In every react project I've worked on, the majority of the business logic was written inside components (the old "code behind" anti-pattern from WinForms days)

In every react project I've worked on, the majority of the business logic was written inside components (the old "code behind" anti-pattern from WinForms days), and about half the files had the extension "jsx" or "tsx". It also required the use of a react specific browser extension to debug effectively. React is a de-jure library, but a de-facto framework.