Extracting State Logic into a Reducer

Consolidate state logic with a reducer

As your components grow in complexity, it can get harder to see at a glance all the different ways in which a component’s state gets updated. For example, the TaskApp component below holds an array of tasks in state and uses three different event handlers to add, remove, and edit tasks:

Each of its event handlers calls setTasks in order to update the state. As this component grows, so does the amount of state logic sprinkled throughout it. To reduce this complexity and keep all your logic in one easy-to-access place, you can move that state logic into a single function outside your component, called a “reducer”.

Reducers are a different way to handle state. You can migrate from useState to useReducer in three steps:

1. Move from setting state to dispatching actions.
2. Write a reducer function.
3. Use the reducer from your component.

Step 1: Move from setting state to dispatching actions

Your event handlers currently specify what to do by setting state:

function handleAddTask(text) {
  setTasks([...tasks, {
    id: nextId++,
    text: text,
    done: false
  }]);
}

function handleChangeTask(task) {
  setTasks(tasks.map(t => {
    if (t.id === task.id) {
      return task;
    } else {
      return t;
    }
  }));
}

function handleDeleteTask(taskId) {
  setTasks(
    tasks.filter(t => t.id !== taskId)
  );
}

Remove all the state setting logic. What you are left with are three event handlers:

• handleAddTask(text) is called when the user presses “Add”.
• handleChangeTask(task) is called when the user toggles a task or presses “Save”.
• handleDeleteTask(taskId) is called when the user presses “Delete”.

Managing state with reducers is slightly different from directly setting state. Instead of telling React “what to do” by setting state, you specify “what the user just did” by dispatching “actions” from your event handlers. (The state update logic will live elsewhere!) So instead of “setting tasks” via an event handler, you’re dispatching an “added/changed/deleted a task” action. This is more descriptive of the user’s intent.

function handleAddTask(text) {
  dispatch({
    type: 'added',
    id: nextId++,
    text: text,
  });
}

function handleChangeTask(task) {
  dispatch({
    type: 'changed',
    task: task
  });
}

function handleDeleteTask(taskId) {
  dispatch({
    type: 'deleted',
    id: taskId
  });
}

The object you pass to dispatch is called an “action”:

function handleDeleteTask(taskId) {
  dispatch(
    // "action" object:
    {
      type: 'deleted',
      id: taskId
    }
  );
}

It is a regular JavaScript object. You decide what to put in it, but generally it should contain the minimal information about what happened. (You will add the dispatch function itself in a later step.)

dispatch({
  // specific to component
  type: 'what_happened',
  // other fields go here
});

Step 2: Write a reducer function

A reducer function is where you will put your state logic. It takes two arguments, the current state and the action object, and it returns the next state:

function yourReducer(state, action) {
  // return next state for React to set
}

React will set the state to what you return from the reducer.

To move your state setting logic from your event handlers to a reducer function in this example, you will:

1. Declare the current state (tasks) as the first argument.
2. Declare the action object as the second argument.
3. Return the next state from the reducer (which React will set the state to).

Here is all the state setting logic migrated to a reducer function:

function tasksReducer(tasks, action) {
  if (action.type === 'added') {
    return [...tasks, {
      id: action.id,
      text: action.text,
      done: false
    }];
  } else if (action.type === 'changed') {
    return tasks.map(t => {
      if (t.id === action.task.id) {
        return action.task;
      } else {
        return t;
      }
    });
  } else if (action.type === 'deleted') {
    return tasks.filter(t => t.id !== action.id);
  } else {
    throw Error('Unknown action: ' + action.type);
  }
}

Because the reducer function takes state (tasks) as an argument, you can declare it outside of your component. This decreases the indentation level and can make your code easier to read.

function tasksReducer(tasks, action) {
  switch (action.type) {
    case 'added': {
      return [...tasks, {
        id: action.id,
        text: action.text,
        done: false
      }];
    }
    case 'changed': {
      return tasks.map(t => {
        if (t.id === action.task.id) {
          return action.task;
        } else {
          return t;
        }
      });
    }
    case 'deleted': {
      return tasks.filter(t => t.id !== action.id);
    }
    default: {
      throw Error('Unknown action: ' + action.type);
    }
  }
}

const arr = [1, 2, 3, 4, 5];
const sum = arr.reduce(
  (result, number) => result + number
); // 1 + 2 + 3 + 4 + 5

import tasksReducer from './tasksReducer.js';

let initialState = [];
let actions = [
  { type: 'added', id: 1, text: 'Visit Kafka Museum' },
  { type: 'added', id: 2, text: 'Watch a puppet show' },
  { type: 'deleted', id: 1 },
  { type: 'added', id: 3, text: 'Lennon Wall pic' },
];

let finalState = actions.reduce(
  tasksReducer,
  initialState
);

const output = document.getElementById('output');
output.textContent = JSON.stringify(
  finalState,
  null,
  2
);

Step 3: Use the reducer from your component

Finally, you need to hook up the tasksReducer to your component. Make sure to import the useReducer Hook from React:

import { useReducer } from 'react';

Then you can replace useState:

const [tasks, setTasks] = useState(initialTasks);

with useReducer like so:

const [tasks, dispatch] = useReducer(tasksReducer, initialTasks);

The useReducer Hook is similar to useState—you must pass it an initial state and it returns a stateful value and a way to set state (in this case, the dispatch function). But it’s a little different.

The useReducer Hook takes two arguments:

1. A reducer function
2. An initial state

And it returns:

1. A stateful value
2. A dispatch function (to “dispatch” user actions to the reducer)

Now it’s fully wired up! Here, the reducer is declared at the bottom of the component file:

If you want, you can even move the reducer to a different file:

Component logic can be easier to read when you separate concerns like this. Now the event handlers only specify what happened by dispatching actions, and the reducer function determines how the state updates in response to them.

Comparing useState and useReducer

Reducers are not without downsides! Here’s a few ways you can compare them:

• Code size: Generally, with useState you have to write less code upfront. With useReducer, you have to write both a reducer function and dispatch actions. However, useReducer can help cut down on the code if many event handlers modify state in a similar way.
• Readability: useState is very easy to read when the state updates are simple. When they get more complex, they can bloat your component’s code and make it difficult to scan. In this case, useReducer lets you cleanly separate the how of update logic from the what happened of event handlers.
• Debugging: When you have a bug with useState, it can be difficult to tell where the state was set incorrectly, and why. With useReducer, you can add a console log into your reducer to see every state update, and why it happened (due to which action). If each action is correct, you’ll know that the mistake is in the reducer logic itself. However, you have to step through more code than with useState.
• Testing: A reducer is a pure function that doesn’t depend on your component. This means that you can export and test it separately in isolation. While generally it’s best to test components in a more realistic environment, for complex state update logic it can be useful to assert that your reducer returns a particular state for a particular initial state and action.
• Personal preference: Some people like reducers, others don’t. That’s okay. It’s a matter of preference. You can always convert between useState and useReducer back and forth: they are equivalent!

We recommend using a reducer if you often encounter bugs due to incorrect state updates in some component, and want to introduce more structure to its code. You don’t have to use reducers for everything: feel free to mix and match! You can even useState and useReducer in the same component.

Writing reducers well

Keep these two tips in mind when writing reducers:

• Reducers must be pure. Similar to state updater functions, reducers run during rendering! (Actions are queued until the next render.) This means that reducers must be pure—same inputs always result in the same output. They should not send requests, schedule timeouts, or perform any side effects (operations that impact things outside the component). They should update objects and arrays without mutations.
• Each action describes a single user interaction, even if that leads to multiple changes in the data. For example, if a user presses “Reset” on a form with five fields managed by a reducer, it makes more sense to dispatch one reset_form action rather than five separate set_field actions. If you log every action in a reducer, that log should be clear enough for you to reconstruct what interactions or responses happened in what order. This helps with debugging!

Writing concise reducers with Immer

Just like with updating objects and arrays in regular state, you can use the Immer library to make reducers more concise. Here, useImmerReducer lets you mutate the state with push or arr[i] = assignment:

Reducers must be pure, so they shouldn’t mutate state. But Immer provides you with a special draft object which is safe to mutate. Under the hood, Immer will create a copy of your state with the changes you made to the draft. This is why reducers managed by useImmerReducer can mutate their first argument and don’t need to return state.