I agree about having more fundamental JS knowledge coming in handy. I see JS frameworks having a multiplier effect - the more JS you know beforehand, the more the benefits of a JS framework will be multiplied.
One recent example is my work with the Ember framework at work. The framework gives good patterns for managing component state and related data to this state in their "controllers." But foundational JS knowledge helped me better see the limits in these controllers. A coworker used a combination of import/export, JS classes, and object-oriented programming to better abstract and manage some data complexity in a way that split the code more and made it reusable. It was standard JS used to make even better use of that framework.
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