Question

After digging the jQuery sources I still can’t understand why the code like:

$("div").attr("someAttr", undefined);

returns jQuery object but not an attribute value, despite the fact that the second value argument is set as undefined.

JSFiddle: http://jsfiddle.net/5jn5D/.

Answer 1

This is, because .attr("someAttr", undefined) (with 2 arguments) is a setter method, and thus returns the object for sake of chainability. Setting the argument to undefined or any other arbitrary value always yields the same effect. Why should a value be returned, if you know what the value is?

If you use attr as getter method (with only 1 argument) .attr("someAttr") it returns a string with the attribute value.

Line 2003 in the source explicitely checks for the length of the arguments:

return jQuery.access( this, jQuery.attr, name, value, arguments.length > 1 );

with the fifth argument being the chainable switch, returning true for arguments > 1.