This year’s heavy focus in core has been on WordPress’s next generation Gutenberg editor. With the call to learn JavaScript deeply, it’s no surprise that Gutenberg is written in a JavaScript-first architecture. Once Gutenberg and its building blocks are in core, focus will broaden to then include Customizer. The Customizer was the first JavaScript single-page application in WordPress (added in 3.4). With the focus transition from Editor to Customizer, while keeping a JavaScript-first mindset, it is important for contributors and plugin authors that the Customizer’s JavaScript API be demystified to facilitate the integration of the new block editing interfaces. In my talk I’ll delve into the inner workings of the Customizer JS API, show how to use React to build custom controls in the Customizer, and demonstrate how to use the Customizer to preview changes to sites that use React-based themes—including previewing changes on headless REST API-driven sites.