Using TypeScript

TypeScript is a popular way to add type definitions to JavaScript codebases. Out of the box, TypeScript supports JSX and you can get full React Web support by adding @types/react and @types/react-dom to your project.

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Installation

All production-grade React frameworks offer support for using TypeScript. Follow the framework specific guide for installation:

Adding TypeScript to an existing React project

To install the latest version of React’s type definitions:

Terminal

npm install @types/react @types/react-dom

The following compiler options need to be set in your tsconfig.json:

dom must be included in lib (Note: If no lib option is specified, dom is included by default).
jsx must be set to one of the valid options. preserve should suffice for most applications. If you’re publishing a library, consult the jsx documentation on what value to choose.

TypeScript with React Components

補足

Every file containing JSX must use the .tsx file extension. This is a TypeScript-specific extension that tells TypeScript that this file contains JSX.

Writing TypeScript with React is very similar to writing JavaScript with React. The key difference when working with a component is that you can provide types for your component’s props. These types can be used for correctness checking and providing inline documentation in editors.

Taking the MyButton component from the Quick Start guide, we can add a type describing the title for the button:

Fork TypeScript Playground

function MyButton({ title }: { title: string }) {
  return (
    <button>{title}</button>
  );
}

export default function MyApp() {
  return (
    <div>
      <h1>Welcome to my app</h1>
      <MyButton title="I'm a button" />
    </div>
  );
}

補足

These sandboxes can handle TypeScript code, but they do not run the type-checker. This means you can amend the TypeScript sandboxes to learn, but you won’t get any type errors or warnings. To get type-checking, you can use the TypeScript Playground or use a more fully-featured online sandbox.

This inline syntax is the simplest way to provide types for a component, though once you start to have a few fields to describe it can become unwieldy. Instead, you can use an interface or type to describe the component’s props:

Fork TypeScript Playground

interface MyButtonProps {
  /** The text to display inside the button */
  title: string;
  /** Whether the button can be interacted with */
  disabled: boolean;
}

function MyButton({ title, disabled }: MyButtonProps) {
  return (
    <button disabled={disabled}>{title}</button>
  );
}

export default function MyApp() {
  return (
    <div>
      <h1>Welcome to my app</h1>
      <MyButton title="I'm a disabled button" disabled={true}/>
    </div>
  );
}

The type describing your component’s props can be as simple or as complex as you need, though they should be an object type described with either a type or interface. You can learn about how TypeScript describes objects in Object Types but you may also be interested in using Union Types to describe a prop that can be one of a few different types and the Creating Types from Types guide for more advanced use cases.

Example Hooks

The type definitions from @types/react include types for the built-in hooks, so you can use them in your components without any additional setup. They are built to take into account the code you write in your component, so you will get inferred types a lot of the time and ideally do not need to handle the minutiae of providing the types.

However, we can look at a few examples of how to provide types for hooks.

`useState`

The useState hook will re-use the value passed in as the initial state to determine what the type of the value should be. For example:

// Infer the type as "boolean"
const [enabled, setEnabled] = useState(false);

Will assign the type of boolean to enabled, and setEnabled will be a function accepting either a boolean argument, or a function that returns a boolean. If you want to explicitly provide a type for the state, you can do so by providing a type argument to the useState call:

// Explicitly set the type to "boolean"
const [enabled, setEnabled] = useState<boolean>(false);

This isn’t very useful in this case, but a common case where you may want to provide a type is when you have a union type. For example, status here can be one of a few different strings:

type Status = "idle" | "loading" | "success" | "error";

const [status, setStatus] = useState<Status>("idle");

Or, as recommended in Principles for structuring state, you can group related state as an object and describe the different possibilities via object types:

type RequestState =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success', data: any }
  | { status: 'error', error: Error };

const [requestState, setRequestState] = useState<RequestState>({ status: 'idle' });

`useReducer`

The useReducer hook is a more complex hook that takes a reducer function and an initial state. The types for the reducer function are inferred from the initial state. You can optionally provide a type argument to the useReducer call to provide a type for the state, but it is often better to set the type on the initial state instead:

Fork TypeScript Playground

import {useReducer} from 'react';

interface State {
   count: number 
};

type CounterAction =
  | { type: "reset" }
  | { type: "setCount"; value: State["count"] }

const initialState: State = { count: 0 };

function stateReducer(state: State, action: CounterAction): State {
  switch (action.type) {
    case "reset":
      return initialState;
    case "setCount":
      return { ...state, count: action.value };
    default:
      throw new Error("Unknown action");
  }
}

export default function App() {
  const [state, dispatch] = useReducer(stateReducer, initialState);

  const addFive = () => dispatch({ type: "setCount", value: state.count + 5 });
  const reset = () => dispatch({ type: "reset" });

  return (
    <div>
      <h1>Welcome to my counter</h1>

      <p>Count: {state.count}</p>
      <button onClick={addFive}>Add 5</button>
      <button onClick={reset}>Reset</button>
    </div>
  );
}

We are using TypeScript in a few key places:

interface State describes the shape of the reducer’s state.
type CounterAction describes the different actions which can be dispatched to the reducer.
const initialState: State provides a type for the initial state, and also the type which is used by useReducer by default.
stateReducer(state: State, action: CounterAction): State sets the types for the reducer function’s arguments and return value.

A more explicit alternative to setting the type on initialState is to provide a type argument to useReducer:

import { stateReducer, State } from './your-reducer-implementation';

const initialState = { count: 0 };

export default function App() {
  const [state, dispatch] = useReducer<State>(stateReducer, initialState);
}

`useContext`

The useContext hook is a technique for passing data down the component tree without having to pass props through components. It is used by creating a provider component and often by creating a hook to consume the value in a child component.

The type of the value provided by the context is inferred from the value passed to the createContext call:

Fork TypeScript Playground

import { createContext, useContext, useState } from 'react';

type Theme = "light" | "dark" | "system";
const ThemeContext = createContext<Theme>("system");

const useGetTheme = () => useContext(ThemeContext);

export default function MyApp() {
  const [theme, setTheme] = useState<Theme>('light');

  return (
    <ThemeContext.Provider value={theme}>
      <MyComponent />
    </ThemeContext.Provider>
  )
}

function MyComponent() {
  const theme = useGetTheme();

  return (
    <div>
      <p>Current theme: {theme}</p>
    </div>
  )
}

This technique works when you have an default value which makes sense - but there are occasionally cases when you do not, and in those cases null can feel reasonable as a default value. However, to allow the type-system to understand your code, you need to explicitly set ContextShape | null on the createContext.

This causes the issue that you need to eliminate the | null in the type for context consumers. Our recommendation is to have the hook do a runtime check for it’s existence and throw an error when not present:

import { createContext, useContext, useState, useMemo } from 'react';

// This is a simpler example, but you can imagine a more complex object here
type ComplexObject = {
  kind: string
};

// The context is created with `| null` in the type, to accurately reflect the default value.
const Context = createContext<ComplexObject | null>(null);

// The `| null` will be removed via the check in the hook.
const useGetComplexObject = () => {
  const object = useContext(Context);
  if (!object) { throw new Error("useGetComplexObject must be used within a Provider") }
  return object;
}

export default function MyApp() {
  const object = useMemo(() => ({ kind: "complex" }), []);

  return (
    <Context.Provider value={object}>
      <MyComponent />
    </Context.Provider>
  )
}

function MyComponent() {
  const object = useGetComplexObject();

  return (
    <div>
      <p>Current object: {object.kind}</p>
    </div>
  )
}

`useMemo`

The useMemo hooks will create/re-access a memorized value from a function call, re-running the function only when dependencies passed as the 2nd parameter are changed. The result of calling the hook is inferred from the return value from the function in the first parameter. You can be more explicit by providing a type argument to the hook.

// The type of visibleTodos is inferred from the return value of filterTodos
const visibleTodos = useMemo(() => filterTodos(todos, tab), [todos, tab]);

`useCallback`

The useCallback provide a stable reference to a function as long as the dependencies passed into the second parameter are the same. Like useMemo, the function’s type is inferred from the return value of the function in the first parameter, and you can be more explicit by providing a type argument to the hook.

const handleClick = useCallback(() => {
  // ...
}, [todos]);

When working in TypeScript strict mode useCallback requires adding types for the parameters in your callback. This is because the type of the callback is inferred from the return value of the function, and without parameters the type cannot be fully understood.

Depending on your code-style preferences, you could use the *EventHandler functions from the React types to provide the type for the event handler at the same time as defining the callback:

import { useState, useCallback } from 'react';

export default function Form() {
  const [value, setValue] = useState("Change me");

  const handleChange = useCallback<React.ChangeEventHandler<HTMLInputElement>>((event) => {
    setValue(event.currentTarget.value);
  }, [setValue])
  
  return (
    <>
      <input value={value} onChange={handleChange} />
      <p>Value: {value}</p>
    </>
  );
}

Useful Types

There is quite an expansive set of types which come from the @types/react package, it is worth a read when you feel comfortable with how React and TypeScript interact. You can find them in React’s folder in DefinitelyTyped. We will cover a few of the more common types here.

DOM Events

When working with DOM events in React, the type of the event can often be inferred from the event handler. However, when you want to extract a function to be passed to an event handler, you will need to explicitly set the type of the event.

Fork TypeScript Playground

import { useState } from 'react';

export default function Form() {
  const [value, setValue] = useState("Change me");

  function handleChange(event: React.ChangeEvent<HTMLInputElement>) {
    setValue(event.currentTarget.value);
  }

  return (
    <>
      <input value={value} onChange={handleChange} />
      <p>Value: {value}</p>
    </>
  );
}

There are many types of events provided in the React types - the full list can be found here which is based on the most popular events from the DOM.

When determining the type you are looking for you can first look at the hover information for the event handler you are using, which will show the type of the event.

If you need to use an event that is not included in this list, you can use the React.SyntheticEvent type, which is the base type for all events.

Children

There are two common paths to describing the children of a component. The first is to use the React.ReactNode type, which is a union of all the possible types that can be passed as children in JSX:

interface ModalRendererProps {
  title: string;
  children: React.ReactNode;
}

This is a very broad definition of children. The second is to use the React.ReactElement type, which is only JSX elements and not JavaScript primitives like strings or numbers:

interface ModalRendererProps {
  title: string;
  children: React.ReactElement;
}

Note, that you cannot use TypeScript to describe that the children are a certain type of JSX elements, so you cannot use the type-system to describe a component which only accepts <li> children.

You can see all an example of both React.ReactNode and React.ReactElement with the type-checker in this TypeScript playground.

Style Props

When using inline styles in React, you can use React.CSSProperties to describe the object passed to the style prop. This type is a union of all the possible CSS properties, and is a good way to ensure you are passing valid CSS properties to the style prop, and to get auto-complete in your editor.

interface MyComponentProps {
  style: React.CSSProperties;
}

Further learning

This guide has covered the basics of using TypeScript with React, but there is a lot more to learn. Individual API pages on the docs may contain more in-depth documentation on how to use them with TypeScript.

We recommend the following resources:

The TypeScript handbook is the official documentation for TypeScript, and covers most key language features.
The TypeScript release notes covers a each new features in-depth.
React TypeScript Cheatsheet is a community-maintained cheatsheet for using TypeScript with React, covering a lot of useful edge cases and providing more breadth than this document.
TypeScript Community Discord is a great place to ask questions and get help with TypeScript and React issues.

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