1. Learn the actual language
Learn from third-party code, but don’t lean on anything. No React and no Angular. Going back to “how they did things in the 90's” may not earn you cool points, but solid fundamentals are more important.
You will also face some of the problems that popular libraries and frameworks are designed to solve. This will help you develop the ability to pick the right — as opposed to fashionable — tools for the job. You may even realize that some solutions are an overkill, and come up with simpler ones.
2. Write your own JS library or framework
Not because the world is in desperate need of yet another one, but because this exercise will make you:
b) confident in your ability to make sense of new libraries and frameworks.
3. Harness the power of new tech