[whatwg] Problems with DOMTokenString

Maciej Stachowiak mjs at apple.com
Fri Feb 2 08:19:31 PST 2007

I looked into DOMTokenString with keen interest, because I believe an  
API for manipulating individual classes of an HTML element is  
increasingly important as we see more dynamic sites that use CSS  
styling. However, I think the design for this is not suitable as-is.

Summary of DOMTokenString

Web Apps 1.0 introduces the DOMTokenString interface, and uses it for  
the className attribute on HTMLElement instead of DOMString; and on a  
few DataGrid methods:

void getCaptionClasses(in unsigned long column, in DOMTokenString  
   void getRowClasses(in RowSpecification row, in DOMTokenString  
   void getCellClasses(in RowSpecification row, in unsigned long  
column, in DOMTokenString classes);

DOMTokenString has the following interface:

interface DOMTokenString : DOMString {
   boolean has(in DOMString token);
   void add(in DOMString token);
   void remove(in DOMString token);

In addition the spec says:

"In the ECMAScript DOM binding, objects implementing the  
DOMTokenString interface must stringify to the object's underlying  
string representation.
DOMTokenString inherits from DOMString, so in bindings where strings  
have attributes or methods, those attributes and methods will also  
operate on DOMTokenString objects."


This approach is clever and solves the long-standing need to  
manipulate individual classes via the DOM API. There are a few  
technical problems, however:

1) In ECMAScript, the DOMString interface represents a String  
primitive value, not a String object. There is no way to "subclass"  
primitive types. (When you invoke a method on a string it actually  
makes a wrapper for it on the fly.) There's no way to make an object  
type appear just like a string, since the typeof operator for  
instance will know the difference. Even to make the String methods  
and properties work, you'd likely have to reimplement them, which is  
annoying for implementors. Java strings are also not subclassable.

2) ECMAScript strings are always immutable. This interface introduces  
operations that mutate the string. This violates the contract of JS  
strings - if you store one of these in an object, its value may  
change before you can retrieve it. Java strings are also immutable.

3) It seems to be implied that the DOMTokenString should be a  
reference to, rather than a copy of the underlying attribute value.  
Presumably doing myElement.className.add("foo") is intended to  
actually modify the elements "class" attribute, and not just a  
temporary DOMTokenString object. However, many implementations copy  
DOM string values before returning them to JS because the underlying  
type is not the same.

4) Semantics of assigning to the className field are odd. The spec  
says to the string value even if you provide a DOMTokenString, but  
this is somewhat inconsistent, as presumably var someClass =  
myElt.className does not copy. While the DOM is already full of magic  
properties that do odd things on assignment, I'm unusure of the  
benefit of making className one of them.

I do not think the cleverness of this idea so great as to be worth  
these problems (especially #2).

Possible alternate designs

Alternative #1: leave className an ordinary string, but add the  
following methods to the HTMLElement interface:

void addClass(DOMString newClass);
void removeClass(DOMString removeClass);
bool hasClass(DOMString possibleClass);

This is no more API surface area, and the semantics are much more  
clear; methods that modify the HTMLElement's class attribute are on  
the element itself. An additional method to get an array of the  
tokenized class strings could also be useful, but that starts getting  
into the territory of the other option.

Alternative #2: leave the className an ordinary string, but add a new  
readonly DOMClassList classList property with something like the  
following interface:

interface DOMClassList {
     void add(DOMString newClass);
     void remove(DOMString removeClass);
     bool has(DOMString possibleClass);

If you add DOMString index(unsigned i) and unsigned length, you would  
also have the ability to enumerate the classes easily, which the API  
as currently specced lacks. Because the classList property would be  
readonly, there would be no question of whether two elements ever  
share a DOMClassList.

I like #1 better, but either seems fine.

In both of these alternatives, the DataGrid methods would be changed  
to return a string instead of taking and modifing an existing  
DOMTokenString. It is in any case highly unusual in DOM APIs for  
getters to mutate a provided "out" object rather than to return a  
value. Also the spec does not appear to provide a way to make a brand  
new empty DOMTokenString, so these methods would otherwise only be  
useful for altering the class of existing elements based on the  
classes of some other elements, which seems unuseful.


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