Data formats

Your choices

• XML
• HTML
• JSON

XML

The X in AJAX

• Returned in the responseXML property
• The server must send the correct header e.g. text/xml
• Can be traversed and manipulated using DOM methods (e.g getElementsByTagName) and properties (e.g. childNode, nodeValue, etc.)
• Examples: People profiles (data) (JS), RSS reader (data) (JS)

Advantages of XML

• Human readable
• Popular

Disadvantages of XML

• Requires a lot of finicky client-side DOM manipulation
• Slow

HTML

• Returned in the responseText property
• Can be dumped into the page using innerHTML
• XMLHttpRequest.responseText + innerHTML = AHAH
• Examples: People profiles (data) (JS), RSS reader (data) (JS)

Advantages of HTML

• Very little client-side processing
• Fast
• The server can send one output format, whether the request comes from Ajax or not

Disdvantages of HTML

• Uses the brute force of the proprietary innerHTML property
• Won't work on XHTML served with the correct mime-type in Firefox 1.0
• Not very useful for changing a document in more than one place

JSON

• Proposed by Douglas Crockford. A nested list of name/value pairs described using JavaScript's object syntax.
• Can be returned in the resonseText property.
• Could also be requested without using XMLHttpRequest!
• Use the DOM to change the src attribute of a <script> tag, pointing it to a resource that returns JSON. Then eval() the returned JavaScript.
• Examples: People profiles (data) (JS), RSS reader (data) (JS)

Examples of JSON

Using JSON with a web service from a different site.

• Yahoo! News Search (view JavaScript)
• Live Yahoo! News Search

Advantages of JSON

• Lightweight
• Some APIS will return JSON (e.g. Yahoo!)
• Can get around the same-site restriction

Disadvantages of JSON

• Not as readable as XML or HTML
• Potential problems with character encoding and escaping quotes
• Uses eval() so the source must be trustworthy

Which format to choose

Choose the best format for the job.

• To update many different parts of the current document, XML may be best.
• To retrieve data directly from another server, JSON (and the <script> tag hack) is the only option.
• To update one part of the page, just use innerHTML to insert a fragment of HTML

Next

Doing the right thing: Hijax