If you store 0, false or null as a value for the data, it will always return undefined. Reason being, if it is stored as a camelized key, the below condition will always fail, and then $.data will always return the nonexisting non-camelized data, which is undefined.

The offending code is this, at the end of the $.data function:

 thisCache[ jQuery.camelCase( name ) ] || thisCache[ name ] 

This problem is especially bad, if 0 is stored as the data, and during retrieval, it is expected to be of a numeric type. With undefined, any numerical expression will result in NaN.

The above code should be changed to :

var camelName = jQuery.camelCase(name);
camelName in thisCache
 ? thisCache[camelName] : thisCache[name]