As someone with 15+ years of experience, a lot of that FE specific, that is the advice I always give newer devs if asked. Learn the fundamentals of Javascript, HTML, CSS (it's like a 3 legged stool, even if the JS leg is oversized in the days of web apps). If you know how to program, and you know the fundamentals, you can work in whatever framework is thrown your way.
Now, practically speaking, that's actually probably better advice for someone with a job and 1-2 years in. To get an initial foothold in the industry, people often need to specialize in one specific thing (React at the moment most likely), in order to be able to demonstrate enough competence to get that initial job and so I understand how fundamentals can be backburnered initially. But I recommend devs don't let that initial success lock them into that framework - that's the time to get back and learn all the fundamentals, go wide, learn a couple other frameworks even so it's easy to compare and contrast the strengths/weaknesses of each.
And you will want to be well-versed in the framework you currently use day to day, knowing best practices, architecture patterns that work and those to avoid, etc. Knowing the fundamentals will help, but there will be framework specific things that will change from framework to framework, even code-base to code-base sometimes. So it's always going to be a bit of a balance. But long-term, IMO, being well-versed in the fundamentals affords you the most flexibility and employ-ability long-term.
Now, practically speaking, that's actually probably better advice for someone with a job and 1-2 years in. To get an initial foothold in the industry, people often need to specialize in one specific thing (React at the moment most likely), in order to be able to demonstrate enough competence to get that initial job and so I understand how fundamentals can be backburnered initially. But I recommend devs don't let that initial success lock them into that framework - that's the time to get back and learn all the fundamentals, go wide, learn a couple other frameworks even so it's easy to compare and contrast the strengths/weaknesses of each.
And you will want to be well-versed in the framework you currently use day to day, knowing best practices, architecture patterns that work and those to avoid, etc. Knowing the fundamentals will help, but there will be framework specific things that will change from framework to framework, even code-base to code-base sometimes. So it's always going to be a bit of a balance. But long-term, IMO, being well-versed in the fundamentals affords you the most flexibility and employ-ability long-term.