Generator function default parameter value initializers should be evaluated when the generator function is called not at first next() call

[ianwjhalliday]

Currently default parameter value initializers are evaluated on the first call to next() on the resulting generator object from a call to a generator function. The ES spec says they should be evaluated on the call to the generator function itself, before it returns the generator object.

E.g.

let x = 0;
function* gf(a = x = 1) { }
let g = gf();
print(x); // should print 1, not 0

One simple solution might be to put a yield between the default parameter initializers and the body in the bytecode and then in the generator function call stub, call next() once, then return the generator object.

[ljharb]

If I've modified %IteratorPrototype%.next, wouldn't that be observable?

[ianwjhalliday]

We would call the generator next function directly for this purpose. Indeed we'd only do it this way if we can guarantee no observable effects. I believe this is possible but haven't explored the idea in depth.

[abchatra]

@ianwjhalliday Any update on this issue?

[ianwjhalliday]

No, have not had and still do not have time to look into this yet. If someone else can take it up that would be great. @aneeshdk?

[aneeshdk]

#2941