JSX is an XML-like syntax extension to ECMAScript without any defined semantics. It’s NOT intended to be implemented by engines or browsers. It’s NOT a proposal to incorporate JSX into the ECMAScript spec itself. It’s intended to be used by various preprocessors (transpilers) to transform these tokens into standard ECMAScript.
https://facebook.github.io/jsx/
https://reactjs.org/docs/introducing-jsx.html
https://reactjs.org/docs/jsx-in-depth.html
Comments