When does an xmlHttpRequest object parse serialized XML into a DOM during its lifecycle?

Question

When does an xmlHttpRequest object parse serialized XML into a DOM during its lifecycle?

When a JavaScript code with xmlHttpRequest.responseXML() runs, it creates a DOM Document object from the XML-structured HTTP response body.

Have you ever wondered at what moment the XML string is turned into the DOM Document by an xmlHttpRequest object?

There are two possibilities:

The parsing could take place when responseXML() is executed. This way, resources are not wasted on converting the XML string until it's actually needed.
Alternatively, the parsing might occur once the HTTP response is received. If the server sends back a text/xml content-type, it indicates that XML was requested and likely needs to be converted into a DOM for proper use of the data received.

Both options have their advantages, but my inclination is that the XML string is parsed only upon calling responseXML.

So, when exactly does the parsing of the XML string happen?

I ask because I want to compare browser performance in deserializing XML versus JSON data.

javascript ajax dom xmlhttprequest

Answer 1

Answer №1

It would be ideal to parse the stream as it is being received rather than waiting until the entire response is complete or the responseXml property is called. This would eliminate unnecessary delays in building the DOM after all bytes have been received. A more efficient approach would involve building the DOM concurrently with receiving the stream, so that once the stream is complete, the DOM is immediately ready for usage.

An important consideration is that MSXML does not generate a DOM if the Content type does not specify an XML type, while Mozilla's implementation allows you to override this and treat the content as XML regardless of the indicated Content-Type.

If the DOM is only parsed when the property is accessed, there would be no need for the above complexities. The property could simply attempt to parse the content directly. It is because a DOM can be constructed on-the-fly as content is received that these considerations come into play.

Answer 2

It would be ideal to parse the stream as it is being received rather than waiting until the entire response is complete or the responseXml property is called. This would eliminate unnecessary delays in building the DOM after all bytes have been received. A more efficient approach would involve building the DOM concurrently with receiving the stream, so that once the stream is complete, the DOM is immediately ready for usage.

An important consideration is that MSXML does not generate a DOM if the Content type does not specify an XML type, while Mozilla's implementation allows you to override this and treat the content as XML regardless of the indicated Content-Type.

If the DOM is only parsed when the property is accessed, there would be no need for the above complexities. The property could simply attempt to parse the content directly. It is because a DOM can be constructed on-the-fly as content is received that these considerations come into play.

Answer 3

Answer №2

It wouldn't shock me if this varies depending on the browser. How about we analyze all three?

Answer 4

It wouldn't shock me if this varies depending on the browser. How about we analyze all three?

Answer 5

Answer №3

When using Chrome, the DOM is generated and parsed upon calling responseXML().

Here is the code snippet for the function:

Document* XMLHttpRequest::responseXML(ExceptionCode& ec)
{
    if (responseTypeCode() != ResponseTypeDefault && responseTypeCode() != ResponseTypeText && responseTypeCode() != ResponseTypeDocument) {
        ec = INVALID_STATE_ERR;
        return 0;
    }

    if (m_state != DONE)
        return 0;

    if (!m_createdDocument) {
        if ((m_response.isHTTP() && !responseIsXML()) || scriptExecutionContext()->isWorkerContext()) {
            // According to W3C specification.
            m_responseXML = 0;
        } else {
            m_responseXML = Document::create(0, m_url);
            // TODO: Implement setting Last-Modified.
            m_responseXML->setContent(m_responseBuilder.toStringPreserveCapacity());
            m_responseXML->setSecurityOrigin(document()->securityOrigin());
            if (!m_responseXML->wellFormed())
                m_responseXML = 0;
        }
        m_createdDocument = true;
    }

    return m_responseXML.get();
}

Answer 6

When using Chrome, the DOM is generated and parsed upon calling responseXML().

Here is the code snippet for the function:

Document* XMLHttpRequest::responseXML(ExceptionCode& ec)
{
    if (responseTypeCode() != ResponseTypeDefault && responseTypeCode() != ResponseTypeText && responseTypeCode() != ResponseTypeDocument) {
        ec = INVALID_STATE_ERR;
        return 0;
    }

    if (m_state != DONE)
        return 0;

    if (!m_createdDocument) {
        if ((m_response.isHTTP() && !responseIsXML()) || scriptExecutionContext()->isWorkerContext()) {
            // According to W3C specification.
            m_responseXML = 0;
        } else {
            m_responseXML = Document::create(0, m_url);
            // TODO: Implement setting Last-Modified.
            m_responseXML->setContent(m_responseBuilder.toStringPreserveCapacity());
            m_responseXML->setSecurityOrigin(document()->securityOrigin());
            if (!m_responseXML->wellFormed())
                m_responseXML = 0;
        }
        m_createdDocument = true;
    }

    return m_responseXML.get();
}

When does an xmlHttpRequest object parse serialized XML into a DOM during its lifecycle?

Answer №1

Answer №2

Answer №3

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