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TypeScript

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Example project: with-typescript
Expo has first-class support for TypeScript. The JavaScript interface of the Expo SDK is completely written in TypeScript.
To get started, create a tsconfig.json in your project root:
touch tsconfig.json
Running expo start will prompt you to install the required dependencies (typescript, @types/react, @types/react-native), and automatically configure your tsconfig.json.
Rename files to convert them to TypeScript. For example, you would rename App.js to App.tsx. Use the .tsx extension if the file includes React components (JSX). If the file did not include any JSX, you can use the .ts file extension.
mv App.js App.tsx
You can now run yarn tsc or npx tsc to typecheck the project.

Base configuration

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You can disable the TypeScript setup in Expo CLI with the environment variable EXPO_NO_TYPESCRIPT_SETUP=1
An Expo app's tsconfig.json should extend the expo/tsconfig.base by default. This sets the following default compiler options (which can be overwritten in your project's tsconfig.json):
  • "allowJs": true
    • Allow JavaScript files to be compiled. If you project requires more strictness, you can disable this.
  • "esModuleInterop": true
    • Improve Babel ecosystem compatibility. This also sets allowSyntheticDefaultImports to true, allowing default imports from modules with no default export.
  • "jsx": "react-native"
    • Preserve JSX, and converts the jsx extension to js. This is optimized for bundlers that transform the JSX internally (like Metro).
  • "lib": ["DOM", "ESNext"]
  • "moduleResolution": "node"
    • Emulate how Metro and Webpack resolve modules.
  • "noEmit": true
    • Only use the TypeScript compiler (TSC) to check the code. The Metro bundler is responsible for compiling TypeScript to JavaScript.
  • "resolveJsonModule": true
    • Enables importing .json files. Metro's default behavior is to allow importing json files as JS objects.
  • "skipLibCheck": true
    • Skip type checking of all declaration files (*.d.ts).
  • "target": "ESNext"
    • Compile to the latest version of ECMAScript.

Expo CLI will automatically modify your tsconfig.json to the preferred default which is optimized for universal React development:
{
  "extends": "expo/tsconfig.base",
  "compilerOptions": {}
}
The default configuration is forgiving and makes it easier to adopt TypeScript. If you'd like to opt-in to more strict type checking, you can add "strict": true to the compilerOptions. We recommend enabling this to minimize the chance of introducing runtime errors.
Certain language features may require additional configuration, for example if you'd like to use decorators you will need to add the experimentalDecorators option. For more information on the available properties see the TypeScript compiler options documentation documentation.

expo init -t expo-template-blank-typescript
The easiest way to get started is to initialize your new project using a TypeScript template. When you run expo init choose one of the templates with TypeScript in the name and then run yarn tsc or npx tsc to typecheck the project.
When you create new source files in your project you should use the .ts extension or the .tsx if the file includes React components.

A good place to start learning TypeScript is the official TypeScript Handbook.

We recommend reading over and referring to the React TypeScript CheatSheet to learn how to type your React components in a variety of common situations.

If you would like to go deeper and learn how to create more expressive and powerful types, we recommend the Advanced Static Types in TypeScript course (this requires an egghead.io subscription).