HTML in XMLHttpRequest

To discourage the synchronous use of XMLHttpRequest, HTML support is not available in the synchronous mode. Also, HTML support is only available if the responseType property has been set to "document". This limitation avoids wasting time parsing HTML uselessly when legacy code uses XMLHttpRequest in the default mode to retrieve responseText for text/html resources. Also, this limitation avoids problems with legacy code that assumes that responseXML is null for HTTP error pages (which often have a text/html response body).

Retrieving an HTML resource as a DOM using XMLHttpRequest works just like retrieving an XML resource as a DOM using XMLHttpRequest, except you can't use the synchronous mode and you have to explicitly request a document by assigning the string "document" to the responseType property of the XMLHttpRequest object after calling open() but before calling send().

const xhr = new XMLHttpRequest();
xhr.onload = () => {
  console.log(xhr.responseXML.title);
};
xhr.open("GET", "file.html");
xhr.responseType = "document";
xhr.send();

This method relies on the "force async" nature of the feature. When you try to set responseType of an XMLHttpRequest object after it is opened as "sync". This throws an error in the browsers that implement the feature and works on others.

function HTMLinXHR() {
  if (!window.XMLHttpRequest) {
    return false;
  }
  const req = new window.XMLHttpRequest();
  req.open("GET", window.location.href, false);
  try {
    req.responseType = "document";
  } catch (e) {
    return true;
  }
  return false;
}

View on JSFiddle

This method is synchronous, does not rely on external assets though it may not be as reliable as method 2 described below since it does not check the actual feature but an indication of that feature.

There are two challenges to detecting exactly if a browser supports HTML parsing in XMLHttpRequest. First, the detection result is obtained asynchronously, because HTML support is only available in the asynchronous mode. Second, you have to actually fetch a test document over HTTP, because testing with a data: URL would end up testing data: URL support at the same time.

Thus, to detect HTML support, a test HTML file is needed on the server. This test file is small and is not well-formed XML:

<title>&amp;&<</title>

If the file is named detect.html, the following function can be used for detecting HTML parsing support:

function detectHtmlInXhr(callback) {
  if (!window.XMLHttpRequest) {
    setTimeout(function () {
      callback(false);
    }, 0);

    return;
  }
  let done = false;
  const xhr = new window.XMLHttpRequest();
  xhr.onreadystatechange = () => {
    if (xhr.readyState === 4 && !done) {
      done = true;
      callback(
        !!(
          xhr.responseXML &&
          xhr.responseXML.title &&
          xhr.responseXML.title === "&&<"
        ),
      );
    }
  };
  xhr.onabort = xhr.onerror = () => {
    if (!done) {
      done = true;
      callback(false);
    }
  };
  try {
    xhr.open("GET", "detect.html");
    xhr.responseType = "document";
    xhr.send();
  } catch (e) {
    setTimeout(function () {
      if (!done) {
        done = true;
        callback(false);
      }
    }, 0);
  }
}

The argument callback is a function that will be called asynchronously with true as the only argument if HTML parsing is supported and false as the only argument if HTML parsing is not supported.

View on JSFiddle

If the character encoding is declared in the HTTP Content-Type header, that character encoding is used. Failing that, if there is a byte order mark, the encoding indicated by the byte order mark is used. Failing that, if there is a <meta> element that declares the encoding within the first 1024 bytes of the file, that encoding is used. Otherwise, the file is decoded as UTF-8.

• XMLHttpRequest
• Using XMLHttpRequest