RegExp.prototype.compile and RegExp instance properties

Mark S. Miller erights at
Tue Jan 15 18:03:04 PST 2013

My answer depends on a prior issue. Do we agree that in ES6,
RegExp.prototype is not itself a RegExp? Like the Date.prototype and
WeakMap.prototype we've already talked about, freezing it and
everything reachable from it must render it immutable, so that it
isn't a communications channel. SES currently removes
RegExp.prototype.compile to prevent that communications channel. SES
includes all variables, properties, and methods normatively defined by
ES5. I would really like to see it include all of normative ES6
without violating ocap rules.

If we agree that RegExp.prototype is not a RegExp and the invariant
breakage of compile were fixed by any of your proposals, then I
believe we would no longer need to remove it.

Although any of your proposals will then work, I'll suggest a
variation of your least favorite (#1):

#4: These properties are {writable: false, configurable: true}.
RegExps may indeed need to be exotic. But they don't refuse freezing.
Rather, if a RegExp is frozen, or if its properties are individually
made non-configurable non-writable, then neither compile nor anything
else can mutate that RegExp. A frozen RegExp (without additional
properties) is an immutable RegExp. This would be pleasant.

On Tue, Jan 15, 2013 at 5:27 PM, Allen Wirfs-Brock
<allen at> 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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