Assignment of global variables deleted by their RHS in strict mode is permitted

[bakkot]

(Copying from bugzilla; see discussion in esdiscuss.)

In short, it is possible to have a Reference to a global variable or object-environment-record property which has been deleted. Normally, bare assignments to undeclared variables in strict mode cause ReferenceErrors. However, calling PutValue on a reference to a global variable which has been deleted since the reference was created does not throw a ReferenceError in strict mode, even though, at the time of writing, that variable does not exist.

This circumvents a protection which strict mode is intended to provide.

For example, in

var global = this;
Object.defineProperty(this, "x", {
  configurable: true,
  value: 1
});

(function() {
  "use strict";
  x = (delete global.x, 2);
})();

the x = goes through PutValue, which in step 5 performs IsUnresolvableReference, which returns false because the Reference was resolvable when it was created. Hence no error is thrown.

However, Firefox, Chrome, and Safari all throw for the above code, in violation of the spec.

I would regard this as a (normative) bug in the spec.

[bakkot]

See also #467, though in this case the right answer is clearer, I think.