useReducer Flashcards
useReducer
useReducer is a React Hook that lets you add a reducer to your component.
const [state, dispatch] = useReducer(reducer, initialArg, init)
Adding a reducer to a component
Call useReducer at the top level of your component to manage state with a reducer.
useReducer returns an array with exactly two items:
- The current state of this state variable, initially set to the initial state you provided.
- 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.
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, you need to 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.
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.
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.
Reference
useReducer(reducer, initialArg, init?)
Call useReducer at the top level of your component to manage its state with a reducer.
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 specifies how the initial state is calculated. 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:
- The current state. During the first render, it’s set to init(initialArg) or initialArg (if there’s no init).
- 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 the logic of your component. The result from one of the calls is ignored.
dispatch functions
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
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.
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.
- 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
tated an existing state object and returned it, so React ignored the update. To fix this, you need to ensure that you’re always replacing objects and 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);
}
- 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
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 and fix it.
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.
