It's really kind of a moot point. The Array conversion trick wouldn't be necessary if String.prototype.repeat was defined, but it's not available and for a one-liner Kata it's not worth trying to polyfill it.

If it was undefined then available[key] / recipe[key] would evaluate to NaN and then default to 0 via the || operator.

Hopefully recipe[key] wouldn't be undefined since you're mapping over Object.keys(recipe).

My solution is about 5 times faster and doesn't rely on array conversion.