Static type checkers like Flow and TypeScript identify certain types of problems before you even run your code. They can also improve developer workflow by adding features like auto-completion. For this reason, we recommend using Flow or TypeScript instead of PropTypes for larger code bases.
Flow is a static type checker for your JavaScript code. It is developed at Facebook and is often used with React. It lets you annotate the variables, functions, and React components with a special type syntax, and catch mistakes early. You can read an introduction to Flow to learn its basics.
To use Flow, you need to:
We will explain these steps below in detail.
First, navigate to your project directory in the terminal. You will need to run the following command:
If you use Yarn, run:
yarn add --dev flow-bin
If you use npm, run:
npm install --save-dev flow-bin
This command installs the latest version of Flow into your project.
Now, add flow to the "scripts" section of your package.json to be able to use this from the terminal:
{// ..."scripts": {"flow": "flow",// ...},// ...}
Finally, run one of the following commands:
If you use Yarn, run:
yarn run flow init
If you use npm, run:
npm run flow init
This command will create a Flow configuration file that you will need to commit.
Flow extends the JavaScript language with a special syntax for type annotations. However, browsers aren’t aware of this syntax, so we need to make sure it doesn’t end up in the compiled JavaScript bundle that is sent to the browser.
The exact way to do this depends on the tools you use to compile JavaScript.
If your project was set up using Create React App, congratulations! The Flow annotations are already being stripped by default so you don’t need to do anything else in this step.
Note:
These instructions are not for Create React App users. Even though Create React App uses Babel under the hood, it is already configured to understand Flow. Only follow this step if you don’t use Create React App.
If you manually configured Babel for your project, you will need to install a special preset for Flow.
If you use Yarn, run:
yarn add --dev @babel/preset-flow
If you use npm, run:
npm install --save-dev @babel/preset-flow
Then add the flow preset to your Babel configuration. For example, if you configure Babel through .babelrc file, it could look like this:
{"presets": ["@babel/preset-flow","react"]}
This will let you use the Flow syntax in your code.
Note:
Flow does not require the
reactpreset, but they are often used together. Flow itself understands JSX syntax out of the box.
If you don’t use either Create React App or Babel, you can use flow-remove-types to strip the type annotations.
If you followed the instructions above, you should be able to run Flow for the first time.
yarn flow
If you use npm, run:
npm run flow
You should see a message like:
No errors!✨ Done in 0.17s.
By default, Flow only checks the files that include this annotation:
// @flow
Typically it is placed at the top of a file. Try adding it to some files in your project and run yarn flow or npm run flow to see if Flow already found any issues.
There is also an option to force Flow to check all files regardless of the annotation. This can be too noisy for existing projects, but is reasonable for a new project if you want to fully type it with Flow.
Now you’re all set! We recommend to check out the following resources to learn more about Flow:
TypeScript is a programming language developed by Microsoft. It is a typed superset of JavaScript, and includes its own compiler. Being a typed language, TypeScript can catch errors and bugs at build time, long before your app goes live. You can learn more about using TypeScript with React here.
To use TypeScript, you need to:
Let’s go over these in detail.
Create React App supports TypeScript out of the box.
To create a new project with TypeScript support, run:
npx create-react-app my-app --template typescript
You can also add it to an existing Create React App project, as documented here.
Note:
If you use Create React App, you can skip the rest of this page. It describes the manual setup which doesn’t apply to Create React App users.
It all begins with running one command in your terminal.
If you use Yarn, run:
yarn add --dev typescript
If you use npm, run:
npm install --save-dev typescript
Congrats! You’ve installed the latest version of TypeScript into your project. Installing TypeScript gives us access to the tsc command. Before configuration, let’s add tsc to the “scripts” section in our package.json:
{// ..."scripts": {"build": "tsc",// ...},// ...}
The compiler is of no help to us until we tell it what to do. In TypeScript, these rules are defined in a special file called tsconfig.json. To generate this file:
If you use Yarn, run:
yarn run tsc --init
If you use npm, run:
npx tsc --init
Looking at the now generated tsconfig.json, you can see that there are many options you can use to configure the compiler. For a detailed description of all the options, check here.
Of the many options, we’ll look at rootDir and outDir. In its true fashion, the compiler will take in typescript files and generate javascript files. However we don’t want to get confused with our source files and the generated output.
We’ll address this in two steps:
src directory.├── package.json├── src│ └── index.ts└── tsconfig.json
// tsconfig.json{"compilerOptions": {// ..."rootDir": "src","outDir": "build"// ...},}
Great! Now when we run our build script the compiler will output the generated javascript to the build folder. The TypeScript React Starter provides a tsconfig.json with a good set of rules to get you started.
Generally, you don’t want to keep the generated javascript in your source control, so be sure to add the build folder to your .gitignore.
In React, you most likely write your components in a .js file. In TypeScript we have 2 file extensions:
.ts is the default file extension while .tsx is a special extension used for files which contain JSX.
If you followed the instructions above, you should be able to run TypeScript for the first time.
yarn build
If you use npm, run:
npm run build
If you see no output, it means that it completed successfully.
To be able to show errors and hints from other packages, the compiler relies on declaration files. A declaration file provides all the type information about a library. This enables us to use javascript libraries like those on npm in our project.
There are two main ways to get declarations for a library:
Bundled - The library bundles its own declaration file. This is great for us, since all we need to do is install the library, and we can use it right away. To check if a library has bundled types, look for an index.d.ts file in the project. Some libraries will have it specified in their package.json under the typings or types field.
DefinitelyTyped - DefinitelyTyped is a huge repository of declarations for libraries that don’t bundle a declaration file. The declarations are crowd-sourced and managed by Microsoft and open source contributors. React for example doesn’t bundle its own declaration file. Instead we can get it from DefinitelyTyped. To do so enter this command in your terminal.
# yarnyarn add --dev @types/react# npmnpm i --save-dev @types/react
Local Declarations
Sometimes the package that you want to use doesn’t bundle declarations nor is it available on DefinitelyTyped. In that case, we can have a local declaration file. To do this, create a declarations.d.ts file in the root of your source directory. A simple declaration could look like this:
declare module 'querystring' {export function stringify(val: object): stringexport function parse(val: string): object}
You are now ready to code! We recommend to check out the following resources to learn more about TypeScript:
Reason is not a new language; it’s a new syntax and toolchain powered by the battle-tested language, OCaml. Reason gives OCaml a familiar syntax geared toward JavaScript programmers, and caters to the existing NPM/Yarn workflow folks already know.
Reason is developed at Facebook, and is used in some of its products like Messenger. It is still somewhat experimental but it has dedicated React bindings maintained by Facebook and a vibrant community.
Kotlin is a statically typed language developed by JetBrains. Its target platforms include the JVM, Android, LLVM, and JavaScript.
JetBrains develops and maintains several tools specifically for the React community: React bindings as well as Create React Kotlin App. The latter helps you start building React apps with Kotlin with no build configuration.
Note there are other statically typed languages that compile to JavaScript and are thus React compatible. For example, F#/Fable with elmish-react. Check out their respective sites for more information, and feel free to add more statically typed languages that work with React to this page!