"use strict"; var ok = false; try { undeclared = (this.undeclared = 5); } catch (exc) { ok = exc instanceof ReferenceError; } assertEq(ok, true); Per ES5, this assignment to undeclared should throw a ReferenceError in strict mode. In detail, undeclared = (this.undeclared = 5); * The LHS evaluates to an unresolveable strict Reference (ES5 §10.2.2.1); * the RHS evaluates to 5; * assignment to the Reference throws, because it is still unresolveable (see IsUnresolvableReference, §8.7), even though undeclared now exists.

If Allen's fix makes it into an erratum, then there'll be a check before RHS evaluation. But we need a check somewhere. /be

The semantics with checking RHS didn't change in ES6 spec, so this still should throw ReferenceError, because LHS is an unresolved reference.