Why, Blog, Why?

I came across a bug in Internet Explorer whereby the document.getElementById() function may return an element with a name attribute equal to the id that is specified. This can cause unexpected results.

According to the W3C’s DOM Level 2 Core Specification:

getElementById introduced in DOM Level 2

Returns the Element whose ID is given by elementId. If no such element exists, returns null. Behavior is not defined if more than one element has this ID.

Note: The DOM implementation must have information that says which attributes are of type ID. Attributes with the name “ID” are not of type ID unless so defined. Implementations that do not know whether attributes are of type ID or not are expected to return null.

Parameters

elementId of type DOMString

The unique id value for an element.

Return Value

Element

The matching element.

No Exceptions

http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-getElBId

However, for the browsers that failed the test, it appears that their implimentation of getElementById returns the first element where the ID or name attribute is equal to the specified ID.

This bug was tested against the following browsers:

• IE 6.0 sp2: failed
• IE 5.5: failed
• IE 5.01: failed
• Netscape 7.2: passed
• Netscape 6: passed
• Mozilla 1.7.3: passed
• Opera 8.2: failed
• Firefox 1.0.6: passed

For instance, the following code should produce an alert with “foo=foo; bar=bar”. However, the browsers that fail the test actually return “foo=foo; bar=foo”.

<form name="form1" id="form1" method="get" action="javascript: void(0)">
<input name="bar" type="text" id="foo" value="foo" /><br />
<input name="foo" type="text" id="bar" value="bar" readonly="readonly" /><br />
<input type="button" name="Button" value="Test" onclick="alert(’foo=’+document.getElementById(’foo’).value+’; bar=’+document.getElementById(’bar’).value)" /><br />
</form>

Test Page

Reported at http://www.quirksmode.org/bugreports/ (permalink)

Noted on http://channel9.msdn.com/wiki/ (Update 2007-04-04: my note has since been replaced by several others