Category:Ajax site tips: Difference between revisions

Ajax-driven or JavaScript-heavy sites often require different strategies than when scripting a regular site: elements appear, disappear and change at any time. This page contains some tips for scripting such sites.

Catch bubbling events

To react on changements of the document, e.g appearing nodes, you can add new event listeners to the document or any other node with the element.addEventListener() function:

target.addEventListener(type, listener, useCapture);

Example: This code will tell you what element you click, unless it or some ancestor element does event.stopPropagation(). This is useful since elements you want to attach listeners to might not be in the DOM yet when the page/script loads.

document.addEventListener('click', function(event) {
        GM_log("Clicked " + event.target);
    }, true);

Note that the second argument has to be a function or the name of a function and not code itself. If you want code to be executed but not define an extra function, you can use an inline function like in the example above. The function is called with one parameter with an attribute target, which is the element that raised the event.

Example scripts

• Gmail attachment reminder

Listen for DOMNodeInserted event

When new nodes are inserted into the DOM tree, an DOMNodeInserted event is raised at the inserted node and bubbles up the tree until its propagation is stopped. You can handle it with an event handler at the document node. The following code shows an alert with the id of any new node in the document.

document.addEventListener('DOMNodeInserted', function (event) { alert(event.target.id + ' has been added'); }, false);

Sometimes, new nodes appear in the document without being inserted in a way that they would raise DOMNodeInserted events. This happens for example when the element.innerHTML attribute is used. In this case, you can add listeners for the DOMAttrModified event. The following code is copied from a userscript for the german StudiVZ:

/* watch for any changed attributes */
document.addEventListener("DOMAttrModified", documentChanged, false);

function documentChanged(event) {
    // alert(event.target.id);                      /* helpfull for debugging */
    if (event.target.id == "get_kds_link" ||        /* if one of those nodes are modified, call myFunction */
        event.target.id == "checkcode") {
            myFunction();
    }
}

There are other event types you might want to listen for, which are listed on the W3C website.

Example scripts

This code is from Jesse Ruderman's Autolink script:

var moddingDOM = false;

window.addEventListener("load", init, false);
function init()
{
  document.addEventListener("DOMNodeInserted", nodeInserted, false);
  setTimeout(go, 50, document.body);
}

// This makes it work at Gmail.
// 20% performance penalty on a plain text file with a link on almost every line.
// Tiny performance penalty on pages with few automatically added links.
function nodeInserted(e)
{
  // our own modifications should not trigger this.
  // (we don't want our regular expression objects getting confused)
  // (we want better control over when we recurse)
  
  //GM_log("Inserted: " + e.target);
  
  if (!moddingDOM)
    go(e.target);
}