This appears to be due to the fact that const is lexically scoped, so it also happens in case of let. kozyatinskiy@ also managed to reproduce this without getting DevTools involved simply by having two scripts:

<script>
let a = nonexistent; // Uncaught ReferenceError: nonexistent is not defined
</script>
<script>
a = 1; // Uncaught ReferenceError: a is not defined
</script>

The second error looks rather counter-intuitive and I'm having a hard time figuring whether this is according to the spec, so over to V8 team to triage.

I'm pretty sure this is the correct behavior per spec (uninitialized lexical declarations throw ReferenceErrors when accessed), but assigning to littledan for confirmation.

Yes, I agree that this is correct behavior per spec. You can also achieve it via classes!

class A extends B { } // where B doesn't exist
// now A is in limbo too

Note that this has previously been called out as a concern for counter-intuitive behavior in the spec, both inside and outside of the committee. If enough developers complain about this issue, it could be changed in a future revision. But any change would start on the spec side, not in V8.

Thanks for the explanation! I agree it seems counter-intuitive but the spec is the spec so as long as it doesn't change there v8 is fine.

 Issue 610817  has been merged into this issue.