XMLHttpRequest
makes sending HTTP requests very easy. You simply create an instance of the object, open a URL, and send the request. The HTTP status of the result, as well as the result's contents, are available in the request object when the transaction is completed. This page outlines some of the common, and even slightly obscure, use cases for this powerful JavaScript object.
function reqListener () { console.log(this.responseText); } var oReq = new XMLHttpRequest(); oReq.addEventListener("load", reqListener); oReq.open("GET", "https://www.example.org/example.txt"); oReq.send();
Types of requests
A request made via XMLHttpRequest can fetch the data in one of two ways, asynchronously or synchronously. The type of request is dictated by the optional async
argument (the third argument) that is set on the XMLHttpRequest open() method. If this argument is true
or not specified, the XMLHttpRequest
is processed asynchronously, otherwise the process is handled synchronously. A detailed discussion and demonstrations of these two types of requests can be found on the synchronous and asynchronous requests page. In general, you should rarely if ever use synchronous requests.
Handling responses
There are several types of response attributes defined by the W3C specification for XMLHttpRequest. These tell the client making the XMLHttpRequest important information about the status of the response. Some cases where dealing with non-text response types may involve some manipulation and analysis are outlined in the following sections.
Analyzing and manipulating the responseXML
property
If you use XMLHttpRequest
to get the content of a remote XML document, the responseXML
property will be a DOM Object containing a parsed XML document. This could prove difficult to manipulate and analyze. There are five primary ways of analyzing this XML document:
- Using XPath to address (or point to) parts of it.
- Using JXON to convert it into a JavaScript Object tree.
- Manually Parsing and serializing XML to strings or objects.
- Using XMLSerializer to serialize DOM trees to strings or to files.
- RegExp can be used if you always know the content of the XML document beforehand. You might want to remove line breaks, if you use RegExp to scan with regard to linebreaks. However, this method is a "last resort" since if the XML code changes slightly, the method will likely fail.
Analyzing and manipulating a responseText
property containing an HTML document
XMLHttpRequest.responseXML
property. Read the article about HTML in XMLHttpRequest for details.If you use XMLHttpRequest
to get the content of a remote HTML webpage, the responseText
property is a string containing a "soup" of all the HTML tags. This could prove difficult to manipulate and analyze. There are three primary ways of analyzing this HTML soup string:
- Use the
XMLHttpRequest.responseXML
property. - Inject the content into the body of a document fragment via
fragment.body.innerHTML
and traverse the DOM of the fragment. - RegExp can be used if you always know the content of the HTML
responseText
beforehand. You might want to remove line breaks, if you use RegExp to scan with regard to linebreaks. However, this method is a "last resort" since if the HTML code changes slightly, the method will likely fail.
Handling binary data
Although XMLHttpRequest
is most commonly used to send and receive textual data, it can be used to send and receive binary content. There are several well tested methods for coercing the response of an XMLHttpRequest into sending binary data. These involve utilizing the .overrideMimeType()
method on the XMLHttpRequest object and is a workable solution.
var oReq = new XMLHttpRequest(); oReq.open("GET", url); // retrieve data unprocessed as a binary string oReq.overrideMimeType("text/plain; charset=x-user-defined"); /* ... */
The XMLHttpRequest Level 2 Specification adds new responseType attributes which make sending and receiving binary data much easier.
var oReq = new XMLHttpRequest(); oReq.onload = function(e) { var arraybuffer = oReq.response; // not responseText /* ... */ } oReq.open("GET", url); oReq.responseType = "arraybuffer"; oReq.send();
For more examples check out the Sending and Receiving Binary Data page
Monitoring progress
XMLHttpRequest
provides the ability to listen to various events that can occur while the request is being processed. This includes periodic progress notifications, error notifications, and so forth.
Support for DOM progress event monitoring of XMLHttpRequest
transfers follows the Web API specification for progress events: these events implement the ProgressEvent
interface.
var oReq = new XMLHttpRequest(); oReq.addEventListener("progress", updateProgress); oReq.addEventListener("load", transferComplete); oReq.addEventListener("error", transferFailed); oReq.addEventListener("abort", transferCanceled); oReq.open(); // ... // progress on transfers from the server to the client (downloads) function updateProgress (oEvent) { if (oEvent.lengthComputable) { var percentComplete = oEvent.loaded / oEvent.total; // ... } else { // Unable to compute progress information since the total size is unknown } } function transferComplete(evt) { console.log("The transfer is complete."); } function transferFailed(evt) { console.log("An error occurred while transferring the file."); } function transferCanceled(evt) { console.log("The transfer has been canceled by the user."); }
Lines 3-6 add event listeners for the various events that are sent while performing a data transfer using XMLHttpRequest
.
open()
on the request. Otherwise the progress events will not fire.The progress event handler, specified by the updateProgress()
function in this example, receives the total number of bytes to transfer as well as the number of bytes transferred so far in the event's total
and loaded
fields. However, if the lengthComputable
field is false, the total length is not known and will be zero.
Progress events exist for both download and upload transfers. The download events are fired on the XMLHttpRequest
object itself, as shown in the above sample. The upload events are fired on the XMLHttpRequest.upload
object, as shown below:
var oReq = new XMLHttpRequest(); oReq.upload.addEventListener("progress", updateProgress); oReq.upload.addEventListener("load", transferComplete); oReq.upload.addEventListener("error", transferFailed); oReq.upload.addEventListener("abort", transferCanceled); oReq.open();
file:
protocol.Note: Starting in Gecko 9.0, progress events can now be relied upon to come in for every chunk of data received, including the last chunk in cases in which the last packet is received and the connection closed before the progress event is fired. In this case, the progress event is automatically fired when the load event occurs for that packet. This lets you now reliably monitor progress by only watching the "progress" event.
Note: As of Gecko 12.0, if your progress event is called with a responseType
of "moz-blob", the value of response is a Blob
containing the data received so far.
One can also detect all three load-ending conditions (abort
, load
, or error
) using the loadend
event:
req.addEventListener("loadend", loadEnd); function loadEnd(e) { console.log("The transfer finished (although we don't know if it succeeded or not)."); }
Note there is no way to be certain, from the information received by the loadend
event, as to which condition caused the operation to terminate; however, you can use this to handle tasks that need to be performed in all end-of-transfer scenarios.
Submitting forms and uploading files
Instances of XMLHttpRequest
can be used to submit forms in two ways:
- using only AJAX
- using the
FormData
API
Using the FormData
API is the simplest and fastest, but has the disadvantage that data collected can not be stringified.
Using only AJAX is more complex, but also lends itself to be the most flexible and powerful way.
Using nothing but XMLHttpRequest
Submitting forms without the FormData
API does not require other APIs for most use cases. The only case where you need an additional API is if you want to upload one or more files, where you use the FileReader
API.
A brief introduction to the submit methods
An html <form>
can be sent in four ways:
- using the
POST
method and setting theenctype
attribute toapplication/x-www-form-urlencoded
(default); - using the
POST
method and setting theenctype
attribute totext/plain
; - using the
POST
method and setting theenctype
attribute tomultipart/form-data
; - using the
GET
method (in this case theenctype
attribute will be ignored).
Now, consider to submit a form containing only two fields, named foo
and baz
. If you are using the POST
method the server will receive a string similar to one of the following three example, depending on the encoding type you are using:
-
Method:
POST
; Encoding type:application/x-www-form-urlencoded
(default):Content-Type: application/x-www-form-urlencoded foo=bar&baz=The+first+line.%0D%0AThe+second+line.%0D%0A
-
Method:
POST
; Encoding type:text/plain
:Content-Type: text/plain foo=bar baz=The first line. The second line.
-
Method:
POST
; Encoding type:multipart/form-data
:Content-Type: multipart/form-data; boundary=---------------------------314911788813839 -----------------------------314911788813839 Content-Disposition: form-data; name="foo" bar -----------------------------314911788813839 Content-Disposition: form-data; name="baz" The first line. The second line. -----------------------------314911788813839--
However, if you are using the GET
method, a string like the following will be simply added to the URL:
?foo=bar&baz=The%20first%20line.%0AThe%20second%20line.
A little vanilla framework
All these effects are done automatically by the web browser whenever you submit a <form>
. If you want to perform the same effects using JavaScript you have to instruct the interpreter about everything. Therefore, how to send forms in pure AJAX is too complex to be explained here in detail. For this reason, here we place a complete (yet didactic) framework, able to use all four ways to submit, and to upload files:
<!doctype html> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Sending forms with pure AJAX – MDN</title> <script type="text/javascript"> "use strict"; /*\ |*| |*| :: XMLHttpRequest.prototype.sendAsBinary() Polyfill :: |*| |*| https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest#sendAsBinary() \*/ if (!XMLHttpRequest.prototype.sendAsBinary) { XMLHttpRequest.prototype.sendAsBinary = function(sData) { var nBytes = sData.length, ui8Data = new Uint8Array(nBytes); for (var nIdx = 0; nIdx < nBytes; nIdx++) { ui8Data[nIdx] = sData.charCodeAt(nIdx) & 0xff; } /* send as ArrayBufferView...: */ this.send(ui8Data); /* ...or as ArrayBuffer (legacy)...: this.send(ui8Data.buffer); */ }; } /*\ |*| |*| :: AJAX Form Submit Framework :: |*| |*| https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest/Using_XMLHttpRequest |*| |*| This framework is released under the GNU Public License, version 3 or later. |*| https://www.gnu.org/licenses/gpl-3.0-standalone.html |*| |*| Syntax: |*| |*| AJAXSubmit(HTMLFormElement); \*/ var AJAXSubmit = (function () { function ajaxSuccess () { /* console.log("AJAXSubmit - Success!"); */ console.log(this.responseText); /* you can get the serialized data through the "submittedData" custom property: */ /* console.log(JSON.stringify(this.submittedData)); */ } function submitData (oData) { /* the AJAX request... */ var oAjaxReq = new XMLHttpRequest(); oAjaxReq.submittedData = oData; oAjaxReq.onload = ajaxSuccess; if (oData.technique === 0) { /* method is GET */ oAjaxReq.open("get", oData.receiver.replace(/(?:\?.*)?$/, oData.segments.length > 0 ? "?" + oData.segments.join("&") : ""), true); oAjaxReq.send(null); } else { /* method is POST */ oAjaxReq.open("post", oData.receiver, true); if (oData.technique === 3) { /* enctype is multipart/form-data */ var sBoundary = "---------------------------" + Date.now().toString(16); oAjaxReq.setRequestHeader("Content-Type", "multipart\/form-data; boundary=" + sBoundary); oAjaxReq.sendAsBinary("--" + sBoundary + "\r\n" + oData.segments.join("--" + sBoundary + "\r\n") + "--" + sBoundary + "--\r\n"); } else { /* enctype is application/x-www-form-urlencoded or text/plain */ oAjaxReq.setRequestHeader("Content-Type", oData.contentType); oAjaxReq.send(oData.segments.join(oData.technique === 2 ? "\r\n" : "&")); } } } function processStatus (oData) { if (oData.status > 0) { return; } /* the form is now totally serialized! do something before sending it to the server... */ /* doSomething(oData); */ /* console.log("AJAXSubmit - The form is now serialized. Submitting..."); */ submitData (oData); } function pushSegment (oFREvt) { this.owner.segments[this.segmentIdx] += oFREvt.target.result + "\r\n"; this.owner.status--; processStatus(this.owner); } function plainEscape (sText) { /* how should I treat a text/plain form encoding? what characters are not allowed? this is what I suppose...: */ /* "4\3\7 - Einstein said E=mc2" ----> "4\\3\\7\ -\ Einstein\ said\ E\=mc2" */ return sText.replace(/[\s\=\\]/g, "\\$&"); } function SubmitRequest (oTarget) { var nFile, sFieldType, oField, oSegmReq, oFile, bIsPost = oTarget.method.toLowerCase() === "post"; /* console.log("AJAXSubmit - Serializing form..."); */ this.contentType = bIsPost && oTarget.enctype ? oTarget.enctype : "application\/x-www-form-urlencoded"; this.technique = bIsPost ? this.contentType === "multipart\/form-data" ? 3 : this.contentType === "text\/plain" ? 2 : 1 : 0; this.receiver = oTarget.action; this.status = 0; this.segments = []; var fFilter = this.technique === 2 ? plainEscape : escape; for (var nItem = 0; nItem < oTarget.elements.length; nItem++) { oField = oTarget.elements[nItem]; if (!oField.hasAttribute("name")) { continue; } sFieldType = oField.nodeName.toUpperCase() === "INPUT" ? oField.getAttribute("type").toUpperCase() : "TEXT"; if (sFieldType === "FILE" && oField.files.length > 0) { if (this.technique === 3) { /* enctype is multipart/form-data */ for (nFile = 0; nFile < oField.files.length; nFile++) { oFile = oField.files[nFile]; oSegmReq = new FileReader(); /* (custom properties:) */ oSegmReq.segmentIdx = this.segments.length; oSegmReq.owner = this; /* (end of custom properties) */ oSegmReq.onload = pushSegment; this.segments.push("Content-Disposition: form-data; name=\"" + oField.name + "\"; filename=\""+ oFile.name + "\"\r\nContent-Type: " + oFile.type + "\r\n\r\n"); this.status++; oSegmReq.readAsBinaryString(oFile); } } else { /* enctype is application/x-www-form-urlencoded or text/plain or method is GET: files will not be sent! */ for (nFile = 0; nFile < oField.files.length; this.segments.push(fFilter(oField.name) + "=" + fFilter(oField.files[nFile++].name))); } } else if ((sFieldType !== "RADIO" && sFieldType !== "CHECKBOX") || oField.checked) { /* field type is not FILE or is FILE but is empty */ this.segments.push( this.technique === 3 ? /* enctype is multipart/form-data */ "Content-Disposition: form-data; name=\"" + oField.name + "\"\r\n\r\n" + oField.value + "\r\n" : /* enctype is application/x-www-form-urlencoded or text/plain or method is GET */ fFilter(oField.name) + "=" + fFilter(oField.value) ); } } processStatus(this); } return function (oFormElement) { if (!oFormElement.action) { return; } new SubmitRequest(oFormElement); }; })(); </script> </head> <body> <h1>Sending forms with pure AJAX</h1> <h2>Using the GET method</h2> <form action="register.php" method="get" onsubmit="AJAXSubmit(this); return false;"> <fieldset> <legend>Registration example</legend> <p> First name: <input type="text" name="firstname" /><br /> Last name: <input type="text" name="lastname" /> </p> <p> <input type="submit" value="Submit" /> </p> </fieldset> </form> <h2>Using the POST method</h2> <h3>Enctype: application/x-www-form-urlencoded (default)</h3> <form action="register.php" method="post" onsubmit="AJAXSubmit(this); return false;"> <fieldset> <legend>Registration example</legend> <p> First name: <input type="text" name="firstname" /><br /> Last name: <input type="text" name="lastname" /> </p> <p> <input type="submit" value="Submit" /> </p> </fieldset> </form> <h3>Enctype: text/plain</h3> <form action="register.php" method="post" enctype="text/plain" onsubmit="AJAXSubmit(this); return false;"> <fieldset> <legend>Registration example</legend> <p> Your name: <input type="text" name="user" /> </p> <p> Your message:<br /> <textarea name="message" cols="40" rows="8"></textarea> </p> <p> <input type="submit" value="Submit" /> </p> </fieldset> </form> <h3>Enctype: multipart/form-data</h3> <form action="register.php" method="post" enctype="multipart/form-data" onsubmit="AJAXSubmit(this); return false;"> <fieldset> <legend>Upload example</legend> <p> First name: <input type="text" name="firstname" /><br /> Last name: <input type="text" name="lastname" /><br /> Sex: <input id="sex_male" type="radio" name="sex" value="male" /> <label for="sex_male">Male</label> <input id="sex_female" type="radio" name="sex" value="female" /> <label for="sex_female">Female</label><br /> Password: <input type="password" name="secret" /><br /> What do you prefer: <select name="image_type"> <option>Books</option> <option>Cinema</option> <option>TV</option> </select> </p> <p> Post your photos: <input type="file" multiple name="photos[]"> </p> <p> <input id="vehicle_bike" type="checkbox" name="vehicle[]" value="Bike" /> <label for="vehicle_bike">I have a bike</label><br /> <input id="vehicle_car" type="checkbox" name="vehicle[]" value="Car" /> <label for="vehicle_car">I have a car</label> </p> <p> Describe yourself:<br /> <textarea name="description" cols="50" rows="8"></textarea> </p> <p> <input type="submit" value="Submit" /> </p> </fieldset> </form> </body> </html>
To test this, create a page named register.php (which is the action
attribute of these sample forms), and put the following minimalistic content:
<?php /* register.php */ header("Content-type: text/plain"); /* NOTE: You should never use `print_r()` in production scripts, or otherwise output client-submitted data without sanitizing it first. Failing to sanitize can lead to cross-site scripting vulnerabilities. */ echo ":: data received via GET ::\n\n"; print_r($_GET); echo "\n\n:: Data received via POST ::\n\n"; print_r($_POST); echo "\n\n:: Data received as \"raw\" (text/plain encoding) ::\n\n"; if (isset($HTTP_RAW_POST_DATA)) { echo $HTTP_RAW_POST_DATA; } echo "\n\n:: Files received ::\n\n"; print_r($_FILES);
The syntax to activate this script is simply:
AJAXSubmit(myForm);
FileReader
API to transmit file uploads. This is a recent API and is not implemented in IE9 or below. For this reason, the AJAX-only upload is considered an experimental technique. If you do not need to upload binary files, this framework works fine in most browsers.send()
method and possibly the readAsArrayBuffer()
method of the FileReader
API. But, since the aim of this script is to work with a stringifiable raw data, we used the sendAsBinary()
method in conjunction with the readAsBinaryString()
method of the FileReader
API. As such, the above script makes sense only when you are dealing with small files. If you do not intend to upload binary content, consider instead using the FormData
API.sendAsBinary
method is considered deprecated as of Gecko 31 (Firefox 31 / Thunderbird 31 / SeaMonkey 2.28) and will be removed soon. The standard send(Blob data)
method can be used instead.Using FormData objects
The FormData
constructor lets you compile a set of key/value pairs to send using XMLHttpRequest
. Its primary use is in sending form data, but can also be used independently from a form in order to transmit user keyed data. The transmitted data is in the same format the form's submit()
method uses to send data, if the form's encoding type were set to "multipart/form-data". FormData objects can be utilized in a number of ways with an XMLHttpRequest. For examples, and explanations of how one can utilize FormData with XMLHttpRequests, see the Using FormData Objects page. For didactic purposes here is a translation of the previous example transformed to use the FormData
API. Note the brevity of the code:
<!doctype html> <html> <head> <meta http-equiv="Content-Type" charset="UTF-8" /> <title>Sending forms with FormData – MDN</title> <script> "use strict"; function ajaxSuccess () { console.log(this.responseText); } function AJAXSubmit (oFormElement) { if (!oFormElement.action) { return; } var oReq = new XMLHttpRequest(); oReq.onload = ajaxSuccess; if (oFormElement.method.toLowerCase() === "post") { oReq.open("post", oFormElement.action); oReq.send(new FormData(oFormElement)); } else { var oField, sFieldType, nFile, sSearch = ""; for (var nItem = 0; nItem < oFormElement.elements.length; nItem++) { oField = oFormElement.elements[nItem]; if (!oField.hasAttribute("name")) { continue; } sFieldType = oField.nodeName.toUpperCase() === "INPUT" ? oField.getAttribute("type").toUpperCase() : "TEXT"; if (sFieldType === "FILE") { for (nFile = 0; nFile < oField.files.length; sSearch += "&" + escape(oField.name) + "=" + escape(oField.files[nFile++].name)); } else if ((sFieldType !== "RADIO" && sFieldType !== "CHECKBOX") || oField.checked) { sSearch += "&" + escape(oField.name) + "=" + escape(oField.value); } } oReq.open("get", oFormElement.action.replace(/(?:\?.*)?$/, sSearch.replace(/^&/, "?")), true); oReq.send(null); } } </script> </head> <body> <h1>Sending forms with FormData</h1> <h2>Using the GET method</h2> <form action="register.php" method="get" onsubmit="AJAXSubmit(this); return false;"> <fieldset> <legend>Registration example</legend> <p> First name: <input type="text" name="firstname" /><br /> Last name: <input type="text" name="lastname" /> </p> <p> <input type="submit" value="Submit" /> </p> </fieldset> </form> <h2>Using the POST method</h2> <h3>Enctype: application/x-www-form-urlencoded (default)</h3> <form action="register.php" method="post" onsubmit="AJAXSubmit(this); return false;"> <fieldset> <legend>Registration example</legend> <p> First name: <input type="text" name="firstname" /><br /> Last name: <input type="text" name="lastname" /> </p> <p> <input type="submit" value="Submit" /> </p> </fieldset> </form> <h3>Enctype: text/plain</h3> <p>The text/plain encoding is not supported by the FormData API.</p> <h3>Enctype: multipart/form-data</h3> <form action="register.php" method="post" enctype="multipart/form-data" onsubmit="AJAXSubmit(this); return false;"> <fieldset> <legend>Upload example</legend> <p> First name: <input type="text" name="firstname" /><br /> Last name: <input type="text" name="lastname" /><br /> Sex: <input id="sex_male" type="radio" name="sex" value="male" /> <label for="sex_male">Male</label> <input id="sex_female" type="radio" name="sex" value="female" /> <label for="sex_female">Female</label><br /> Password: <input type="password" name="secret" /><br /> What do you prefer: <select name="image_type"> <option>Books</option> <option>Cinema</option> <option>TV</option> </select> </p> <p> Post your photos: <input type="file" multiple name="photos[]"> </p> <p> <input id="vehicle_bike" type="checkbox" name="vehicle[]" value="Bike" /> <label for="vehicle_bike">I have a bike</label><br /> <input id="vehicle_car" type="checkbox" name="vehicle[]" value="Car" /> <label for="vehicle_car">I have a car</label> </p> <p> Describe yourself:<br /> <textarea name="description" cols="50" rows="8"></textarea> </p> <p> <input type="submit" value="Submit" /> </p> </fieldset> </form> </body> </html>
FormData
objects are not stringifiable objects. If you want to stringify a submitted data, use the previous pure-AJAX example. Note also that, although in this example there are some file
<input>
fields, when you submit a form through the FormData
API you do not need to use the FileReader
API also: files are automatically loaded and uploaded.Get last modified date
function getHeaderTime () { console.log(this.getResponseHeader("Last-Modified")); /* A valid GMTString date or null */ } var oReq = new XMLHttpRequest(); oReq.open("HEAD" /* use HEAD if you only need the headers! */, "yourpage.html"); oReq.onload = getHeaderTime; oReq.send();
Do something when last modified date changes
Let's create two functions:
function getHeaderTime () { var nLastVisit = parseFloat(window.localStorage.getItem('lm_' + this.filepath)); var nLastModif = Date.parse(this.getResponseHeader("Last-Modified")); if (isNaN(nLastVisit) || nLastModif > nLastVisit) { window.localStorage.setItem('lm_' + this.filepath, Date.now()); isFinite(nLastVisit) && this.callback(nLastModif, nLastVisit); } } function ifHasChanged(sURL, fCallback) { var oReq = new XMLHttpRequest(); oReq.open("HEAD" /* use HEAD - we only need the headers! */, sURL); oReq.callback = fCallback; oReq.filepath = sURL; oReq.onload = getHeaderTime; oReq.send(); }
And to test:
/* Let's test the file "yourpage.html"... */ ifHasChanged("yourpage.html", function (nModif, nVisit) { console.log("The page '" + this.filepath + "' has been changed on " + (new Date(nModif)).toLocaleString() + "!"); });
If you want to know if the current page has changed, please read the article about document.lastModified
.
Cross-site XMLHttpRequest
Modern browsers support cross-site requests by implementing the Web Applications (WebApps) Working Group's Access Control for Cross-Site Requests standard. As long as the server is configured to allow requests from your web application's origin, XMLHttpRequest
will work. Otherwise, an INVALID_ACCESS_ERR
exception is thrown.
Bypassing the cache
A cross-browser compatible approach to bypassing the cache is appending a timestamp to the URL, being sure to include a "?" or "&" as appropriate. For example:
https://foo.com/bar.html -> https://foo.com/bar.html?12345 https://foo.com/bar.html?foobar=baz -> https://foo.com/bar.html?foobar=baz&12345
As the local cache is indexed by URL, this causes every request to be unique, thereby bypassing the cache.
You can automatically adjust URLs using the following code:
var oReq = new XMLHttpRequest(); oReq.open("GET", url + ((/\?/).test(url) ? "&" : "?") + (new Date()).getTime()); oReq.send(null);
Security
Versions of Firefox prior to Firefox 3 allowed you to set the preference capability.policy.<policyname>.XMLHttpRequest.open</policyname>
to allAccess
to give specific sites cross-site access. This is no longer supported.
Versions of Firefox prior to Firefox 5 could use netscape.security.PrivilegeManager.enablePrivilege("UniversalBrowserRead");
to request cross-site access. This is no longer supported, even though it produces no warning and permission dialog is still presented.
The recommended way to enable cross-site scripting is to use the Access-Control-Allow-Origin
HTTP header in the response to the XMLHttpRequest.
XMLHttpRequests being stopped
If you conclude with an XMLHttpRequest receiving status=0
and statusText=null
, this means the request was not allowed to be performed. It was UNSENT
. A likely cause for this is when the XMLHttpRequest
origin (at the creation of the XMLHttpRequest) has changed when the XMLHttpRequest is subsequently open()
. This case can happen, for example, when one has an XMLHttpRequest that gets fired on an onunload event for a window, the expected XMLHttpRequest is created when the window to be closed is still there, and finally sending the request (in otherwords, open()
) when this window has lost its focus and another window gains focus. The most effective way to avoid this problem is to set a listener on the new window's "activate" event which is set once the terminated window has its "unload" event triggered.
Using XMLHttpRequest from JavaScript modules / XPCOM components
Instantiating XMLHttpRequest
from a JavaScript module or an XPCOM component works a little differently; it can't be instantiated using the XMLHttpRequest()
constructor. The constructor is not defined inside components, and the code results in an error. The best way to work around this is to use the XPCOM component constructor.
const XMLHttpRequest = Components.Constructor("@mozilla.org/xmlextras/xmlhttprequest;1", "nsIXMLHttpRequest");
In earlier versions of Gecko, prior to Gecko 16, a bug is known to cause requests created in this manner to be cancelled for no reason. If you require your code to work on Gecko 15, or earlier, you can get the XMLHttpRequest constructor from the hidden DOM window.
const { XMLHttpRequest } = Components.classes["@mozilla.org/appshell/appShellService;1"] .getService(Components.interfaces.nsIAppShellService) .hiddenDOMWindow; var oReq = new XMLHttpRequest();
See also
- MDN AJAX introduction
- HTTP access control
- How to check the security state of an XMLHTTPRequest over SSL
- XMLHttpRequest - REST and the Rich User Experience
- Microsoft documentation
- Apple developers' reference
- "Using the XMLHttpRequest Object" (jibbering.com)
- The XMLHttpRequest Object: W3C Specification
- Web Progress Events specification