For Front-end frameworks and libraries React has the highest satisfaction ratio and a higher usage by salary range .
But interestingly, Svelte has gained lots of interest in 2020.
For the Data Layer (loading and managing data in your app). Redux has decreased in Satisfaction, and Interest. GraphQL and Apollo have taken first and second in these categories.
Still, Redux has to have a high salary range in lager companies.
In Testing (tools for testing your code) Jest seems to be the dominant tool.
For Mobile & Desktop development a safe bet would be on React Native, Electron, and Native Development.
Fun fact, most people reported to use Udemy and Egghead.io for courses: