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Showcase of an Expo app coded in TypeScript
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README.md

React Native using Expo and Typescript

This is a React Native demo app using the Expo framework and written in TypeScript.

This repo used to also contain a type definition file for Expo, but that work has been merged into the @types/expo package. The expo npm package now contains its own set of type definitions, so @types/expo is now ignore, and has been removed. There is currently a type check issue with the SVGs that I haven't figured out how to solve.

Screen shot

Prerequisites

You don't need to install any global packages for this repo. expo-cli is included as a dev dependency.

Warnings When Installing Packages

There are a surprising amount of of warnings when installing Node packages, because of peer dependencies not being correct. My guess is that the Expo team has a tough time getting all the added React Native libraries to play well together. This app seems to run fine, so I believe it's safe to ignore the warnings.

Running the App

Start the local server. This will give you a QR code that you can scan using the Expo Client app on your mobile device.

yarn start

If you're on a Mac and have Xcode installed, you can run the app using the iOS Simulator with the following command. I am sure you can do something similar with Android.

yarn ios

Setting Expo and TypeScript With Expo Version ≥ 33

Since version 33 Expo includes great types, so there is no longer a need for @types/expo. I am really glad that TypeScript support has become official, but also admittedly a bit sad, that all the work in @types/expo will no longer be used.

Setting Expo and TypeScript With Expo Version ≥ 31

Since version 31 Expo includes Babel 7, and Babel is used out of the box to transform the TypeScript into JavaScript. Note that Babel doesn't "perform type-checking on TypeScript code; it [only transforms] your code, and it will compile regardless of whether type errors are present.", so if you want to type check your code, you have to run the TypeScript compiler separately. The npm/Yarn command script check-types in this repo does this. tsconfig.json is only used when checking the types. Babel is configured with babel.config.js.

The Expo team is currently working on native type definitions, but this works is not yet done. Look for the types property in node_modules/expo/package.json. If this property exists then TypeScript will use there instead of @types/expo. You can add the lines below to your tsconfig.json if you want to use @types/expo instead of the official type definitions. I would recommend trying to the use the built-in type definition once they are released, and only install @types/expo if they aren't good enough for your project.

"paths": {
  "expo": ["../node_modules/@types/expo", "../node_modules/expo"]
}

Setting Expo and TypeScript With Expo Version ≤ 30

This guide show how to transform and Expo app written in JavaScript in to TypeScript. While Expo supports TypeScript since version 31, it is still a work in progress, and namely the type definition files are not yet included.

Setting breakpoints in TypeScript files works, and hot reloading works, so it's a pretty solid development experience as long as you're lucky enough to not hit a missing type definition.

Add TypeScript

Add TypeScript and the helper library, tslib, to the project. I prefer micro managing the version of the packages that I'm using so I add --exact, but this is optional. You can, of course, also use npm instead of yarn.

yarn add --dev --exact typescript react-native-typescript-transformer
yarn add --exact tslib

Configure TypeScript by adding a tsconfig.json file in the root of your project. If you are stating a new project, I would definitely recommend including "strict": true and some of the other compiler checks. If you are converting a JavaScript project to TypeScript, you might want to loosen up the rules to start with, and apply them gradually.

Main App Component in TypeScript

Create a src folder, move App.js to that folder, and rename the file to App.tsx. Since TypeScript has a syntax that is very similar to JavaScript it is probably not necessary to make any modifications to App.tsx to make it valid TypeScript. It is not required to put the sources files in a src folder, but that is the de facto standard.

Create a new App.js in the root of the project, and insert the following lines. Expo will still be looking for App.js in the root of the project, and these lines simply loads src/App.tsx.

import App from "./src/App";
export default App;

Add Type Definitions

Finally add the type definitions for React, React Native, Expo and the Expo icons.

yarn add --dev --exact @types/react @types/react-native @types/expo @types/expo__vector-icons

Since Expo is evolving quickly it is not uncommon that the type definitions are outdated or simply missing. AudioScreen.tsx line 59 shows how to deal with an incorrect type definition and [MapViewScreen.tsx line 5] show how to deal with missing type definitions.

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