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React+TypeScript Cheatsheets

react + ts logo

Cheatsheets for experienced React developers getting started with TypeScript

Basic | Advanced | Migrating | HOC | 中文翻译 | Español | Contribute! | Ask!

👋 This repo is maintained by @swyx, @ferdaber, @eps1lon, @IslamAttrash, and @jsjoeio, we're so happy you want to try out TypeScript with React! If you see anything wrong or missing, please file an issue! 👍

All Contributors

All React + TypeScript Cheatsheets

  • The Basic Cheatsheet (/README.md) is focused on helping React devs just start using TS in React apps
    • focus on opinionated best practices, copy+pastable examples
    • explains some basic TS types usage and setup along the way
    • answers the most Frequently Asked Questions
    • does not cover generic type logic in detail. Instead we prefer to teach simple troubleshooting techniques for newbies.
    • The goal is to get effective with TS without learning too much TS.
  • The Advanced Cheatsheet (/ADVANCED.md) helps show and explain advanced usage of generic types for people writing reusable type utilities/functions/render prop/higher order components and TS+React libraries.
    • It also has miscellaneous tips and tricks for pro users.
    • Advice for contributing to DefinitelyTyped
    • The goal is to take full advantage of TypeScript.
  • The Migrating Cheatsheet (/MIGRATING.md) helps collate advice for incrementally migrating large codebases from JS or Flow, from people who have done it.
    • We do not try to convince people to switch, only to help people who have already decided
    • ⚠️This is a new cheatsheet, all assistance is welcome
  • The HOC Cheatsheet (/HOC.md) specifically teaches people to write HOCs with examples.
    • Familiarity with Generics is necessary.
    • ⚠️This is the newest cheatsheet, all assistance is welcome

Basic Cheatsheet Table of Contents

Expand Table of Contents

Section 1: Setup

Prerequisites

  1. good understanding of React
  2. familiarity with TypeScript Types (2ality's guide is helpful. If you’re an absolute beginner in TypeScript, check out chibicode’s tutorial.)
  3. having read the TypeScript section in the official React docs.
  4. having read the React section of the new TypeScript playground (optional: also step through the 40+ examples under the playground's Examples section)

This guide will always assume you are starting with the latest TypeScript version. Notes for older versions will be in expandable <details> tags.

React + TypeScript Starter Kits

  1. Create React App v2.1+ with Typescript: npx create-react-app my-app --template typescript
  1. Basarat's guide for manual setup of React + TypeScript + Webpack + Babel

Import React

import * as React from "react";
import * as ReactDOM from "react-dom";

In TypeScript 2.7+, you can run TypeScript with --allowSyntheticDefaultImports (or add "allowSyntheticDefaultImports": true to tsconfig) to import like in regular jsx:

import React from "react";
import ReactDOM from "react-dom";
Explanation

Why allowSyntheticDefaultImports over esModuleInterop? Daniel Rosenwasser has said that it's better for webpack/parcel. For more discussion check out wmonk/create-react-app-typescript#214

You should also check the new TypeScript docs for official descriptions between each compiler flag!

Section 2: Getting Started

Function Components

These can be written as normal functions that take a props argument and return a JSX element.

type AppProps = { message: string }; /* could also use interface */
const App = ({ message }: AppProps) => <div>{message}</div>;
What about `React.FC`/`React.FunctionComponent`?

You can also write components with React.FunctionComponent (or the shorthand React.FC):

const App: React.FC<{ message: string }> = ({ message }) => (
  <div>{message}</div>
);

Some differences from the "normal function" version:

  • It provides typechecking and autocomplete for static properties like displayName, propTypes, and defaultProps - However, there are currently known issues using defaultProps with React.FunctionComponent. See this issue for details - scroll down to our defaultProps section for typing recommendations there.

  • It provides an implicit definition of children (see below) - however there are some issues with the implicit children type (e.g. DefinitelyTyped#33006), and it might considered better style to be explicit about components that consume children, anyway.

const Title: React.FunctionComponent<{ title: string }> = ({
  children,
  title
}) => <div title={title}>{children}</div>;

  • In the future, it may automatically mark props as readonly, though that's a moot point if the props object is destructured in the parameter list.

  • React.FunctionComponent is explicit about the return type, while the normal function version is implicit (or else needs additional annotation).

In most cases it makes very little difference which syntax is used, but the React.FC syntax is slightly more verbose without providing clear advantage, so precedence was given to the "normal function" syntax.

Minor Pitfalls

These patterns are not supported:

Conditional rendering

const MyConditionalComponent = ({ shouldRender = false }) =>
  shouldRender ? <div /> : false; // don't do this in JS either
const el = <MyConditionalComponent />; // throws an error

This is because due to limitations in the compiler, function components cannot return anything other than a JSX expression or null, otherwise it complains with a cryptic error message saying that the other type is not assignable to Element.

const MyArrayComponent = () => Array(5).fill(<div />);
const el2 = <MyArrayComponent />; // throws an error

Array.fill

Unfortunately just annotating the function type will not help so if you really need to return other exotic types that React supports, you'd need to perform a type assertion:

const MyArrayComponent = () => (Array(5).fill(<div />) as any) as JSX.Element;

See commentary by @ferdaber here.

Hooks

Hooks are supported in @types/react from v16.8 up.

useState

Type inference works very well most of the time:

const [val, toggle] = React.useState(false); // `val` is inferred to be a boolean, `toggle` only takes booleans

See also the Using Inferred Types section if you need to use a complex type that you've relied on inference for.

However, many hooks are initialized with null-ish default values, and you may wonder how to provide types. Explicitly declare the type, and use a union type:

const [user, setUser] = React.useState<IUser | null>(null);

// later...
setUser(newUser);

useRef

When using useRef, you have two options when creating a ref container that does not have an initial value:

const ref1 = useRef<HTMLElement>(null!);
const ref2 = useRef<HTMLElement | null>(null);

The first option will make ref1.current read-only, and is intended to be passed in to built-in ref attributes that React will manage (because React handles setting the current value for you).

The second option will make ref2.current mutable, and is intended for "instance variables" that you manage yourself.

useEffect

When using useEffect, take care not to return anything other than a function or undefined, otherwise both TypeScript and React will yell at you. This can be subtle when using arrow functions:

function DelayedEffect(props: { timerMs: number }) {
  const { timerMs } = props;
  // bad! setTimeout implicitly returns a number because the arrow function body isn't wrapped in curly braces
  useEffect(
    () =>
      setTimeout(() => {
        /* do stuff */
      }, timerMs),
    [timerMs]
  );
  return null;
}

useRef

function TextInputWithFocusButton() {
  // initialise with null, but tell TypeScript we are looking for an HTMLInputElement
  const inputEl = React.useRef<HTMLInputElement>(null);
  const onButtonClick = () => {
    // strict null checks need us to check if inputEl and current exist.
    // but once current exists, it is of type HTMLInputElement, thus it
    // has the method focus! ✅
    if (inputEl && inputEl.current) {
      inputEl.current.focus();
    }
  };
  return (
    <>
      {/* in addition, inputEl only can be used with input elements. Yay! */}
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

View in the TypeScript Playground

example from Stefan Baumgartner

useReducer

You can use Discriminated Unions for reducer actions. Don't forget to define the return type of reducer, otherwise Typescript will infer it.

type AppState = {};
type Action =
  | { type: "SET_ONE"; payload: string }
  | { type: "SET_TWO"; payload: number };

export function reducer(state: AppState, action: Action): AppState {
  switch (action.type) {
    case "SET_ONE":
      return {
        ...state,
        one: action.payload // `payload` is string
      };
    case "SET_TWO":
      return {
        ...state,
        two: action.payload // `payload` is number
      };
    default:
      return state;
  }
}

View in the TypeScript Playground

Custom Hooks

If you are returning an array in your Custom Hook, you will want to avoid type inference as Typescript will infer a union type (when you actually want different types in each position of the array). Instead, use TS 3.4 const assertions:

export function useLoading() {
  const [isLoading, setState] = React.useState(false);
  const load = (aPromise: Promise<any>) => {
    setState(true);
    return aPromise.finally(() => setState(false));
  };
  return [isLoading, load] as const; // infers [boolean, typeof load] instead of (boolean | typeof load)[]
}

View in the TypeScript Playground

This way, when you destructure you actually get the right types based on destructure position.

Alternative: Asserting a tuple return type

If you are having trouble with const assertions, you can also assert or define the function return types:

export function useLoading() {
  const [isLoading, setState] = React.useState(false);
  const load = (aPromise: Promise<any>) => {
    setState(true);
    return aPromise.finally(() => setState(false));
  };
  return [isLoading, load] as [
    boolean,
    (aPromise: Promise<any>) => Promise<any>
  ];
}

A helper function that automatically types tuples can also be helpful if you write a lot of custom hooks:

function tuplify<T extends any[]>(...elements: T) {
  return elements;
}

function useArray() {
  const numberValue = useRef(3).current;
  const functionValue = useRef(() => {}).current;
  return [numberValue, functionValue]; // type is (number | (() => void))[]
}

function useTuple() {
  const numberValue = useRef(3).current;
  const functionValue = useRef(() => {}).current;
  return tuplify(numberValue, functionValue); // type is [number, () => void]
}

Note that the React team recommends that custom hooks that return more than two values should use proper objects instead of tuples, however.

More Hooks + TypeScript reading:

If you are writing a React Hooks library, don't forget that you should also expose your types for users to use.

Example React Hooks + TypeScript Libraries:

Something to add? File an issue.

Class Components

Within TypeScript, React.Component is a generic type (aka React.Component<PropType, StateType>), so you want to provide it with (optional) prop and state type parameters:

type MyProps = {
  // using `interface` is also ok
  message: string;
};
type MyState = {
  count: number; // like this
};
class App extends React.Component<MyProps, MyState> {
  state: MyState = {
    // optional second annotation for better type inference
    count: 0
  };
  render() {
    return (
      <div>
        {this.props.message} {this.state.count}
      </div>
    );
  }
}

View in the TypeScript Playground

Don't forget that you can export/import/extend these types/interfaces for reuse.

Why annotate `state` twice?

It isn't strictly necessary to annotate the state class property, but it allows better type inference when accessing this.state and also initializing the state.

This is because they work in two different ways, the 2nd generic type parameter will allow this.setState() to work correctly, because that method comes from the base class, but initializing state inside the component overrides the base implementation so you have to make sure that you tell the compiler that you're not actually doing anything different.

See commentary by @ferdaber here.

No need for readonly

You often see sample code include readonly to mark props and state immutable:

type MyProps = {
  readonly message: string;
};
type MyState = {
  readonly count: number;
};

This is not necessary as React.Component<P,S> already marks them as immutable. (See PR and discussion!)

Class Methods: Do it like normal, but just remember any arguments for your functions also need to be typed:

class App extends React.Component<{ message: string }, { count: number }> {
  state = { count: 0 };
  render() {
    return (
      <div onClick={() => this.increment(1)}>
        {this.props.message} {this.state.count}
      </div>
    );
  }
  increment = (amt: number) => {
    // like this
    this.setState(state => ({
      count: state.count + amt
    }));
  };
}

View in the TypeScript Playground

Class Properties: If you need to declare class properties for later use, just declare it like state, but without assignment:

class App extends React.Component<{
  message: string;
}> {
  pointer: number; // like this
  componentDidMount() {
    this.pointer = 3;
  }
  render() {
    return (
      <div>
        {this.props.message} and {this.pointer}
      </div>
    );
  }
}

View in the TypeScript Playground

Something to add? File an issue.

Typing defaultProps

For Typescript 3.0+, type inference should work, although some edge cases are still problematic. Just type your props like normal, except don't use React.FC.

// ////////////////
// function components
// ////////////////
type Props = { age: number } & typeof defaultProps;
const defaultProps = {
  who: "Johny Five"
};

const Greet = (props: Props) => {
  /*...*/
};
Greet.defaultProps = defaultProps;

For Class components, there are a couple ways to do it(including using the Pick utility type) but the recommendation is to "reverse" the props definition:

type GreetProps = typeof Greet.defaultProps & {
  age: number;
};

class Greet extends React.Component<GreetProps> {
  static defaultProps = {
    name: "world"
  };
  /*...*/
}

// Type-checks! No type assertions needed!
let el = <Greet age={3} />;
Why does React.FC break defaultProps?

You can check the discussions here:

This is just the current state and may be fixed in future.

Typescript 2.9 and earlier

For Typescript 2.9 and earlier, there's more than one way to do it, but this is the best advice we've yet seen:

type Props = Required<typeof MyComponent.defaultProps> & {
  /* additional props here */
};

export class MyComponent extends React.Component<Props> {
  static defaultProps = {
    foo: "foo"
  };
}

Our former recommendation used the Partial type feature in TypeScript, which means that the current interface will fulfill a partial version on the wrapped interface. In that way we can extend defaultProps without any changes in the types!

interface IMyComponentProps {
  firstProp?: string;
  secondProp: IPerson[];
}

export class MyComponent extends React.Component<IMyComponentProps> {
  public static defaultProps: Partial<IMyComponentProps> = {
    firstProp: "default"
  };
}

The problem with this approach is it causes complex issues with the type inference working with JSX.LibraryManagedAttributes. Basically it causes the compiler to think that when creating a JSX expression with that component, that all of its props are optional.

See commentary by @ferdaber here.

Something to add? File an issue.

Types or Interfaces?

interfaces are different from types in TypeScript, but they can be used for very similar things as far as common React uses cases are concerned. Here's a helpful rule of thumb:

  • always use interface for public API's definition when authoring a library or 3rd party ambient type definitions.

  • consider using type for your React Component Props and State, because it is more constrained.

Types are useful for union types (e.g. type MyType = TypeA | TypeB) whereas Interfaces are better for declaring dictionary shapes and then implementing or extending them.

Useful table for Types vs Interfaces It's a nuanced topic, don't get too hung up on it. Here's a handy graphic:

https://pbs.twimg.com/media/DwV-oOsXcAIct2q.jpg (source: Karol Majewski)

Something to add? File an issue.

Basic Prop Types Examples

type AppProps = {
  message: string;
  count: number;
  disabled: boolean;
  /** array of a type! */
  names: string[];
  /** string literals to specify exact string values, with a union type to join them together */
  status: "waiting" | "success";
  /** any object as long as you dont use its properties (not common) */
  obj: object;
  obj2: {}; // almost the same as `object`, exactly the same as `Object`
  /** an object with defined properties (preferred) */
  obj3: {
    id: string;
    title: string;
  };
  /** array of objects! (common) */
  objArr: {
    id: string;
    title: string;
  }[];
  /** any function as long as you don't invoke it (not recommended) */
  onSomething: Function;
  /** function that doesn't take or return anything (VERY COMMON) */
  onClick: () => void;
  /** function with named prop (VERY COMMON) */
  onChange: (id: number) => void;
  /** alternative function type syntax that takes an event (VERY COMMON) */
  onClick(event: React.MouseEvent<HTMLButtonElement>): void;
  /** an optional prop (VERY COMMON!) */
  optional?: OptionalType;
};

Notice we have used the TSDoc /** comment */ style here on each prop. You can and are encouraged to leave descriptive comments on reusable components. For a fuller example and discussion, see our Commenting Components section in the Advanced Cheatsheet.

Useful React Prop Type Examples

export declare interface AppProps {
  children1: JSX.Element; // bad, doesnt account for arrays
  children2: JSX.Element | JSX.Element[]; // meh, doesnt accept functions
  children3: React.ReactChildren; // despite the name, not at all an appropriate type; it is a utility
  children4: React.ReactChild[]; // better
  children: React.ReactNode; // best, accepts everything
  functionChildren: (name: string) => React.ReactNode; // recommended function as a child render prop type
  style?: React.CSSProperties; // to pass through style props
  onChange?: React.FormEventHandler<HTMLInputElement>; // form events! the generic parameter is the type of event.target
  props: Props & React.PropsWithoutRef<JSX.IntrinsicElements["button"]>; // to impersonate all the props of a button element without its ref
}
JSX.Element vs React.ReactNode?

Quote @ferdaber: A more technical explanation is that a valid React node is not the same thing as what is returned by React.createElement. Regardless of what a component ends up rendering, React.createElement always returns an object, which is the JSX.Element interface, but React.ReactNode is the set of all possible return values of a component.

  • JSX.Element -> Return value of React.createElement
  • React.ReactNode -> Return value of a component

More discussion: Where ReactNode does not overlap with JSX.Element

Something to add? File an issue.

getDerivedStateFromProps

Before you start using getDerivedStateFromProps, please go through the documentation and You Probably Don't Need Derived State. Derived State can be easily achieved using hooks which can also help set up memoization easily.

Here are a few ways in which you can annotate getDerivedStateFromProps

  1. If you have explicitly typed your derived state and want to make sure that the return value from getDerivedStateFromProps conforms to it.
class Comp extends React.Component<Props, State> {
  static getDerivedStateFromProps(
    props: Props,
    state: State
  ): Partial<State> | null {
    //
  }
}
  1. When you want the function's return value to determine your state.
class Comp extends React.Component<
  Props,
  ReturnType<typeof Comp["getDerivedStateFromProps"]>
> {
  static getDerivedStateFromProps(props: Props) {}
}
  1. When you want derived state with other state fields and memoization
type CustomValue = any;
interface Props {
  propA: CustomValue;
}
interface DefinedState {
  otherStateField: string;
}
type State = DefinedState & ReturnType<typeof transformPropsToState>;
function transformPropsToState(props: Props) {
  return {
    savedPropA: props.propA, // save for memoization
    derivedState: props.propA
  };
}
class Comp extends React.PureComponent<Props, State> {
  constructor(props: Props) {
    super(props);
    this.state = {
      otherStateField: "123",
      ...transformPropsToState(props)
    };
  }
  static getDerivedStateFromProps(props: Props, state: State) {
    if (isEqual(props.propA, state.savedPropA)) return null;
    return transformPropsToState(props);
  }
}

View in the TypeScript Playground

Forms and Events

If performance is not an issue, inlining handlers is easiest as you can just use type inference and contextual typing:

const el = (
  <button
    onClick={event => {
      /* ... */
    }}
  />
);

But if you need to define your event handler separately, IDE tooling really comes in handy here, as the @type definitions come with a wealth of typing. Type what you are looking for and usually the autocomplete will help you out. Here is what it looks like for an onChange for a form event:

class App extends React.Component<
  {},
  {
    // no props
    text: string;
  }
> {
  state = {
    text: ""
  };

  // typing on RIGHT hand side of =
  onChange = (e: React.FormEvent<HTMLInputElement>): void => {
    this.setState({ text: e.currentTarget.value });
  };
  render() {
    return (
      <div>
        <input type="text" value={this.state.text} onChange={this.onChange} />
      </div>
    );
  }
}

View in the TypeScript Playground

Instead of typing the arguments and return values with React.FormEvent<> and void, you may alternatively apply types to the event handler itself (contributed by @TomasHubelbauer):

  // typing on LEFT hand side of =
  onChange: React.ChangeEventHandler<HTMLInputElement> = (e) => {
    this.setState({text: e.currentTarget.value})
  }
Why two ways to do the same thing?

The first method uses an inferred method signature (e: React.FormEvent<HTMLInputElement>): void and the second method enforces a type of the delegate provided by @types/react. So React.ChangeEventHandler<> is simply a "blessed" typing by @types/react, whereas you can think of the inferred method as more... artisanally hand-rolled. Either way it's a good pattern to know. See our Github PR for more.

Typing onSubmit, with Uncontrolled components in a Form

If you don't quite care about the type of the event, you can just use React.SyntheticEvent. If your target form has custom named inputs that you'd like to access, you can use type widening:

<form
  ref={formRef}
  onSubmit={(e: React.SyntheticEvent) => {
    e.preventDefault();
    const target = e.target as typeof e.target & {
      email: { value: string };
      password: { value: string };
    };
    const email = target.email.value; // typechecks!
    const password = target.password.value; // typechecks!
    // etc...
  }}
>
  <div>
    <label>
      Email:
      <input type="email" name="email" />
    </label>
  </div>
  <div>
    <label>
      Password:
      <input type="password" name="password" />
    </label>
  </div>
  <div>
    <input type="submit" value="Log in" />
  </div>
</form>

View in the TypeScript Playground

Of course, if you're making any sort of significant form, you should use Formik, which is written in TypeScript.

Context

Using React.createContext and context getters to make a createCtx with no defaultValue, yet no need to check for undefined:

// create context with no upfront defaultValue
// without having to do undefined check all the time
function createCtx<A>() {
  const ctx = React.createContext<A | undefined>(undefined);
  function useCtx() {
    const c = React.useContext(ctx);
    if (!c) throw new Error("useCtx must be inside a Provider with a value");
    return c;
  }
  return [useCtx, ctx.Provider] as const; // make TypeScript infer a tuple, not an array of union types
}

// usage

export const [useCtx, SettingProvider] = createCtx<string>(); // specify type, but no need to specify value upfront!
export function App() {
  const key = useCustomHook("key"); // get a value from a hook, must be in a component
  return (
    <SettingProvider value={key}>
      <Component />
    </SettingProvider>
  );
}
export function Component() {
  const key = useCtx(); // can still use without null check!
  return <div>{key}</div>;
}

View in the TypeScript Playground

Using React.createContext and useContext to make a createCtx with unstated-like context setters:

export function createCtx<A>(defaultValue: A) {
  type UpdateType = React.Dispatch<React.SetStateAction<typeof defaultValue>>;
  const defaultUpdate: UpdateType = () => defaultValue;
  const ctx = React.createContext({
    state: defaultValue,
    update: defaultUpdate
  });
  function Provider(props: React.PropsWithChildren<{}>) {
    const [state, update] = React.useState(defaultValue);
    return <ctx.Provider value={{ state, update }} {...props} />;
  }
  return [ctx, Provider] as const; // alternatively, [typeof ctx, typeof Provider]
}

// usage

const [ctx, TextProvider] = createCtx("someText");
export const TextContext = ctx;
export function App() {
  return (
    <TextProvider>
      <Component />
    </TextProvider>
  );
}
export function Component() {
  const { state, update } = React.useContext(TextContext);
  return (
    <label>
      {state}
      <input type="text" onChange={e => update(e.target.value)} />
    </label>
  );
}

View in the TypeScript Playground

A useReducer-based version may also be helpful.

Mutable Context Using a Class component wrapper

Contributed by: @jpavon

interface ProviderState {
  themeColor: string;
}

interface UpdateStateArg {
  key: keyof ProviderState;
  value: string;
}

interface ProviderStore {
  state: ProviderState;
  update: (arg: UpdateStateArg) => void;
}

const Context = React.createContext({} as ProviderStore); // type assertion on empty object

class Provider extends React.Component<{}, ProviderState> {
  public readonly state = {
    themeColor: "red"
  };

  private update = ({ key, value }: UpdateStateArg) => {
    this.setState({ [key]: value });
  };

  public render() {
    const store: ProviderStore = {
      state: this.state,
      update: this.update
    };

    return (
      <Context.Provider value={store}>{this.props.children}</Context.Provider>
    );
  }
}

const Consumer = Context.Consumer;

Something to add? File an issue.

forwardRef/createRef

Check the Hooks section for useRef.

createRef:

class CssThemeProvider extends React.PureComponent<Props> {
  private rootRef = React.createRef<HTMLDivElement>(); // like this
  render() {
    return <div ref={this.rootRef}>{this.props.children}</div>;
  }
}

forwardRef:

type Props = { children: React.ReactNode; type: "submit" | "button" };
export type Ref = HTMLButtonElement;
export const FancyButton = React.forwardRef<Ref, Props>((props, ref) => (
  <button ref={ref} className="MyClassName" type={props.type}>
    {props.children}
  </button>
));

If you are grabbing the props of a component that forwards refs, use ComponentPropsWithRef.

More info: https://medium.com/@martin_hotell/react-refs-with-typescript-a32d56c4d315

You may also wish to do Conditional Rendering with forwardRef.

Something to add? File an issue.

Portals

Using ReactDOM.createPortal:

const modalRoot = document.getElementById("modal-root") as HTMLElement;
// assuming in your html file has a div with id 'modal-root';

export class Modal extends React.Component {
  el: HTMLElement = document.createElement("div");

  componentDidMount() {
    modalRoot.appendChild(this.el);
  }

  componentWillUnmount() {
    modalRoot.removeChild(this.el);
  }

  render() {
    return ReactDOM.createPortal(this.props.children, this.el);
  }
}

View in the TypeScript Playground

Context of Example

This example is based on the Event Bubbling Through Portal example of React docs.

Error Boundaries

Not written yet.

Something to add? File an issue.

Concurrent React/React Suspense

Not written yet. watch https://github.com/sw-yx/fresh-async-react for more on React Suspense and Time Slicing.

Something to add? File an issue.

Basic Troubleshooting Handbook: Types

⚠️ Have you read the TypeScript FAQ?) Your answer might be there!

Facing weird type errors? You aren't alone. This is the hardest part of using TypeScript with React. Be patient - you are learning a new language after all. However, the more you get good at this, the less time you'll be working against the compiler and the more the compiler will be working for you!

Try to avoid typing with any as much as possible to experience the full benefits of typescript. Instead, let's try to be familiar with some of the common strategies to solve these issues.

Union Types and Type Guarding

Union types are handy for solving some of these typing problems:

class App extends React.Component<
  {},
  {
    count: number | null; // like this
  }
> {
  state = {
    count: null
  };
  render() {
    return <div onClick={() => this.increment(1)}>{this.state.count}</div>;
  }
  increment = (amt: number) => {
    this.setState(state => ({
      count: (state.count || 0) + amt
    }));
  };
}

View in the TypeScript Playground

Type Guarding: Sometimes Union Types solve a problem in one area but create another downstream. If A and B are both object types, A | B isn't "either A or B", it is "A or B or both at once", which causes some confusion if you expected it to be the former. Learn how to write checks, guards, and assertions (also see the Conditional Rendering section below). For example:

interface Admin {
  role: string;
}
interface User {
  email: string;
}

// Method 1: use `in` keyword
function redirect(user: Admin | User) {
  if ("role" in user) {
    // use the `in` operator for typeguards since TS 2.7+
    routeToAdminPage(user.role);
  } else {
    routeToHomePage(user.email);
  }
}

// Method 2: custom type guard, does the same thing in older TS versions or where `in` isnt enough
function isAdmin(user: Admin | User): user is Admin {
  return (user as any).role !== undefined;
}

View in the TypeScript Playground

Method 2 is also known as User-Defined Type Guards and can be really handy for readable code. This is how TS itself refines types with typeof and instanceof.

If you need if...else chains or the switch statement instead, it should "just work", but look up Discriminated Unions if you need help. (See also: Basarat's writeup). This is handy in typing reducers for useReducer or Redux.

Optional Types

If a component has an optional prop, add a question mark and assign during destructure (or use defaultProps).

class MyComponent extends React.Component<{
  message?: string; // like this
}> {
  render() {
    const { message = "default" } = this.props;
    return <div>{message}</div>;
  }
}

You can also use a ! character to assert that something is not undefined, but this is not encouraged.

Something to add? File an issue with your suggestions!

Enum Types

Enums in TypeScript default to numbers. You will usually want to use them as strings instead:

export enum ButtonSizes {
  default = "default",
  small = "small",
  large = "large"
}

Usage:

export const PrimaryButton = (
  props: Props & React.HTMLProps<HTMLButtonElement>
) => <Button size={ButtonSizes.default} {...props} />;

A simpler alternative to enum is just declaring a bunch of strings with union:

export declare type Position = "left" | "right" | "top" | "bottom";

This is handy because TypeScript will throw errors when you mistype a string for your props.

Type Assertion

Sometimes you know better than TypeScript that the type you're using is narrower than it thinks, or union types need to be asserted to a more specific type to work with other APIs, so assert with the as keyword. This tells the compiler you know better than it does.

class MyComponent extends React.Component<{
  message: string;
}> {
  render() {
    const { message } = this.props;
    return (
      <Component2 message={message as SpecialMessageType}>{message}</Component2>
    );
  }
}

View in the TypeScript Playground

Note that you cannot assert your way to anything - basically it is only for refining types. Therefore it is not the same as "casting" a type.

You can also assert a property is non-null, when accessing it:

element.parentNode!.removeChild(element) // ! before the period
myFunction(document.getElementById(dialog.id!)! // ! after the property accessing
let userID!: string // definite assignment assertion... be careful!

Of course, try to actually handle the null case instead of asserting :)

Simulating Nominal Types

TS' structural typing is handy, until it is inconvenient. However you can simulate nominal typing with type branding:

type OrderID = string & { readonly brand: unique symbol };
type UserID = string & { readonly brand: unique symbol };
type ID = OrderID | UserID;

We can create these values with the Companion Object Pattern:

function OrderID(id: string) {
  return id as OrderID;
}
function UserID(id: string) {
  return id as UserID;
}

Now TypeScript will disallow you from using the wrong ID in the wrong place:

function queryForUser(id: UserID) {
  // ...
}
queryForUser(OrderID("foobar")); // Error, Argument of type 'OrderID' is not assignable to parameter of type 'UserID'

In future you can use the unique keyword to brand. See this PR.

Intersection Types

Adding two types together can be handy, for example when your component is supposed to mirror the props of a native component like a button:

export interface Props {
  label: string;
}
export const PrimaryButton = (
  props: Props & React.HTMLProps<HTMLButtonElement> // adding my Props together with the @types/react button provided props
) => <Button {...props} />;

You can also use Intersection Types to make reusable subsets of props for similar components:

type BaseProps = {
   className?: string,
   style?: React.CSSProperties
   name: string // used in both
}
type DogProps = {
  tailsCount: number
}
type HumanProps = {
  handsCount: number
}
export const Human: React.FC<BaseProps & HumanProps> = // ...
export const Dog: React.FC<BaseProps & DogProps> = // ...

View in the TypeScript Playground

Make sure not to confuse Intersection Types (which are and operations) with Union Types (which are or operations).

Union Types

This section is yet to be written (please contribute!). Meanwhile, see our commentary on Union Types usecases.

The ADVANCED cheatsheet also has information on Discriminated Union Types, which are helpful when TypeScript doesn't seem to be narrowing your union type as you expect.

Overloading Function Types

Specifically when it comes to functions, you may need to overload instead of union type. The most common way function types are written uses the shorthand:

type FunctionType1 = (x: string, y: number) => number;

But this doesn't let you do any overloading. If you have the implementation, you can put them after each other with the function keyword:

function pickCard(x: { suit: string; card: number }[]): number;
function pickCard(x: number): { suit: string; card: number };
function pickCard(x): any {
  // implementation with combined signature
  // ...
}

However, if you don't have an implementation and are just writing a .d.ts definition file, this won't help you either. In this case you can forego any shorthand and write them the old-school way. The key thing to remember here is as far as TypeScript is concerned, functions are just callable objects with no key:

type pickCard = {
  (x: { suit: string; card: number }[]): number;
  (x: number): { suit: string; card: number };
  // no need for combined signature in this form
  // you can also type static properties of functions here eg `pickCard.wasCalled`
};

Note that when you implement the actual overloaded function, the implementation will need to declare the combined call signature that you'll be handling, it won't be inferred for you. You can see readily see examples of overloads in DOM APIs, e.g. createElement.

Read more about Overloading in the Handbook.

Using Inferred Types

Leaning on Typescript's Type Inference is great... until you realize you need a type that was inferred, and have to go back and explicitly declare types/interfaces so you can export them for reuse.

Fortunately, with typeof, you won't have to do that. Just use it on any value:

const [state, setState] = React.useState({
  foo: 1,
  bar: 2
}); // state's type inferred to be {foo: number, bar: number}

const someMethod = (obj: typeof state) => {
  // grabbing the type of state even though it was inferred
  // some code using obj
  setState(obj); // this works
};

Using Partial Types

Working with slicing state and props is common in React. Again, you don't really have to go and explicitly redefine your types if you use the Partial generic type:

const [state, setState] = React.useState({
  foo: 1,
  bar: 2
}); // state's type inferred to be {foo: number, bar: number}

// NOTE: stale state merging is not actually encouraged in React.useState
// we are just demonstrating how to use Partial here
const partialStateUpdate = (obj: Partial<typeof state>) =>
  setState({ ...state, ...obj });

// later on...
partialStateUpdate({ foo: 2 }); // this works
Minor caveats on using Partial

Note that there are some TS users who don't agree with using Partial as it behaves today. See subtle pitfalls of the above example here, and check out this long discussion on why @types/react uses Pick instead of Partial.

The Types I need weren't exported!

This can be annoying but here are ways to grab the types!

  • Grabbing the Prop types of a component: Use React.ComponentProps and typeof, and optionally Omit any overlapping types
import { Button } from "library"; // but doesn't export ButtonProps! oh no!
type ButtonProps = React.ComponentProps<typeof Button>; // no problem! grab your own!
type AlertButtonProps = Omit<ButtonProps, "onClick">; // modify
const AlertButton: React.FC<AlertButtonProps> = props => (
  <Button onClick={() => alert("hello")} {...props} />
);

You may also use ComponentPropsWithoutRef (instead of ComponentProps) and ComponentPropsWithRef (if your component specifically forwards refs)

  • Grabbing the return type of a function: use ReturnType:
// inside some library - return type { baz: number } is inferred but not exported
function foo(bar: string) {
  return { baz: 1 };
}

//  inside your app, if you need { baz: number }
type FooReturn = ReturnType<typeof foo>; // { baz: number }

In fact you can grab virtually anything public: see this blogpost from Ivan Koshelev

function foo() {
  return {
    a: 1,
    b: 2,
    subInstArr: [
      {
        c: 3,
        d: 4
      }
    ]
  };
}

type InstType = ReturnType<typeof foo>;
type SubInstArr = InstType["subInstArr"];
type SubIsntType = SubInstArr[0];

let baz: SubIsntType = {
  c: 5,
  d: 6 // type checks ok!
};

//You could just write a one-liner,
//But please make sure it is forward-readable
//(you can understand it from reading once left-to-right with no jumps)
type SubIsntType2 = ReturnType<typeof foo>["subInstArr"][0];
let baz2: SubIsntType2 = {
  c: 5,
  d: 6 // type checks ok!
};
  • TS also ships with a Parameters utility type for extracting the parameters of a function
  • for anything more "custom", the infer keyword is the basic building block for this, but takes a bit of getting used to. Look at the source code for the above utility types, and this example to get the idea.

Troubleshooting Handbook: Images and other non-TS/TSX files

Use declaration merging:

// declaration.d.ts
// anywhere in your project, NOT the same name as any of your .ts/tsx files
declare module "*.png";

// importing in a tsx file
import * as logo from "./logo.png";

Note that tsc cannot bundle these files for you, you will have to use Webpack or Parcel.

Related issue: microsoft/TypeScript-React-Starter#12 and StackOverflow

Troubleshooting Handbook: Operators

  • typeof and instanceof: type query used for refinement
  • keyof: get keys of an object
  • O[K]: property lookup
  • [K in O]: mapped types
  • + or - or readonly or ?: addition and subtraction and readonly and optional modifiers
  • x ? Y : Z: Conditional types for generic types, type aliases, function parameter types
  • !: Nonnull assertion for nullable types
  • =: Generic type parameter default for generic types
  • as: type assertion
  • is: type guard for function return types

Conditional Types are a difficult topic to get around so here are some extra resources:

Troubleshooting Handbook: Utilities

these are all built in, see source in es5.d.ts:

  • ConstructorParameters: a tuple of class constructor's parameter types
  • Exclude: exclude a type from another type
  • Extract: select a subtype that is assignable to another type
  • InstanceType: the instance type you get from a newing a class constructor
  • NonNullable: exclude null and undefined from a type
  • Parameters: a tuple of a function's parameter types
  • Partial: Make all properties in an object optional
  • Readonly: Make all properties in an object readonly
  • ReadonlyArray: Make an immutable array of the given type
  • Pick: A subtype of an object type with a subset of its keys
  • Record: A map from a key type to a value type
  • Required: Make all properties in an object required
  • ReturnType A function's return type

This section needs writing, but you can probably find a good starting point with Wes Bos' ESLint config (which comes with a YouTube intro).

Troubleshooting Handbook: tsconfig.json

You can find all the Compiler options in the Typescript docs. The new TS docs also has per-flag annotations of what each does. This is the setup I roll with for APPS (not libraries - for libraries you may wish to see the settings we use in tsdx):

{
  "compilerOptions": {
    "incremental": true,
    "outDir": "build/lib",
    "target": "es5",
    "module": "esnext",
    "lib": ["dom", "esnext"],
    "sourceMap": true,
    "importHelpers": true,
    "declaration": true,
    "rootDir": "src",
    "strict": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "allowJs": false,
    "jsx": "react",
    "moduleResolution": "node",
    "baseUrl": "src",
    "forceConsistentCasingInFileNames": true,
    "esModuleInterop": true,
    "suppressImplicitAnyIndexErrors": true,
    "allowSyntheticDefaultImports": true,
    "experimentalDecorators": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "build", "scripts"]
}

Please open an issue and discuss if there are better recommended choices for React.

Selected flags and why we like them:

  • esModuleInterop: disables namespace imports (import * as foo from "foo") and enables CJS/AMD/UMD style imports (import fs from "fs")
  • strict: strictPropertyInitialization forces you to initialize class properties or explicitly declare that they can be undefined. You can opt out of this with a definite assignment assertion.
  • "typeRoots": ["./typings", "./node_modules/@types"]: By default, TypeScript looks in node_modules/@types and parent folders for third party type declarations. You may wish to override this default resolution so you can put all your global type declarations in a special typings folder.

Compilation speed grows linearly with size of codebase. For large projects, you will want to use Project References. See our ADVANCED cheatsheet for commentary.

Troubleshooting Handbook: Bugs in official typings

If you run into bugs with your library's official typings, you can copy them locally and tell TypeScript to use your local version using the "paths" field. In your tsconfig.json:

{
  "compilerOptions": {
    "paths": {
      "mobx-react": ["../typings/modules/mobx-react"]
    }
  }
}

Thanks to @adamrackis for the tip.

If you just need to add an interface, or add missing members to an existing interface, you don't need to copy the whole typing package. Instead, you can use declaration merging:

// my-typings.ts
declare module "plotly.js" {
  interface PlotlyHTMLElement {
    removeAllListeners(): void;
  }
}

// MyComponent.tsx
import { PlotlyHTMLElement } from "plotly.js";

const f = (e: PlotlyHTMLElement) => {
  e.removeAllListeners();
};

You dont always have to implement the module, you can simply import the module as any for a quick start:

// my-typings.ts
declare module "plotly.js"; // each of its imports are `any`

Because you don't have to explicitly import this, this is known as an ambient module declaration. You can do AMD's in a script-mode .ts file (no imports or exports), or a .d.ts file anywhere in your project.

You can also do ambient variable and ambient type declarations:

// ambient utiltity type
type ToArray<T> = T extends unknown[] ? T : T[];
// ambient variable
declare let process: {
  env: {
    NODE_ENV: "development" | "production";
  };
};
process = {
  env: {
    NODE_ENV: "production"
  }
};

You can see examples of these included in the built in type declarations in the lib field of tsconfig.json

Recommended React + TypeScript codebases to learn from

React Boilerplates:

React Native Boilerplates: contributed by @spoeck

Editor Tooling and Integration

Linting

⚠️Note that TSLint is now in maintenance and you should try to use ESLint instead. If you are interested in TSLint tips, please check this PR from @azdanov. The rest of this section just focuses on ESLint. You can convert TSlint to ESlint with this tool.

⚠️This is an evolving topic. typescript-eslint-parser is no longer maintained and work has recently begun on typescript-eslint in the ESLint community to bring ESLint up to full parity and interop with TSLint.

Follow the TypeScript + ESLint docs at https://github.com/typescript-eslint/typescript-eslint:

yarn add -D @typescript-eslint/eslint-plugin @typescript-eslint/parser eslint

add a lint script to your package.json:

  "scripts": {
    "lint": "eslint 'src/**/*.ts'"
  },

and a suitable .eslintrc.js (using .js over .json here so we can add comments):

module.exports = {
  env: {
    es6: true,
    node: true,
    jest: true
  },
  extends: "eslint:recommended",
  parser: "@typescript-eslint/parser",
  plugins: ["@typescript-eslint"],
  parserOptions: {
    ecmaVersion: 2017,
    sourceType: "module"
  },
  rules: {
    indent: ["error", 2],
    "linebreak-style": ["error", "unix"],
    quotes: ["error", "single"],
    "no-console": "warn",
    "no-unused-vars": "off",
    "@typescript-eslint/no-unused-vars": [
      "error",
      { vars: "all", args: "after-used", ignoreRestSiblings: false }
    ],
    "@typescript-eslint/explicit-function-return-type": "warn", // Consider using explicit annotations for object literals and function return types even when they can be inferred.
    "no-empty": "warn"
  }
};

Most of this is taken from the tsdx PR which is for libraries.

More .eslintrc.json options to consider with more options you may want for apps:

{
  "extends": [
    "airbnb",
    "prettier",
    "prettier/react",
    "plugin:prettier/recommended",
    "plugin:jest/recommended",
    "plugin:unicorn/recommended"
  ],
  "plugins": ["prettier", "jest", "unicorn"],
  "parserOptions": {
    "sourceType": "module",
    "ecmaFeatures": {
      "jsx": true
    }
  },
  "env": {
    "es6": true,
    "browser": true,
    "jest": true
  },
  "settings": {
    "import/resolver": {
      "node": {
        "extensions": [".js", ".jsx", ".ts", ".tsx"]
      }
    }
  },
  "overrides": [
    {
      "files": ["**/*.ts", "**/*.tsx"],
      "parser": "typescript-eslint-parser",
      "rules": {
        "no-undef": "off"
      }
    }
  ]
}

You can read a fuller TypeScript + ESLint setup guide here from Matterhorn, in particular check https://github.com/MatterhornDev/learn-typescript-linting.

Another great resource is "Using ESLint and Prettier in a TypeScript Project" by @robertcoopercode.

If you're looking for information on Prettier, check out the Prettier.

Other React + TypeScript resources

Recommended React + TypeScript talks

  • Please help contribute this new section!

Time to Really Learn TypeScript

Believe it or not, we have only barely introduced TypeScript here in this cheatsheet. There is a whole world of generic type logic that you will eventually get into, however it becomes far less dealing with React than just getting good at TypeScript so it is out of scope here. But at least you can get productive in React now :)

It is worth mentioning some resources to help you get started:

Example App

My question isn't answered here!

Contributors

This project follows the all-contributors specification. See CONTRIBUTORS.md for the full list. Contributions of any kind welcome!

About

Cheatsheets for experienced React developers getting started with TypeScript

twitter.com/swyx/status/1002776892154445826

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