[Silhouette]

What, like a couple more weeks?

For a junior, sure, but I would expect a mid-to-senior developer working on front-end JS for a living to have some experience of at least one of the popular rendering libraries/frameworks and therefore to be able to understand at least the essentials of any of the others after a few hours of training. They're relatively simple interfaces, and many of the same concepts come up in most or all of them. Of course it will take a bit longer to understand the edge cases, any unusual or unique aspects, and any common traps to avoid, but you don't need all of that to be productive with everyday front-end development work. Presumably anyone new and/or below senior level is going to be supervised/mentored while they're getting up to speed and have their code reviewed anyway.