Using XMLHttpRequest
In this guide, we'll take a look at how to use
XMLHttpRequest
to issue HTTP
requests in order to exchange data between the website and a server.
Examples of both common and more obscure use cases for XMLHttpRequest
are included.
To send an HTTP request, create an XMLHttpRequest
object, open a URL, and
send the request. After the transaction completes, the object will contain useful
information such as the response body and the HTTP status of the result.
js
function reqListener() {
console.log(this.responseText);
}
const req = new XMLHttpRequest();
req.addEventListener("load", reqListener);
req.open("GET", "http://www.example.org/example.txt");
req.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. You can't use synchronous requests outside web
workers as it freezes the main interface.
Note: The constructor XMLHttpRequest
isn't limited to only XML documents.
It starts with "XML" because when it was created the main format
that was originally used for asynchronous data exchange was XML.
Handling responses
There are several types of response attributes defined for the
XMLHttpRequest()
constructor. 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 four primary ways of analyzing this XML document:
- Using XPath to address (or point to) parts of it.
- 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 useRegExp
to scan with regard to line breaks. However, this method is a "last resort" since if the XML code changes slightly, the method will likely fail.
Note: XMLHttpRequest
can now interpret HTML for you
using the responseXML
property. Read the
article about HTML in XMLHttpRequest to learn how to do this.
Processing a responseText property containing an HTML document
If you use XMLHttpRequest
to get the content of a remote HTML webpage, the
responseText
property is a string
containing the raw HTML. This could prove difficult to manipulate and analyze. There are
three primary ways to analyze and parse this raw HTML string:
-
Use the
XMLHttpRequest.responseXML
property as covered in the article HTML in XMLHttpRequest. -
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 HTMLresponseText
beforehand. You might want to remove line breaks, if you useRegExp
to scan with regard to line breaks. 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.
js
const req = new XMLHttpRequest();
req.open("GET", url);
// retrieve data unprocessed as a binary string
req.overrideMimeType("text/plain; charset=x-user-defined");
/* ... */
However, more modern techniques are available, since the
responseType
attribute now supports a
number of additional content types, which makes sending and receiving binary data much
easier.
For example, consider this snippet, which uses the responseType
of
"arraybuffer
" to fetch the remote content into a ArrayBuffer
object, which stores the raw binary data.
js
const req = new XMLHttpRequest();
req.onload = (e) => {
const arraybuffer = req.response; // not responseText
/* ... */
};
req.open("GET", url);
req.responseType = "arraybuffer";
req.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 specification for progress events: these events implement the ProgressEvent
interface. The
actual events you can monitor to determine the state of an ongoing transfer are:
progress
-
The amount of data that has been retrieved has changed.
load
-
The transfer is complete; all data is now in the
response
js
const req = new XMLHttpRequest();
req.addEventListener("progress", updateProgress);
req.addEventListener("load", transferComplete);
req.addEventListener("error", transferFailed);
req.addEventListener("abort", transferCanceled);
req.open();
// ...
// progress on transfers from the server to the client (downloads)
function updateProgress(event) {
if (event.lengthComputable) {
const percentComplete = (event.loaded / event.total) * 100;
// ...
} 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
.
Note: You need to add the event listeners before
calling 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:
js
const req = new XMLHttpRequest();
req.upload.addEventListener("progress", updateProgress);
req.upload.addEventListener("load", transferComplete);
req.upload.addEventListener("error", transferFailed);
req.upload.addEventListener("abort", transferCanceled);
oReq.open();
Note: Progress events are not available for the
file:
protocol.
Progress events 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.
One can also detect all three load-ending conditions (abort
,
load
, or error
) using the loadend
event:
js
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.
Get last modified date
js
function getHeaderTime() {
console.log(this.getResponseHeader("Last-Modified")); // A valid GMTString date or null
}
const req = new XMLHttpRequest();
req.open(
"HEAD", // use HEAD when you only need the headers
"yourpage.html"
);
req.onload = getHeaderTime;
req.send();
Do something when last modified date changes
Let's create two functions:
js
function getHeaderTime() {
const lastVisit = parseFloat(
window.localStorage.getItem(`lm_${this.filepath}`)
);
const lastModified = Date.parse(this.getResponseHeader("Last-Modified"));
if (isNaN(lastVisit) || lastModified > lastVisit) {
window.localStorage.setItem(`lm_${this.filepath}`, Date.now());
isFinite(lastVisit) && this.callback(lastModified, lastVisit);
}
}
function ifHasChanged(URL, callback) {
const req = new XMLHttpRequest();
req.open("HEAD" /* use HEAD - we only need the headers! */, URL);
req.callback = callback;
req.filepath = URL;
req.onload = getHeaderTime;
req.send();
}
And to test:
js
// Let's test the file "yourpage.html"
ifHasChanged("yourpage.html", function (modified, visit) {
console.log(
`The page '${this.filepath}' has been changed on ${new Date(
nModified
).toLocaleString()}!`
);
});
If you want to know if the current page has changed, refer to the article about document.lastModified
.
Cross-site XMLHttpRequest
Modern browsers support cross-site requests by implementing the Cross-Origin Resource Sharing (CORS) 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:
http://foo.com/bar.html -> http://foo.com/bar.html?12345 http://foo.com/bar.html?foobar=baz -> http://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:
js
const req = new XMLHttpRequest();
req.open("GET", url + (/\?/.test(url) ? "&" : "?") + new Date().getTime());
req.send(null);
Security
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 other words, 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 DOMActivate
event which is set once the
terminated window has its unload
event triggered.
Workers
Setting overrideMimeType
does not work from a Worker
. See
Firefox bug 678057 for more details. Other browsers may handle this differently.
Specifications
Specification |
---|
XMLHttpRequest Standard # interface-xmlhttprequest |
Browser compatibility
BCD tables only load in the browser