottingham, Mark S. Miller, Martin Hassman, Mohamed Zergaoui, Ms2ger, Odin Hørthe Omdal, Olli Pettay, Pawel Glowacki, Peter Michaux, Philip Jägenstedt, Philip Taylor, Robin Berjon, Rune F. Halvorsen, Ruud Steltenpool, 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, 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 (Mozilla, annevk@annevk.nl).

Per CC0, to the extent possible under law, the editor has waived all copyright and related or neighboring rights to this work.

Index

Terms defined by this specification

Terms defined by reference

IDL Index

[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"
};

[Constructor,
 Exposed=(Window,DedicatedWorker,SharedWorker)]
interface XMLHttpRequest : XMLHttpRequestEventTarget {
  // 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
  void open(ByteString method, USVString url);
  void open(ByteString method, USVString url, boolean async, optional USVString? username = null, optional USVString? password = null);
  void setRequestHeader(ByteString name, ByteString value);
           attribute unsigned long timeout;
           attribute boolean withCredentials;
  [SameObject] readonly attribute XMLHttpRequestUpload upload;
  void send(optional (Document or BodyInit)? body = null);
  void abort();

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

[Constructor(optional HTMLFormElement form),
 Exposed=(Window,Worker)]
interface FormData {
  void append(USVString name, USVString value);
  void append(USVString name, Blob blobValue, optional USVString filename);
  void delete(USVString name);
  FormDataEntryValue? get(USVString name);
  sequence<FormDataEntryValue> getAll(USVString name);
  boolean has(USVString name);
  void set(USVString name, USVString value);
  void set(USVString name, Blob blobValue, optional USVString filename);
  iterable<USVString, FormDataEntryValue>;
};
[Constructor(DOMString type, optional ProgressEventInit eventInitDict),
 Exposed=(Window,DedicatedWorker,SharedWorker)]
interface ProgressEvent : Event {
  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;
};