The Tor Browser / file revision

 // The LHS of a for-of loop is not evaluated until after the .next() method returns.

 var s;

 function f() {

     s += 'f';

     return {};

 }

 // Test 1: .next() throws StopIteration right away. f is never called.

 s = '';

 for (f().x of [])

     s += '.';

 assertEq(s, '');

 // Test 2: check proper interleaving of f calls, iterator.next() calls, and the loop body.

 function g() {

     s += 'g';

     yield 0;

     s += 'g';

     yield 1;

     s += 'g';

 }

 for (f().x of g())

     s += '.';

 assertEq(s, 'gf.gf.g');