RegExp.prototype.compile and RegExp instance properties

Brendan Eich brendan at mozilla.com
Tue Jan 15 18:01:42 PST 2013


#3. If any ES5 or __lookupGetter__ code breaks, I'll be surprised, but 
also willing to say "too bad".

/be

Allen Wirfs-Brock wrote:
> We've previously concluded that RegExp.prototype.compile is part of web reality and should be included in the ES6 spec.  Whether it is put in the main spec. or in Annex B doesn't make a difference for the technical issue I'm about to discuss.
>
> Currently RegExp instances are specified to have four own data properties (source, global, ignoreCase, multiline) that are non-writable and non-configurable.
>
> This is fine because the standard spec. for RegExp initializes these properties when the object is constructed and according to the spec. they never change.
>
> However, RegExp.prototype.compile, as widely implemented in browsers allows the pattern and flags associated with an RegExp instance to be modified after object construction is completed and changing the pattern/flags also changes the values of these four properties.  Implementation just change their values even though the properties are non-writable/non-configurable.
>
> If we are going to include the compile method in the spec. we need to do so without violating the invariant that a non-configurable/non-writable property never changes its visible value.  There are several ways we might do this:
>
> 1) Change the specified attributes of these properties to {writable: false, configurable: true} or perhaps {writable: true, configurable: false}.  However, if we want to enforce that the only way to modify them was via the compile method we would have to make RegExp instances a special kind of exotic object in order to prevent assignment or Object.defineProperty from being used to modify the values of the property.
>
> Compatibility impact: change of property attribute from ES5
> Spec/implementation impact: requires introduction of a new kind of exotic object
>
> 2) Change these properties to instance own accessor properties with a get function but no set function.  The accessors would produce values based upon the current pattern and flags.
>
> Compatibility impact:  visibly changes the properties from data properties to accessor properties.
> Spec./implementation issues:  Do all instances share the same get functions or do 4 new  get function need to be created for every RegExp instance?  (Observable via get function identify)
>
> 3) Change these properties to prototype level accessor get-only accessor properties
>
> Compatibility impact: visible change from data to accessor property.  Inherited prototype property rather than instance property.  Inherited property could be over-ridden at instance level.
>
>
> #3 is similar to how WebIDL now represents similar properties.  It is probably the easiest to spec. and implement.  It is the biggest change from a compat. perspective but I suspect that the actual compat. impact of any of these is small.  If I was designing RegExp from scratch and still had these requirements I would probably go with #3.
>
> What do people think?  #3 or #2?  I really don't want to go the exotic object route.
>
> Allen
>
>
>
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