When used as a getter, $.fn.data will "fall back" to the DOM data-

attribute. Additionally, it will save this value to the internal data lookup

table for that element. This means that $.fn.data is an accessor with a side

effect.

var $el = $( "<div>" );
$el.attr( "data-john", 23 );
$el.data( "john" );			// 23
$el.attr( "data-john", 45 );
$el.data( "john" );			// 23

This behavior is surprising--querying the state of the element changes it.

Granted, the API documentation contains a caveat about this detail:

The data- attributes are pulled in the first time the data property is accessed and then are no longer accessed or mutated (all data values are then stored internally in jQuery).

...but one could argue that the need for such documentation is a sign of an

unintuitive interface.

Modifying this behavior would certainly be a breaking change, but I believe it

is in jQuery's interest to implement an API that is consistent and easy to

reason about.