Devtrium

React & TypeScript: how to type hooks (a complete guide)

October 12, 202110 min read

Introduction

Using hooks is one of the most common things you do when writing React applications. If you use TypeScript in your apps, knowing how to type hooks properly is very important (and if you don't use TypeScript, you should seriously think about it!).

In this article we'll go over how to type each usual hook React provides us. There are also a few bonuses sprinkled in there, like how to type forwardRef and how to type custom hooks.

This post is meant both as a tutorial that you can read linearly, but also as a reference that you can go back to whenever you have doubts about any of the topics covered. Feel free to bookmark this page for easy access later on.

Info

Typing hooks in React relies heavily on the concept of "generics" in TypeScript. If you're not familiar with that topic, check out this article going over what generic types are in TypeScript, and their application to React before you continue with this article.

Rely on type inference if you can

Before diving into the meat of the subject, we have to talk a bit about type inference.

In a lot of cases, TypeScript can infer the type of your hooks itself without your help. In those cases, you don't need to do anything yourself.

For example, the following is perfectly valid TypeScript, and works as you would want:

const [greeting, setGreeting] = useState('Hello World');

TypeScript will understand that greeting is supposed to be a string, and will also assign the right type to setGreeting as a result.

When to not rely on type inference

Type inference fails in two main cases.

  • The inferred type is too permissive for your use case.
  • The inferred type is wrong.

Let's explain both of these with examples.

Inferred type is too permissive

Let's look back at our previous example - we had a greeting of type string. But let's say the greeting could only have one of three predetermined values.

In that case, we would want greeting to have a very specific type, for example this one:

type Greeting = 'Hello World' | 'Hey!' | "What's up?";
Info

This type uses both a union type and literal types.

In that case, the inferred type is too permissive (string, instead of the specific subset of 3 strings we want), and you have to specify the type yourself:

const [greeting, setGreeting] = useState<Greeting>('Hello World');

The inferred type is wrong

Sometimes, the inferred type is wrong (or at least too restrictive/not the one you want). This happens frequently in React with default values in useState. Let's say that the initial value of greeting is null:

const [greeting, setGreeting] = useState(null);

In this case, TypeScript will infer that greeting is of type null (which means that it's always null). That's obviously wrong: we will want to set greeting to a string later on.

So you have to tell TypeScript that it can be something else:

const [greeting, setGreeting] = useState<string | null>(null);

With this, TypeScript will properly understand that greeting can be either null or a string.

Tip

One thing important to mention here is that when type inference doesn't work you should rely on type arguments over type assertions.

So prefer this:

const [greeting, setGreeting] = useState<Greeting>('Hello World');

over this:

const [greeting, setGreeting] = useState('Hello World' as Greeting);

Now that we are clear on when we can and can't rely on inferred types, let's see how to type each hook! I've already spoiled the first one a bit...

How to type useState

This will be short, as it's pretty simple and I've already shown multiple examples in the previous section.

const [greeting, setGreeting] = useState<string | null>(null);

That's it: you just have to specify the type of the state in the generic.

How to type useReducer

useReducer is a bit more complex to type than useState, because it has more moving parts.

There are two things to type: the state and the action.

Tip

If you're not comfortable with useReducer and the associated concepts, I would suggest giving How to use React useReducer like a pro a read.

Here is a useReducer example from my article on it. We will learn how to add proper types to it:

import { useReducer } from 'react';

const initialValue = {
  username: '',
  email: '',
};

const reducer = (state, action) => {
  switch (action.type) {
    case 'username':
      return { ...state, username: action.payload };
    case 'email':
      return { ...state, email: action.payload };
    case 'reset':
      return initialValue;
    default:
      throw new Error(`Unknown action type: ${action.type}`);
  }
};

const Form = () => {
  const [state, dispatch] = useReducer(reducer, initialValue);
  return (
    <div>
      <input
        type="text"
        value={state.username}
        onChange={(event) =>
          dispatch({ type: 'username', payload: event.target.value })
        }
      />
      <input
        type="email"
        value={state.email}
        onChange={(event) =>
          dispatch({ type: 'email', payload: event.target.value })
        }
      />
    </div>
  );
};

export default Form;

Let's start with typing the state.

How to type the reducer state

We have two choices to type the state:

Here's the typeof initialValue option:

const initialValue = {
  username: '',
  email: '',
};

const reducer = (state: typeof initialValue, action) => {
switch (action.type) { case 'username': return {...state, username: action.payload }; case 'email': return {...state, email: action.payload }; case 'reset': return initialValue; default: throw new Error(`Unknown action type: ${action.type}`); } };

And here's the type alias option:

type State = {
username: string;
email: string;
};
const initialValue = { username: '', email: '', };
const reducer = (state: State, action) => {
switch (action.type) { case 'username': return { ...state, username: action.payload }; case 'email': return { ...state, email: action.payload }; case 'reset': return initialValue; default: throw new Error(`Unknown action type: ${action.type}`); } };

How to type the reducer action

The reducer action is a tad harder to type than the state because its structure changes depending on the exact action.

For example for the 'username' action we might expect the following type:

type UsernameAction = {
  type: 'username';
  payload: string;
};

But for the 'reset' action we don't need a payload:

type ResetAction = {
  type: 'reset';
};

So how do we tell TypeScript that the action object can have very different structures depending on its exact type? With the help of discrimated unions!

And they even have a very nice syntax (in my opinion):

const initialValue = {
  username: "",
  email: ""
};

type Action =
| { type: "username"; payload: string }
| { type: "email"; payload: string }
| { type: "reset" };
const reducer = (state: typeof initialValue, action: Action) => {
switch (action.type) { case "username": return {...state, username: action.payload }; case "email": return { ...state, email: action.payload }; case "reset": return initialValue; default: throw new Error(`Unknown action type: ${action.type}`); } };

The Action type is saying that it can take any of the three types contained in the discriminated union. So if TypeScript sees that action.type is the string "username", it will automatically know that it should be in the first case and that the payload should be a string. Handy, isn't it?

And that's it! You have your useReducer fully typed, as it's able to infer everything it needs from the types of the reducer function.

Here is the example in its entirety:

import { useReducer } from 'react';

const initialValue = {
  username: '',
  email: '',
};

type Action =
  | { type: 'username'; payload: string }
  | { type: 'email'; payload: string }
  | { type: 'reset' };

const reducer = (state: typeof initialValue, action: Action) => {
  switch (action.type) {
    case 'username':
      return { ...state, username: action.payload };
    case 'email':
      return { ...state, email: action.payload };
    case 'reset':
      return initialValue;
    default:
      throw new Error(`Unknown action type: ${action.type}`);
  }
};

const Form = () => {
  const [state, dispatch] = useReducer(reducer, initialValue);

  return (
    <div>
      <input
        type="text"
        value={state.username}
        onChange={(event) =>
          dispatch({ type: 'username', payload: event.target.value })
        }
      />
      <input
        type="email"
        value={state.email}
        onChange={(event) =>
          dispatch({ type: 'email', payload: event.target.value })
        }
      />
    </div>
  );
};

export default Form;
Tip

The above types work well for useReducer, but not when using using Redux. Here is a good resource on that subject: Do not create union types with Redux Action Types. It's most likely an antipattern.

How to type useRef

useRef has two main uses:

  1. To hold a custom mutable value, a bit like useState (but with important differences)
  2. To keep a reference to a DOM object

Both of those uses require different types. Let's begin with the simpler one (at least type-wise).

How to type useRef for mutable values

It's basically the same as useState. You want useRef to hold a custom value, so you tell it the type.

Let's take this example directly from the React documentation:

function Timer() {
const intervalRef = useRef<number | undefined>();
useEffect(() => { const id = setInterval(() => { // ... }); intervalRef.current = id; return () => { clearInterval(intervalRef.current); }; }); // ... }
Info

If you want a reminder on how the setInterval function works (in general but also in React), I've got you covered.

How to type useRef for DOM nodes

A classic use case for using useRef with DOM nodes is focusing input elements:

import { useRef, useEffect } from 'react';

const AutoFocusInput = () => {
  const inputRef = useRef(null);

  useEffect(() => {
    inputRef.current.focus();
  }, []);

  return <input ref={inputRef} type="text" value="Hello World" />;
};

export default AutoFocusInput;

TypeScript has built-in DOM element types that we can make use of. The structure of those types is always the same: if name is the name of the HTML tag you're using, the corresponding type will be HTMLNameElement.

Tip

Knowing HTML element type names is usually straightforward, but I've always struggled to remember the name of the a tag type for some reason. It's HTMLAnchorElement.

Another one that isn't obvious is the one for <h1> tags (and all the others). It's HTMLHeadingElement.

You can also directly use HTMLElementTagNameMap['a'] to get the type from the tag name. It works for all HTML elements!

For our input, the name of the type will thus be HTMLInputElement:

import { useRef, useEffect } from 'react';

const AutoFocusInput = () => {
const inputRef = useRef<HTMLInputElement>(null);
useEffect(() => { inputRef.current?.focus(); }, []); return <input ref={inputRef} type="text" value="Hello World" />; }; export default AutoFocusInput;

Note that we've added a question mark to inputRef.current?.focus(). This is because for TypeScript, inputRef.current is possibly null. In this case, we know that it won't be null because it's populated by React before useEffect first runs. Adding the question mark is the simplest way to make TypeScript happy about that issue.

Info

I find it pretty fun to go look at the type definitions as they are defined in the TypeScript source code. Here is the link for HTMLInputElement!

Bonus: how to type forwardRef

Sometimes you want to forward refs to children components. To do that in React we have to wrap the component with forwardRef.

Here's the link to the React documentation on forwardRef if you want more info.

We'll see a simple example using a controlled Input component (that doesn't do much but hey, it's just an example):

import { ChangeEvent } from 'react';

type Props = {
  value: string,
  handleChange: (event: ChangeEvent<HTMLInputElement>) => void,
};

const TextInput = ({ value, handleChange }: Props) => {
  return <input type="text" value={value} onChange={handleChange} />;
};
Info

You can learn all about using events with TypeScript and React in this article!

Now here it is with forwardRef:

import { fowardRef, ChangeEvent } from 'react';

type Props = {
  value: string;
  handleChange: (event: ChangeEvent<HTMLInputElement>) => void;
};

const TextInput = forwardRef<HTMLInputElement, Props>(
({ value, handleChange }, ref) => { return ( <input ref={ref} type="text" value={value} onChange={handleChange} /> ); } );

The syntax isn't very pretty in my opinion, but overall it's very similar to typing useRef. You just have to provide forwardRef with what HTMLElement it should expect (in that case, HTMLInputElement).

The one thing which is a bit weird and that can be tricky is that the order of the component arguments ref and props is not the same as in the generic <HTMLInputElement, Props>.

It's an easy mistake to make, but TypeScript will yell at you if you do it so you should notice it quickly enough. 😁

How to type useEffect and useLayoutEffect

Those are pretty easy since you don't have to give them any type.

The only thing to be aware of is implicit returns. What I mean by that is that the callback inside useEffect is expected to either return nothing or a Destructor function that will clean up any side effect (and Destructor is the actual name of the type! See for yourself).

Sometimes you might implicitly return things, which won't make TypeScript happy (so you should avoid it). This is an example of such a case:

const doSomething = () => {
  return 'hey there';
};

useEffect(() => doSomething(), []);

In the code above, doSomething returns a string, which means that the return type of the callback inside useEffect is also a string. TypeScript doesn't want that, so it's not happy.

How to type useMemo and useCallback

Those are even easier than useEffect and useLayoutEffect since you don't need to type anything. Everything is inferred for you.

How to type React contexts

Info

I wrote a guide on using context in React. Check it out if you have questions about using contexts in React!

It's not an intro to contexts though, so if you're a beginner you should rather check the React documentation article on contexts.

This is the example we'll be using:

import { createContext, useState } from 'react';

const AuthContext = createContext(undefined);

const AuthContextProvider = ({ children }) => {
  const [user, setUser] = useState(null);

  const signOut = () => {
    setUser(null);
  };

  useEffect(() => {
    // fetch user and setUser
  }, []);

  return (
    <AuthContext.Provider value={{ user, signOut }}>
      {children}
    </AuthContext.Provider>
  );
};

export default AuthContextProvider;

It's a basic form of the context you might have in your app to manage the authentification state of users.

Providing types to the context is pretty easy. First, create a type for the value of the context, then provide it to the createContext function as a generic type:

import React, { createContext, useEffect, useState, ReactNode } from 'react';

type User = {
name: string;
email: string;
freeTrial: boolean;
};
type AuthValue = {
user: User | null;
signOut: () => void;
};
const AuthContext = createContext<AuthValue | undefined>(undefined);
type Props = { children: ReactNode; }; const AuthContextProvider = ({ children }: Props) => { const [user, setUser] = useState(null); const signOut = () => { setUser(null); }; useEffect(() => { // fetch user and setUser }, []); return ( <AuthContext.Provider value={{ user, signOut }}> {children} </AuthContext.Provider> ); }; export default AuthContextProvider;

Once you have provided the type to createContext, everything else will be populated for you thanks to the magic of generics.

One issue with the implementation above is that as far as TypeScript is concerned, the context value can be undefined. We know that it won't be because we are populating it directly in the AuthContextProvider. So how can we tell TypeScript that?

There are several solutions to that, but the one I prefer is using a custom context getter hook. This is something you should be probably doing anyway, regardless of TypeScript:

export const useAuthContext = () => {
  const context = useContext(AuthContext);

  if (context === undefined) {
    throw new Error('useAuthContext must be used inside AuthContext');
  }

  return context;
};

The condition in this hook acts as a type guard and guarantees that the returned context won't be undefined.

Bonus: how to type custom hooks

Typing custom hooks is basically the same as typing normal functions, there's nothing to be aware of in particular.

It's relatively common to be using generic types when using custom hooks, so be sure to check out my article about generics if you want to learn more about them and their usage in React.

There is one thing that is worth mentioning with respect to typing hooks, because it's a pattern common in React but less so elsewhere.

I'm speaking about the case when the hook is returning tuple values, such as in useState and the following example:

const useCustomHook = () => {
  return ['abc', 123];
};

TypeScipt will widen the return type of useCustomHook to be (number | string)[]: an array that can contain either numbers or strings. Usually, this isn't what you want since the first argument will always be a string and the second example always a number (in this example).

This is how to correctly type the example above:

const useCustomHook = (): [string, number] => {
  return ['abc', 123];
};

Wrap up

That's it folks!

I hope this article helped you (and will help you in the future) to know how to type hooks in React.

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