useReducer

const [state, dispatch] = useReducer(reducer, initialArg, init?)

• Reference
  • useReducer(reducer, initialArg, init?)
  • dispatch function
• Usage
  • Adding a reducer to a component
  • Writing the reducer function
  • Avoiding recreating the initial state
• Troubleshooting
  • I’ve dispatched an action, but logging gives me the old state value
  • I’ve dispatched an action, but the screen doesn’t update
  • A part of my reducer state becomes undefined after dispatching
  • My entire reducer state becomes undefined after dispatching
  • I’m getting an error: “Too many re-renders”
  • My reducer or initializer function runs twice

Reference

useReducer(reducer, initialArg, init?)

Call useReducer at the top level of your component to manage its state with a reducer.

import { useReducer } from 'react';

function reducer(state, action) {
  // ...
}

function MyComponent() {
  const [state, dispatch] = useReducer(reducer, { age: 42 });
  // ...

See more examples below.

Parameters

• reducer: The reducer function that specifies how the state gets updated. It must be pure, should take the state and action as arguments, and should return the next state. State and action can be of any types.
• initialArg: The value from which the initial state is calculated. It can be a value of any type. How the initial state is calculated from it depends on the next init argument.
• optional init: The initializer function that should return the initial state. If it’s not specified, the initial state is set to initialArg. Otherwise, the initial state is set to the result of calling init(initialArg).

Returns

useReducer returns an array with exactly two values:

1. The current state. During the first render, it’s set to init(initialArg) or initialArg (if there’s no init).
2. The dispatch function that lets you update the state to a different value and trigger a re-render.

Caveats

• useReducer is a Hook, so you can only call it at the top level of your component or your own Hooks. You can’t call it inside loops or conditions. If you need that, extract a new component and move the state into it.
• In Strict Mode, React will call your reducer and initializer twice in order to help you find accidental impurities. This is development-only behavior and does not affect production. If your reducer and initializer are pure (as they should be), this should not affect your logic. The result from one of the calls is ignored.

dispatch function

The dispatch function returned by useReducer lets you update the state to a different value and trigger a re-render. You need to pass the action as the only argument to the dispatch function:

const [state, dispatch] = useReducer(reducer, { age: 42 });

function handleClick() {
  dispatch({ type: 'incremented_age' });
  // ...

React will set the next state to the result of calling the reducer function you’ve provided with the current state and the action you’ve passed to dispatch.

Parameters

• action: The action performed by the user. It can be a value of any type. By convention, an action is usually an object with a type property identifying it and, optionally, other properties with additional information.

Returns

dispatch functions do not have a return value.

Caveats

• The dispatch function only updates the state variable for the next render. If you read the state variable after calling the dispatch function, you will still get the old value that was on the screen before your call.

• If the new value you provide is identical to the current state, as determined by an Object.is comparison, React will skip re-rendering the component and its children. This is an optimization. React may still need to call your component before ignoring the result, but it shouldn’t affect your code.

• React batches state updates. It updates the screen after all the event handlers have run and have called their set functions. This prevents multiple re-renders during a single event. In the rare case that you need to force React to update the screen earlier, for example to access the DOM, you can use flushSync.

Usage

Adding a reducer to a component

Call useReducer at the top level of your component to manage state with a reducer.

import { useReducer } from 'react';

function reducer(state, action) {
  // ...
}

function MyComponent() {
  const [state, dispatch] = useReducer(reducer, { age: 42 });
  // ...

useReducer returns an array with exactly two items:

1. The current state of this state variable, initially set to the initial state you provided.
2. The dispatch function that lets you change it in response to interaction.

To update what’s on the screen, call dispatch with an object representing what the user did, called an action:

function handleClick() {
  dispatch({ type: 'incremented_age' });
}

React will pass the current state and the action to your reducer function. Your reducer will calculate and return the next state. React will store that next state, render your component with it, and update the UI.

useReducer is very similar to useState, but it lets you move the state update logic from event handlers into a single function outside of your component. Read more about choosing between useState and useReducer.

Writing the reducer function

A reducer function is declared like this:

function reducer(state, action) {
  // ...
}

Then you need to fill in the code that will calculate and return the next state. By convention, it is common to write it as a switch statement. For each case in the switch, calculate and return some next state.

function reducer(state, action) {
  switch (action.type) {
    case 'incremented_age': {
      return {
        name: state.name,
        age: state.age + 1
      };
    }
    case 'changed_name': {
      return {
        name: action.nextName,
        age: state.age
      };
    }
  }
  throw Error('Unknown action: ' + action.type);
}

Actions can have any shape. By convention, it’s common to pass objects with a type property identifying the action. It should include the minimal necessary information that the reducer needs to compute the next state.

function Form() {
  const [state, dispatch] = useReducer(reducer, { name: 'Taylor', age: 42 });
  
  function handleButtonClick() {
    dispatch({ type: 'incremented_age' });
  }

  function handleInputChange(e) {
    dispatch({
      type: 'changed_name',
      nextName: e.target.value
    });
  }
  // ...

The action type names are local to your component. Each action describes a single interaction, even if that leads to multiple changes in data. The shape of the state is arbitrary, but usually it’ll be an object or an array.

Read extracting state logic into a reducer to learn more.

function reducer(state, action) {
  switch (action.type) {
    case 'incremented_age': {
      // 🚩 Don't mutate an object in state like this:
      state.age = state.age + 1;
      return state;
    }

function reducer(state, action) {
  switch (action.type) {
    case 'incremented_age': {
      // ✅ Instead, return a new object
      return {
        ...state,
        age: state.age + 1
      };
    }

Avoiding recreating the initial state

React saves the initial state once and ignores it on the next renders.

function createInitialState(username) {
  // ...
}

function TodoList({ username }) {
  const [state, dispatch] = useReducer(reducer, createInitialState(username));
  // ...

Although the result of createInitialState(username) is only used for the initial render, you’re still calling this function on every render. This can be wasteful if it’s creating large arrays or performing expensive calculations.

To solve this, you may pass it as an initializer function to useReducer as the third argument instead:

function createInitialState(username) {
  // ...
}

function TodoList({ username }) {
  const [state, dispatch] = useReducer(reducer, username, createInitialState);
  // ...

Notice that you’re passing createInitialState, which is the function itself, and not createInitialState(), which is the result of calling it. This way, the initial state does not get re-created after initialization.

In the above example, createInitialState takes a username argument. If your initializer doesn’t need any information to compute the initial state, you may pass null as the second argument to useReducer.

Troubleshooting

I’ve dispatched an action, but logging gives me the old state value

Calling the dispatch function does not change state in the running code:

function handleClick() {
  console.log(state.age);  // 42

  dispatch({ type: 'incremented_age' }); // Request a re-render with 43
  console.log(state.age);  // Still 42!

  setTimeout(() => {
    console.log(state.age); // Also 42!
  }, 5000);
}

This is because states behaves like a snapshot. Updating state requests another render with the new state value, but does not affect the state JavaScript variable in your already-running event handler.

If you need to guess the next state value, you can calculate it manually by calling the reducer yourself:

const action = { type: 'incremented_age' };
dispatch(action);

const nextState = reducer(state, action);
console.log(state);     // { age: 42 }
console.log(nextState); // { age: 43 }

I’ve dispatched an action, but the screen doesn’t update

React will ignore your update if the next state is equal to the previous state, as determined by an Object.is comparison. This usually happens when you change an object or an array in state directly:

function reducer(state, action) {
  switch (action.type) {
    case 'incremented_age': {
      // 🚩 Wrong: mutating existing object
      state.age++;
      return state;
    }
    case 'changed_name': {
      // 🚩 Wrong: mutating existing object
      state.name = action.nextName;
      return state;
    }
    // ...
  }
}

You mutated an existing state object and returned it, so React ignored the update. To fix this, you need to ensure that you’re always updating objects in state and updating arrays in state instead of mutating them:

function reducer(state, action) {
  switch (action.type) {
    case 'incremented_age': {
      // ✅ Correct: creating a new object
      return {
        ...state,
        age: state.age + 1
      };
    }
    case 'changed_name': {
      // ✅ Correct: creating a new object
      return {
        ...state,
        name: action.nextName
      };
    }
    // ...
  }
}

A part of my reducer state becomes undefined after dispatching

Make sure that every case branch copies all of the existing fields when returning the new state:

function reducer(state, action) {
  switch (action.type) {
    case 'incremented_age': {
      return {
        ...state, // Don't forget this!
        age: state.age + 1
      };
    }
    // ...

Without ...state above, the returned next state would only contain the age field and nothing else.

My entire reducer state becomes undefined after dispatching

If your state unexpectedly becomes undefined, you’re likely forgetting to return state in one of the cases, or your action type doesn’t match any of the case statements. To find why, throw an error outside the switch:

function reducer(state, action) {
  switch (action.type) {
    case 'incremented_age': {
      // ...
    }
    case 'edited_name': {
      // ...
    }
  }
  throw Error('Unknown action: ' + action.type);
}

You can also use a static type checker like TypeScript to catch such mistakes.

I’m getting an error: “Too many re-renders”

You might get an error that says: Too many re-renders. React limits the number of renders to prevent an infinite loop. Typically, this means that you’re unconditionally dispatching an action during render, so your component enters a loop: render, dispatch (which causes a render), render, dispatch (which causes a render), and so on. Very often, this is caused by a mistake in specifying an event handler:

// 🚩 Wrong: calls the handler during render
return <button onClick={handleClick()}>Click me</button>

// ✅ Correct: passes down the event handler
return <button onClick={handleClick}>Click me</button>

// ✅ Correct: passes down an inline function
return <button onClick={(e) => handleClick(e)}>Click me</button>

If you can’t find the cause of this error, click on the arrow next to the error in the console and look through the JavaScript stack to find the specific dispatch function call responsible for the error.

My reducer or initializer function runs twice

In Strict Mode, React will call your reducer and initializer functions twice. This shouldn’t break your code.

This development-only behavior helps you keep components pure. React uses the result of one of the calls, and ignores the result of the other call. As long as your component, initializer, and reducer functions are pure, this shouldn’t affect your logic. However, if they are accidentally impure, this helps you notice the mistakes.

For example, this impure reducer function mutates an array in state:

function reducer(state, action) {
  switch (action.type) {
    case 'added_todo': {
      // 🚩 Mistake: mutating state
      state.todos.push({ id: nextId++, text: action.text });
      return state;
    }
    // ...
  }
}

Because React calls your reducer function twice, you’ll see the todo was added twice, so you’ll know that there is a mistake. In this example, you can fix the mistake by replacing the array instead of mutating it:

function reducer(state, action) {
  switch (action.type) {
    case 'added_todo': {
      // ✅ Correct: replacing with new state
      return {
        ...state,
        todos: [
          ...state.todos,
          { id: nextId++, text: action.text }
        ]
      };
    }
    // ...
  }
}

Now that this reducer function is pure, calling it an extra time doesn’t make a difference in behavior. This is why React calling it twice helps you find mistakes. Only component, initializer, and reducer functions need to be pure. Event handlers don’t need to be pure, so React will never call your event handlers twice.

Read Czyste komponenty to learn more.