The "R" in MERN stands for React, which means writing JavaScript (or TypeScript) using the React ecosystem, usually including JSX (which is an XML/HTML-like syntax for representing UI) React is what you'd use instead of writing HTML and vanilla JS or jQuery. Anything you can do with React, you can do with plain HTML, CSS, and JS, though it might take a lot more work. If you use Mongo, Express, and Node, without React, that would work perfectly fine, but it wouldn't be the MERN stack.
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