Consider the following function:

function f(bar) {    
    var $div = $("<div />").data("foo", bar);
}

The $div variable should be the DOM object, since data with two parameters is a setter and returns itself. However if the function f is called without a parameter, "bar" is undefined, and data triggers get method, and returns undefined instead.

See jsFiddle here:

http://jsfiddle.net/mrmamen/3Xqk7/

This applies not only to data, but also to attr, css, and probably most if not all of jQuerys methods with have setter and getter.

Note:

You can detect the difference between these:

test(foo) and

test(foo, undefined)

be checking arguments.length.