In this scenario, he is making 2.5 cakes. The description is super misleading.

It seems that author doesn't gona fix or explain this issue.
I also agree that {"eggs":1,"flour":200,"sugar":100} is a correct answer.
No, seriously, WTF?

That's exactly what came to my mind!
He just added enough ingredients to bake 2.5 cakes. He has to add some anyway.
I also think

{"eggs":1,"flour":200,"sugar":100}

is the correct output for this case.

also for ruby

And not just for JS. I believe it is no higher than 7 kyu for Python.