Question

http://jsbin.com/ifabem/2/edit

"use strict";

window.x = "Hello World";
alert(x); // this does't throw an exception in strict mode

Why doesn’t accessing x directly in the last statement violate strict mode and throw an exception?

Is it the case that a violating is only writing a new value into the global object, but reading and even modifying an existing object allowed?

Answer 1

The phrase “gaining access to the global object” does not occur in the spec, so doesn’t have a clear definition; but looking through Appendix C of the spec (a non-normative listing of the effects of strict-mode), the only restriction that I think could be described that way is this one:

Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the global object. When a simple assignment occurs within strict mode code, its LeftHandSide must not evaluate to an unresolvable Reference. If it does a ReferenceError exception is thrown (8.7.2).

So you’re not allowed to add a property to the global object without using var or window. or whatnot, but there’s nothing to stop you from getting a property of the global object — or even setting the value of an existing property.

(By the way, the relevant bit of §8.7.2 is not terribly meaningful out-of-context, but for completeness’ sake, it’s this:

3. If IsUnresolvableReference(V), then
    a. If IsStrictReference(V) is true, then
        i. Throw ReferenceError exception.
    b. Call the [[Put]] internal method of the global object, passing
        GetReferencedName(V) for the property name, W for the value,
        and false for the Throw flag.

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