How can I add ReactJS to the child theme?

I’d suggest the cleanest option if you want to do a bit more development to make your calendar reusable is to build it as a plugin which implements a shortcode. You’d have to do a bit of PHP programming to build your plugin, but this would be the cleanest solution. It’d give you a nice shortcode like [mycalendar] to put in any content anywhere in WordPress to render your calendar at that point.

If you just want to do it once as a quick hack you could continue as you are and load the CSS/JS manually then pop the HTML/JS directly in to page content with the Code Editor in the UI.

Either way, you may have some research to do about how to integrate React components into WordPress.

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