I know WP is moving towards Rest API with JSON instead of MySQL
The REST API and JSON are not ‘instead’ of MySQL. All of WordPress’ data is still MySQL, and there a no plans for it to be anything else. Nothing needs to be converted. They’re not instead of PHP either. What do you think powers the REST API? PHP and MySQL. The REST API just allows developers to interact with that data and the PHP back-end remotely by sending and receiving JSON.
While more and more of the front-end side of the admin is being built in JavaScript, the back-end is all still PHP, and there are no plans to change that. For the foreseeable future the vast majority of what you know will continue to be relevant.
That being said, there is new stuff being introduced into WordPress all the time, and the front-end side of that is going to involve a lot of JavaScript. So if you want to integrate and extend these new features (such as the upcoming new editor) you will probably need to learn JavaScript. This is why Matt Mullenweg, the creator of WordPress, publicly told developers in 2015, “Learn JavaScript, deeply”.
Learning WordPress (or any platform) isn’t something you finish and then you’re done. It’s a constant process as WordPress evolves. That’s always been true regardless of the database or programming language. Part of that process is now becoming more comfortable with JavaScript, but even if it wasn’t JavaScript there’d still be stuff you’d need to invest time in learning.
So rather than thinking about hiring a developer for a sudden changeover where everything will be converted to something new, you should thinking about having yourself or your employees start taking JavaScript seriously as a core part of WordPress and learn the skills necessary to work with it.
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