No, JSX is not mandatory for React development.
JSX is simply a syntax sugar that makes writing React components easier and more readable. Instead of using JSX, you can directly use React.createElement() to build your UI. However, writing complex UIs without JSX can quickly become tedious and harder to maintain.

For example: Without JSX:

const element = React.createElement('h1', null, 'Hello, World!');

With JSX:

const element = <h1>Hello, World!</h1>;

Most developers prefer using JSX for better code clarity, but technically, you can build full React applications without it.

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