TypeScript syntax support (#4837)

docusaurus/docs/adding-typescript.md

@@ -0,0 +1,45 @@
+---
+id: adding-typescript
+title: Adding TypeScript
+---
+
+TypeScript is a typed superset of JavaScript that compiles to plain JavaScript.
+
+Recent versions of [TypeScript](https://www.typescriptlang.org/) work with Create React App projects out of the box thanks to Babel 7. Beware that Babel 7 TypeScript does not allow some features of TypeScript such as constant enum and namespaces.
+
+To add TypeScript to a Create React App project, follow these steps:
+
+1. Run `npm install --save typescript fork-ts-checker-webpack-plugin @types/react @types/react-dom @types/jest` (or `yarn add typescript fork-ts-checker-webpack-plugin @types/react @types/react-dom @types/jest`).
+2. Rename the `.js` files you want to convert. Use `.tsx` if they use JSX or `.ts` if not (e.g. `git mv src/index.js src/index.tsx`).
+
+3. Create a [`tsconfig.json` file](https://www.typescriptlang.org/docs/handbook/tsconfig-json.html) at the root directory with the following content:
+
+```json
+{
+  "compilerOptions": {
+    "target": "es5",
+    "module": "esnext",
+    "moduleResolution": "node",
+    "lib": ["esnext", "dom", "dom.iterable"],
+    "allowJs": true,
+    "allowSyntheticDefaultImports": true,
+    "esModuleInterop": true,
+    "isolatedModules": true,
+    "jsx": "preserve",
+    "noEmit": true,
+    "skipLibCheck": true,
+    "strict": true
+  },
+  "include": ["src"]
+}
+```
+
+4. Copy [loaders.d.ts](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/src/loaders.d.ts) from the template to your `src` directory.
+
+Type errors will show up in the same console as the build one.
+
+> Note: If you prefer to run type checking separately from the build process, you can run `npm uninstall fork-ts-checker-webpack-plugin` (or `yarn remove fork-ts-checker-webpack-plugin`) to remove the `fork-ts-checker-webpack-plugin` dependency and then `npx tsc -w` on a new terminal tab.
+
+We recommend using [VSCode](https://code.visualstudio.com/) for a better integrated experience.
+
+To learn more about TypeScript, check out [its documentation](https://www.typescriptlang.org/).