515478 - Change JSScope/JSScopeProperty to better reflect ES5's [[Writable]], [[Configurable]], [[Extensible]] attributes

Status: ASSIGNED

Description

[Jeff Walden [:Waldo]]

ES5 has three object-preservation APIs:

  * Object.preventExtensions(O)
  * Object.seal(O)
  * Object.freeze(O)

These APIs make the given object increasingly immutable, each subsuming the behavior of the previous methods.  The first prevents new properties from being added.  The second goes further by making all properties already on the object non-configurable (incapable of being deleted, i.e. permanent).  The third goes furthest of all by making all properties unwritable (value can't be changed, i.e. readonly).

Of these paradigms, SpiderMonkey supports only the third, through the false cognate JS_SealObject(cx, obj, JS_FALSE).  We need implementation support for the first two capabilities as well.