XMLHttpRequest - Web APIs 编辑

XMLHttpRequest (XHR) objects are used to interact with servers. You can retrieve data from a URL without having to do a full page refresh. This enables a Web page to update just part of a page without disrupting what the user is doing. XMLHttpRequest is used heavily in AJAX programming.

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Despite its name, XMLHttpRequest can be used to retrieve any type of data, not just XML.

If your communication needs to involve receiving event data or message data from a server, consider using server-sent events through the EventSource interface. For full-duplex communication, WebSockets may be a better choice.

Note:

Constructor

XMLHttpRequest()

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

Properties

This interface also inherits properties of XMLHttpRequestEventTarget and of EventTarget.

XMLHttpRequest.onreadystatechange

An EventHandler that is called whenever the readyState attribute changes.

XMLHttpRequest.readyState Read only

Returns an unsigned short, the state of the request.

XMLHttpRequest.response Read only

Returns an ArrayBuffer, Blob, Document, JavaScript object, or a DOMString, depending on the value of XMLHttpRequest.responseType, that contains the response entity body.

XMLHttpRequest.responseText Read only

Returns a DOMString that contains the response to the request as text, or null if the request was unsuccessful or has not yet been sent.

XMLHttpRequest.responseType

Is an enumerated value that defines the response type.

XMLHttpRequest.responseURL Read only

Returns the serialized URL of the response or the empty string if the URL is null.

XMLHttpRequest.responseXML Read only

Returns a Document containing the response to the request, or null if the request was unsuccessful, has not yet been sent, or cannot be parsed as XML or HTML. Not available in workers.

XMLHttpRequest.status Read only

Returns an unsigned short with the status of the response of the request.

XMLHttpRequest.statusText Read only

Returns a DOMString containing the response string returned by the HTTP server. Unlike XMLHttpRequest.status, this includes the entire text of the response message ("200 OK", for example).

Note: According to the HTTP/2 specification (8.1.2.4 Response Pseudo-Header Fields), HTTP/2 does not define a way to carry the version or reason phrase that is included in an HTTP/1.1 status line.

XMLHttpRequest.timeout

Is an unsigned long representing the number of milliseconds a request can take before automatically being terminated.

XMLHttpRequestEventTarget.ontimeout

Is an EventHandler that is called whenever the request times out.

XMLHttpRequest.upload Read only

Is an XMLHttpRequestUpload, representing the upload process.

XMLHttpRequest.withCredentials

Is a Boolean that indicates whether or not cross-site Access-Control requests should be made using credentials such as cookies or authorization headers.

Non-standard properties

XMLHttpRequest.channelRead only

Is a nsIChannel. The channel used by the object when performing the request.

XMLHttpRequest.mozAnonRead only

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

XMLHttpRequest.mozSystemRead only

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

XMLHttpRequest.mozBackgroundRequest

Is a boolean. It indicates whether or not the object represents a background service request.

XMLHttpRequest.mozResponseArrayBuffer Obsolete since Gecko 6 Read only

ArrayBuffer. The response to the request, as a JavaScript typed array.

XMLHttpRequest.multipartObsolete since Gecko 22

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

Event handlers

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

Since then, a number of additional event handlers have been implemented in various browsers (onload, onerror, onprogress, etc.). See 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.

Methods

XMLHttpRequest.abort()

Aborts the request if it has already been sent.

XMLHttpRequest.getAllResponseHeaders()

Returns all the response headers, separated by CRLF, as a string, or null if no response has been received.

XMLHttpRequest.getResponseHeader()

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.

XMLHttpRequest.open()

Initializes a request.

XMLHttpRequest.overrideMimeType()

Overrides the MIME type returned by the server.

XMLHttpRequest.send()

Sends the request. If the request is asynchronous (which is the default), this method returns as soon as the request is sent.

XMLHttpRequest.setRequestHeader()

Sets the value of an HTTP request header. You must call setRequestHeader()after open(), but before send().

Non-standard methods

XMLHttpRequest.init()

Initializes the object for use from C++ code. Warning: This method must not be called from JavaScript.

XMLHttpRequest.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().

XMLHttpRequest.sendAsBinary()

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

Events

abort

Fired when a request has been aborted, for example because the program called XMLHttpRequest.abort().
Also available via the onabort property.

error

Fired when the request encountered an error.
Also available via the onerror property.

load

Fired when an XMLHttpRequest transaction completes successfully.
Also available via the onload property.

loadend

Fired when a request has completed, whether successfully (after load) or unsuccessfully (after abort or error).
Also available via the onloadend property.

loadstart

Fired when a request has started to load data.
Also available via the onloadstart property.

progress

Fired periodically when a request receives more data.
Also available via the onprogress property.

timeout

Fired when progress is terminated due to preset time expiring.
Also available via the ontimeout property.

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• XMLSerializer: Serializing a DOM tree into XML
• MDN tutorials covering XMLHttpRequest:
  • Ajax — Getting Started
  • Using XMLHttpRequest
  • HTML in XMLHttpRequest
  • Fetch API
• HTML5 Rocks — New Tricks in XMLHttpRequest2
• Feature-Policy directive sync-xhr