diff --git a/beta/src/pages/apis/usestate.md b/beta/src/pages/apis/usestate.md
index cac12521..9fa0916c 100644
--- a/beta/src/pages/apis/usestate.md
+++ b/beta/src/pages/apis/usestate.md
@@ -1078,7 +1078,7 @@ button { margin-bottom: 10px; }
Note that if you call a `set` function while rendering, it must be inside a condition like `prevCount !== count`, and there must be a call like `setPrevCount(count)` inside of the condition. Otherwise, your component would re-render in a loop until it crashes. Also, you can only update the state of the *currently rendering* component like this. Calling the `set` function of *another* component during rendering is an error. Finally, your `set` call should still [update state without mutation](#updating-objects-and-arrays-in-state) -- this special case doesn't mean you can break other rules of [pure functions](/learn/keeping-components-pure).
-This pattern can be hard to understand and is usually best avoided. However, it's better than updating state in an effect. When you call the `set` function during render, React will re-render that component immediately after your component exits with a `return` statement, and before rendering the children. This way, children don't need to render twice. The rest of your component function will still execute (and the result will be thrown away), but if your condition is below all the calls to Hooks, you may add `return null` inside it to restart rendering earlier.
+This pattern can be hard to understand and is usually best avoided. However, it's better than updating state in an effect. When you call the `set` function during render, React will re-render that component immediately after your component exits with a `return` statement, and before rendering the children. This way, children don't need to render twice. The rest of your component function will still execute (and the result will be thrown away), but if your condition is below all the calls to Hooks, you may add an early `return;` inside it to restart rendering earlier.
---
diff --git a/beta/src/pages/learn/you-might-not-need-an-effect.md b/beta/src/pages/learn/you-might-not-need-an-effect.md
new file mode 100644
index 00000000..16cb9653
--- /dev/null
+++ b/beta/src/pages/learn/you-might-not-need-an-effect.md
@@ -0,0 +1,1635 @@
+---
+title: 'You Might Not Need an Effect'
+---
+
+
+
+Effects are an escape hatch from the React paradigm. They let you "step outside" of React and synchronize your components with some external system like a non-React widget, network, or the browser DOM. If there is no external system involved (for example, if you want to update a component's state when some props or state change), you shouldn't need an Effect. Removing unnecessary Effects will make your code easier to follow, faster to run, and less error-prone.
+
+
+
+
+
+* Why and how to remove unnecessary Effects from your components
+* How to cache expensive computations without Effects
+* How to reset and adjust component state without Effects
+* How to share logic between event handlers
+* Which logic should be moved to event handlers
+* How to notify parent components about changes
+
+
+
+## How to remove unnecessary Effects {/*how-to-remove-unnecessary-effects*/}
+
+There are two common cases in which you don't need Effects:
+
+* **You don't need Effects to transform data for rendering.** For example, let's say you want to filter a list before displaying it. You might feel tempted to write an Effect that updates a state variable when the list changes. However, this is inefficient. When you update your component's state, React will first call your component functions to calculate what should be on the screen. Then React will ["commit"](/learn/render-and-commit) these changes to the DOM, updating the screen. Then React will run your Effects. If your Effect *also* immediately updates the state, this restarts the whole process from scratch! To avoid the unnecessary render passes, transform all the data at the top level of your components. That code will automatically re-run whenever your props or state change.
+* **You don't need Effects to handle user events.** For example, let's say you want to send an `/api/buy` POST request and show a notification when the user buys a product. In the Buy button click event handler, you know exactly what happened. By the time an Effect runs, you don't know *what* the user did (for example, which button was clicked). This is why you'll usually handle user events in the corresponding event handlers.
+
+You *do* need Effects to [synchronize](/learn/synchronizing-with-effects#what-are-effects-and-how-are-they-different-from-events) with external systems. For example, you can write an Effect that keeps a jQuery widget synchronized with the React state. You can also fetch data with Effects: for example, you can synchronize the search results with the current search query. Keep in mind that modern [frameworks](/learn/start-a-new-react-project#building-with-a-full-featured-framework) provide more efficient built-in data fetching mechanisms than writing Effects directly in your components.
+
+To help you gain the right intuition, let's look at some common concrete examples!
+
+### Updating state based on props or state {/*updating-state-based-on-props-or-state*/}
+
+Suppose you have a component with two state variables: `firstName` and `lastName`. You want to calculate a `fullName` from them by concatenating them. Moreover, you'd like `fullName` to update whenever `firstName` or `lastName` change. Your first instinct might be to add a `fullName` state variable and update it in an effect:
+
+```js {5-9}
+function Form() {
+ const [firstName, setFirstName] = useState('Taylor');
+ const [lastName, setLastName] = useState('Swift');
+
+ // π΄ Avoid: redundant state and unnecessary Effect
+ const [fullName, setFullName] = useState('');
+ useEffect(() => {
+ setFullName(firstName + ' ' + lastName);
+ }, [firstName, lastName]);
+ // ...
+}
+```
+
+This is more complicated than necessary. It is inefficient too: it does an entire render pass with a stale value for `fullName`, then immediately re-renders with the updated value. Remove both the state variable and the Effect:
+
+```js {4-5}
+function Form() {
+ const [firstName, setFirstName] = useState('Taylor');
+ const [lastName, setLastName] = useState('Swift');
+ // β Good: calculated during rendering
+ const fullName = firstName + ' ' + lastName;
+ // ...
+}
+```
+
+**When something can be calculated from the existing props or state, [don't put it in state.](/learn/choosing-the-state-structure#avoid-redundant-state) Instead, calculate it during rendering.** This makes your code faster (you avoid the extra "cascading" updates), simpler (you remove some code), and less error-prone (you avoid bugs caused by different state variables getting out of sync with each other). If this approach feels new to you, [Thinking in React](/learn/thinking-in-react#step-3-find-the-minimal-but-complete-representation-of-ui-state) has some guidance on what should go into state.
+
+### Caching expensive calculations {/*caching-expensive-calculations*/}
+
+This component computes `visibleTodos` by taking the `todos` it receives by props and filtering them according to the `filter` prop. You might feel tempted to store the result in a state variable and update it in an Effect:
+
+```js {4-8}
+function TodoList({ todos, filter }) {
+ const [newTodo, setNewTodo] = useState('');
+
+ // π΄ Avoid: redundant state and unnecessary Effect
+ const [visibleTodos, setVisibleTodos] = useState([]);
+ useEffect(() => {
+ setVisibleTodos(getFilteredTodos(todos, filter));
+ }, [todos, filter]);
+
+ // ...
+}
+```
+
+Like in the earlier example, this is both unnecessary and inefficient. First, remove the state and the Effect:
+
+```js {3-4}
+function TodoList({ todos, filter }) {
+ const [newTodo, setNewTodo] = useState('');
+ // β This is fine if getFilteredTodos() is not slow.
+ const visibleTodos = getFilteredTodos(todos, filter);
+ // ...
+}
+```
+
+In many cases, this code is fine! But maybe `getFilteredTodos()` is slow or you have a lot of `todos`. In that case you don't want to recalculate `getFilteredTodos()` if some unrelated state variable like `newTodo` has changed.
+
+You can cache (or ["memoize"](https://en.wikipedia.org/wiki/Memoization)) an expensive calculation by wrapping it [`useMemo`](/apis/usememo) Hook:
+
+```js {5-8}
+import { useMemo, useState } from 'react';
+
+function TodoList({ todos, filter }) {
+ const [newTodo, setNewTodo] = useState('');
+ const visibleTodos = useMemo(() => {
+ // β Does not re-run unless todos or filter change
+ return getFilteredTodos(todos, filter);
+ }, [todos, filter]);
+ // ...
+}
+```
+
+Or, written as a single line:
+
+```js {5-6}
+import { useMemo, useState } from 'react';
+
+function TodoList({ todos, filter }) {
+ const [newTodo, setNewTodo] = useState('');
+ // β Does not re-run getFilteredTodos() unless todos or filter change
+ const visibleTodos = useMemo(() => getFilteredTodos(todos, filter), [todos, filter]);
+ // ...
+}
+```
+
+**This tells React that you don't want the inner function to re-run unless either `todos` or `filter` have changed.** React will remember the return value of `getFilteredTodos()` during the initial render. During the next renders, it will check if `todos` or `filter` are different. If they're the same as last time, `useMemo` will return the last result it has stored. But if they are different, React will call the wrapped function again (and store _that_ result instead).
+
+The function you wrap in [`useMemo`](/apis/usememo) runs during rendering, so this only works for [pure calculations](/learn/keeping-components-pure).
+
+
+
+In general, unless you're creating or looping over thousands of objects, it's probably not expensive. If you want to get more confidence, you can add a console log to measure the time spent in a piece of code:
+
+```js {1,3}
+console.time('filter array');
+const visibleTodos = getFilteredTodos(todos, filter);
+console.timeEnd('filter array');
+```
+
+Perform the interaction you're measuring (for example, typing into the input). You will then see logs like `filter array: 0.15ms` in your console. If the overall logged time adds up to a significant amount (say, `1ms` or more), it might make sense to memoize that calculation. As an experiment, you can then wrap the calculation in `useMemo` to verify whether the total logged time has decreased for that interaction or not:
+
+```js
+console.time('filter array');
+const visibleTodos = useMemo(() => {
+ return getFilteredTodos(todos, filter); // Skipped if todos and filter haven't changed
+}, [todos, filter]);
+console.timeEnd('filter array');
+```
+
+`useMemo` won't make the *first* render faster. It only helps you skip unnecessary work on updates.
+
+Keep in mind that your machine is probably faster than your users' so it's a good idea to test the performance with an artificial slowdown. For example, Chrome offers a [CPU Throttling](https://developer.chrome.com/blog/new-in-devtools-61/#throttling) option for this.
+
+Also note that measuring performance in development will not give you the most accurate results. (For example, when [Strict Mode](/apis/strictmode) is on, you will see each component render twice rather than once.) To get the most accurate timings, build your app for production and test it on a device like your users have.
+
+
+
+### Resetting all state when a prop changes {/*resetting-all-state-when-a-prop-changes*/}
+
+This `ProfilePage` component receives a `userId` prop. The page contains a comment input, and you use a `comment` state variable to hold its value. One day, you notice a problem: when you navigate from one profile to another, the `comment` state does not get reset. As a result, it's easy to accidentally post a comment on a wrong user's profile. To fix the issue, you want to clear out the `comment` state variable whenever the `userId` changes:
+
+```js {4-7}
+export default function ProfilePage({ userId }) {
+ const [comment, setComment] = useState('');
+
+ // π΄ Avoid: Resetting state on prop change in an Effect
+ useEffect(() => {
+ setComment('');
+ }, [userId]);
+ // ...
+}
+```
+
+This is inefficient because `ProfilePage` and its children will first render with the stale value, and then render again. It is also complicated because you'd need to do this in *every* component that has some state inside `ProfilePage`. For example, if the comment UI is nested, you'd want to clear out nested comment state too.
+
+Instead, you can tell React that each user's profile is conceptually a _different_ profile by giving it an explicit key. Split your component in two and pass a `key` attribute from the outer component to the inner one:
+
+```js {5,11-12}
+export default function ProfilePage({ userId }) {
+ return (
+
+ );
+}
+
+function Profile({ userId }) {
+ // β This and any other state below will reset on key change automatically
+ const [comment, setComment] = useState('');
+ // ...
+}
+```
+
+Normally, React preserves the state when the same component is rendered in the same spot. **By passing `userId` as a `key` to the `Profile` component, you're asking React to treat two `Profile` components with different `userId` as two different components that should not share any state.** Whenever the key (which you've set to `userId`) changes, React will recreate the DOM and [reset the state](/learn/preserving-and-resetting-state#option-2-resetting-state-with-a-key) of the `Profile` component and all of its children. As a result, the `comment` field will clear out automatically when navigating between profiles.
+
+Note that in this example, only the outer `ProfilePage` component is exported and visible to other files in the project. Components rendering `ProfilePage` don't need to pass the key to it: they pass `userId` as a regular prop. The fact `ProfilePage` passes it as a `key` to the inner `Profile` component is an implementation detail.
+
+### Adjusting some state when a prop changes {/*adjusting-some-state-when-a-prop-changes*/}
+
+Sometimes, you might want to reset or adjust a part of the state on a prop change, but not all of it.
+
+This `List` component receives a list of `items` as a prop, and maintains the selected item in the `selection` state variable. You want to reset the `selection` to `null` whenever the `items` prop receives a different array:
+
+```js {5-8}
+function List({ items }) {
+ const [isReverse, setIsReverse] = useState(false);
+ const [selection, setSelection] = useState(null);
+
+ // π΄ Avoid: Adjusting state on prop change in an Effect
+ useEffect(() => {
+ setSelection(null);
+ }, [items]);
+ // ...
+}
+```
+
+This, too, is not ideal. Every time the `items` change, the `List` and its child components will render with a stale `selection` value at first. Then React will update the DOM and run the Effects. Finally, the `setSelection(null)` call will cause another re-render of the `List` and its child components, restarting this whole process again.
+
+Start by deleting the Effect. Instead, adjust the state directly during rendering:
+
+```js {5-11}
+function List({ items }) {
+ const [isReverse, setIsReverse] = useState(false);
+ const [selection, setSelection] = useState(null);
+
+ // Better: Adjust the state while rendering
+ const [prevItems, setPrevItems] = useState(items);
+ if (items !== prevItems) {
+ setPrevItems(items);
+ setSelection(null);
+ }
+ // ...
+}
+```
+
+**This pattern can be hard to understand, but itβs better than updating state in an Effect.** In the above example, `setSelection` is called directly during a render. React will re-render the `List` *immediately* after it exits with a `return` statement. By that point, React hasn't rendered the `List` children or updated the DOM yet, so this lets the `List` children skip rendering the stale `selection` value. [Read more about using this pattern correctly.](/apis/usestate#storing-information-from-previous-renders)
+
+Before moving on, consider whether you can further simplify the requirements to calculate everything during rendering. For example, instead of storing (and resetting) the selected *item*, you can store the selected *item ID:*
+
+```js {3-5}
+function List({ items }) {
+ const [isReverse, setIsReverse] = useState(false);
+ const [selectedId, setSelectedId] = useState(null);
+ // β Best: Calculate everything during rendering
+ const selection = items.find(item => item.id === selectedId) ?? null;
+ // ...
+}
+```
+
+Now there is no need to "adjust" the state at all. If the item with the selected ID is in the list, it remains selected. If it's not, the `selection` calculated during rendering will be `null` because no matching item was found. This behavior is a bit different, but arguably it's better because most changes to `items` now preserve the selection. However, you'd need to use `selection` in all the logic below because an item with `selectedId` might not exist.
+
+### Sharing logic between event handlers {/*sharing-logic-between-event-handlers*/}
+
+Let's say you have a product page with two buttons (Buy and Checkout) that both let you buy that product. You want to show a notification [toast](https://uxdesign.cc/toasts-or-snack-bars-design-organic-system-notifications-1236f2883023) whenever the user puts the product in the cart. Adding the `showToast()` call to both button's click handlers feels repetitive so you might be tempted to place this logic in an Effect:
+
+```js {2-7}
+function ProductPage({ product, addToCart }) {
+ // π΄ Avoid: Event-specific logic inside an Effect
+ useEffect(() => {
+ if (product.isInCart) {
+ showToast(`Added ${product.name} to the shopping cart!`);
+ }
+ }, [product]);
+
+ function handleBuyClick() {
+ addToCart(product);
+ }
+
+ function handleCheckoutClick() {
+ addToCart(product);
+ navigateTo('/checkout');
+ }
+ // ...
+}
+```
+
+This Effect is unnecessary. It will also most likely cause bugs. For example, let's say that your app "remembers" the shopping cart between the page reloads. If you add a product to the cart once and refresh the page, the notification toast will appear again. It will keep appearing every time you refresh that product's page. This is because `product.isInCart` will already be `true` on the page load, so the Effect above will call `showToast()`.
+
+**When you're not sure whether some code should be in an Effect or in an event handler, ask yourself *why* this code needs to run. Use Effects only for code that should run *because* the component was displayed to the user.** In this example, the toast should appear because the user *pressed the button*, not because the product page was displayed! Delete the Effect and put the shared logic into a function that you call from both event handlers:
+
+```js {2-6,9,13}
+function ProductPage({ product, addToCart }) {
+ // β Good: Event-specific logic is called from event handlers
+ function buyProduct() {
+ addToCart(product);
+ showToast(`Added ${product.name} to the shopping cart!`);
+ }
+
+ function handleBuyClick() {
+ buyProduct();
+ }
+
+ function handleCheckoutClick() {
+ buyProduct();
+ navigateTo('/checkout');
+ }
+ // ...
+}
+```
+
+This both removes the unnecessary Effect and fixes the bug.
+
+### Sending a POST request {/*sending-a-post-request*/}
+
+This `Form` component sends two kinds of POST requests. It sends an analytics event when it mounts. When you fill in the form and click the Submit button, it will send a POST request to the `/api/register` endpoint:
+
+```js {5-8,10-16}
+function Form() {
+ const [firstName, setFirstName] = useState('');
+ const [lastName, setLastName] = useState('');
+
+ // β Good: This logic should run because the component was displayed
+ useEffect(() => {
+ post('/analytics/event', { eventName: 'visit_form' });
+ }, []);
+
+ // π΄ Avoid: Event-specific logic inside an Effect
+ const [jsonToSubmit, setJsonToSubmit] = useState(null);
+ useEffect(() => {
+ if (jsonToSubmit !== null) {
+ post('/api/register', jsonToSubmit);
+ }
+ }, [jsonToSubmit]);
+
+ function handleSubmit(e) {
+ e.preventDefault();
+ setJsonToSubmit({ firstName, lastName });
+ }
+ // ...
+}
+```
+
+Let's apply the same criteria as in the example before.
+
+The analytics POST request should remain in an Effect. This is because the _reason_ to send the analytics event is that the form was displayed. (It would fire twice in development, but [see here](/learn/synchronizing-with-effects#sending-analytics) for how to deal with that.)
+
+However, the `/api/register` POST request is not caused by the form being _displayed_. You only want to send the request at one specific moment in time: when the user presses the button. It should only ever happen _on that particular interaction_. Delete the second Effect and move that POST request into the event handler:
+
+```js {12-13}
+function Form() {
+ const [firstName, setFirstName] = useState('');
+ const [lastName, setLastName] = useState('');
+
+ // β Good: This logic runs because the component was displayed
+ useEffect(() => {
+ post('/analytics/event', { eventName: 'visit_form' });
+ }, []);
+
+ function handleSubmit(e) {
+ e.preventDefault();
+ // β Good: Event-specific logic is in the event handler
+ post('/api/register', { firstName, lastName });
+ }
+ // ...
+}
+```
+
+When you choose whether to put some logic into an event handler or an Effect, the main question you need to answer is _what kind of logic_ it is from the user's perspective. If this logic is caused by a particular interaction, keep it in the event handler. If it's caused by the user _seeing_ the component on the screen, keep it in the Effect.
+
+### Initializing the application {/*initializing-the-application*/}
+
+Some logic should only runs once when the app loads. You might place it in an Effect in the top-level component:
+
+```js {2-6}
+function App() {
+ // π΄ Avoid: Effects with logic that should only ever run once
+ useEffect(() => {
+ loadDataFromLocalStorage();
+ checkAuthToken();
+ }, []);
+ // ...
+}
+```
+
+However, you'll quickly discover that it [runs twice in development](/learn/synchronizing-with-effects#how-to-handle-the-effect-firing-twice-in-development). This can cause issues--for example, maybe it invalidates the authentication token because the function wasn't designed to be called twice. In general, your components should be resilient to being remounted. This includes your top-level `App` component. Although it may not ever get remounted in practice in production, following the same constraints in all components makes it easier to move and reuse code. If some logic must run *once per app load* rather than *once per component mount*, you can add a top-level variable to track whether it has already executed, and always skip re-running it:
+
+```js {1,5-6,10}
+let didInit = false;
+
+function App() {
+ useEffect(() => {
+ if (!didInit) {
+ didInit = true;
+ // β Only runs once per app load
+ loadDataFromLocalStorage();
+ checkAuthToken();
+ }
+ }, []);
+ // ...
+}
+```
+
+You can also run it during module initialization and before the app renders:
+
+```js {1,5}
+if (typeof window !== 'undefined') { // Check if we're running in the browser.
+ // β Only runs once per app load
+ checkAuthToken();
+ loadDataFromLocalStorage();
+}
+
+function App() {
+ // ...
+}
+```
+
+Code at the top level runs once when your component is imported--even if it doesn't end up being rendered. To avoid slowdown or surprising behavior when importing arbitrary components, don't overuse this pattern. Keep app-wide initialization logic to root component modules like `App.js` or in your application's entry point module.
+
+### Notifying parent components about state changes {/*notifying-parent-components-about-state-changes*/}
+
+Let's say you're writing a `Toggle` component with an internal `isOn` state which can be either `true` or `false`. There are a few different ways to toggle it (by clicking or dragging). You want to notify the parent component whenever the `Toggle` internal state changes, so you expose an `onChange` event and call it from an Effect:
+
+```js {4-7}
+function Toggle({ onChange }) {
+ const [isOn, setIsOn] = useState(false);
+
+ // π΄ Avoid: The onChange handler runs too late
+ useEffect(() => {
+ onChange(isOn);
+ }, [isOn, onChange])
+
+ function handleClick() {
+ setIsOn(!isOn);
+ }
+
+ function handleDragEnd(e) {
+ if (isCloserToRightEdge(e)) {
+ setIsOn(true);
+ } else {
+ setIsOn(false);
+ }
+ }
+
+ // ...
+}
+```
+
+Like earlier, this is not ideal. The `Toggle` updates its state first, and React updates the screen. Then React runs the Effect, which calls the `onChange` function passed from a parent component. Now the parent component will update its own state, starting another render pass. It would be better to do everything in a single pass instead.
+
+Delete the Effect and instead update the state of *both* components within the same event handler:
+
+```js {5-7,11,16,18}
+function Toggle({ onChange }) {
+ const [isOn, setIsOn] = useState(false);
+
+ function updateToggle(nextIsOn) {
+ // β Good: Perform all updates during the event that caused them
+ setIsOn(nextIsOn);
+ onChange(nextIsOn);
+ }
+
+ function handleClick() {
+ updateToggle(!isOn);
+ }
+
+ function handleDragEnd(e) {
+ if (isCloserToRightEdge(e)) {
+ updateToggle(true);
+ } else {
+ updateToggle(false);
+ }
+ }
+
+ // ...
+}
+```
+
+With this approach, both the `Toggle` component and its parent component update their state during the event. React [batches updates](/learn/queueing-a-series-of-state-updates) from different components together, so there will only be one render pass as a result.
+
+You might also be able to remove the state altogether, and instead receive `isOn` from the parent component:
+
+```js {1,2}
+// β Also good: the component is fully controlled by its parent
+function Toggle({ isOn, onChange }) {
+ function handleClick() {
+ onChange(!isOn);
+ }
+
+ function handleDragEnd(e) {
+ if (isCloserToRightEdge(e)) {
+ onChange(true);
+ } else {
+ onChange(false);
+ }
+ }
+
+ // ...
+}
+```
+
+["Lifting state up"](/learn/sharing-state-between-components) lets the parent component fully control the `Toggle` by toggling the parent's own state. This means the parent component will have to contain more logic, but there will be less state overall to worry about. Whenever you try to keep two different state variables synchronized, it's a sign to try lifting state up instead!
+
+### Passing data to the parent {/*passing-data-to-the-parent*/}
+
+This `Child` component fetches some data and then passes it to the `Parent` component in an Effect:
+
+```js {9-14}
+function Parent() {
+ const [data, setData] = useState(null);
+ // ...
+ return ;
+}
+
+function Child({ onFetched }) {
+ const data = useSomeAPI();
+ // π΄ Avoid: Passing data to the parent in an Effect
+ useEffect(() => {
+ if (data) {
+ onFetched(data);
+ }
+ }, [onFetched, data]);
+ // ...
+}
+```
+
+In React, data flows from the parent components to their children. When you see something wrong on the screen, you can trace where the information comes from by going up the component chain until you find which component passes the wrong prop or has the wrong state. When child components update the state of their parent components in Effects, the data flow becomes very difficult to trace. Since both the child and the parent component need the same data, let the parent component fetch that data, and *pass it down* to the child instead:
+
+```js {4-5}
+function Parent() {
+ const data = useSomeAPI();
+ // ...
+ // β Good: Passing data down to the child
+ return ;
+}
+
+function Child({ data }) {
+ // ...
+}
+```
+
+This is simpler and keeps the data flow predictable: the data flows down from the parent to the child.
+
+### Subscribing to an external store {/*subscribing-to-an-external-store*/}
+
+Sometimes, your components may need to subscribe to some data outside of the React state. This data could be from a third-party library or a built-in browser API. Since this data can change without React's knowledge, you need to manually subscribe your components to it. This is often done with an Effect, for example:
+
+```js {2-17}
+function useOnlineStatus() {
+ // Not ideal: Manual store subscription in an Effect
+ const [isOnline, setIsOnline] = useState(true);
+ useEffect(() => {
+ function updateState() {
+ setIsOnline(navigator.onLine);
+ }
+
+ updateState();
+
+ window.addEventListener('online', updateState);
+ window.addEventListener('offline', updateState);
+ return () => {
+ window.removeEventListener('online', updateState);
+ window.removeEventListener('offline', updateState);
+ };
+ }, []);
+ return isOnline;
+}
+
+function ChatIndicator() {
+ const isOnline = useOnlineStatus();
+ // ...
+}
+```
+
+Here, the component subscribes to an external data store (in this case, the browser `navigator.onLine` API). Since this API does not exist on the server (so it can't be used to generate the initial HTML), initially the state is set to `true`. Whenever the value of that data store changes in the browser, the component updates its state.
+
+Although it's common to use Effects for this, React has a purpose-built Hook for subscribing to an external store that is preferred instead. Delete the Effect and replace it with a call to [`useSyncExternalStore`](/apis/usesyncexternalstore):
+
+```js {11-16}
+function subscribe(callback) {
+ window.addEventListener('online', callback);
+ window.addEventListener('offline', callback);
+ return () => {
+ window.removeEventListener('online', callback);
+ window.removeEventListener('offline', callback);
+ };
+}
+
+function useOnlineStatus() {
+ // β Good: Subscribing to an external store with a built-in Hook
+ return useSyncExternalStore(
+ subscribe, // React won't resubscribe for as long as you pass the same function
+ () => navigator.onLine, // How to get the value on the client
+ () => true // How to get the value on the server
+ );
+}
+
+function ChatIndicator() {
+ const isOnline = useOnlineStatus();
+ // ...
+}
+```
+
+This approach is less error-prone than manually syncing mutable data to React state with an Effect. Typically, you'll write a custom Hook like `useOnlineStatus()` above so that you don't need to repeat this code in the individual components. [Read more about subscribing to external stores from React components.](/apis/usesyncexternalstore)
+
+### Fetching data {/*fetching-data*/}
+
+Many apps use Effects to kick off data fetching. It is quite common to write a data fetching Effect like this:
+
+```js {5-10}
+function SearchResults({ query }) {
+ const [results, setResults] = useState([]);
+ const [page, setPage] = useState(1);
+
+ useEffect(() => {
+ // π΄ Avoid: Fetching without cleanup logic
+ fetchResults(query, page).then(json => {
+ setResults(json);
+ });
+ }, [query, page]);
+
+ function handleNextPageClick() {
+ setPage(page + 1);
+ }
+ // ...
+}
+```
+
+You *don't* need to move this fetch to an event handler.
+
+This might seem like a contradiction with the earlier examples where you needed to put the logic into the event handlers! However, consider that it's not *the typing event* that's the main reason to fetch. Search inputs are often prepopulated from the URL, and the user might navigate Back and Forward without touching the input. It doesn't matter where `page` and `query` come from. While this component is visible, you want to keep `results` [synchronized](/learn/synchronizing-with-effects) with data from the network according to the current `page` and `query`. This is why it's an Effect.
+
+However, the code above has a bug. Imagine you type `"hello"` fast. Then the `query` will change from `"h"`, to `"he"`, `"hel"`, `"hell"`, and `"hello"`. This will kick off separate fetches, but there is no guarantee about which order the responses will arrive in. For example, the `"hell"` response may arrive *after* the `"hello"` response. Since it will call `setResults()` last, you will be displaying the wrong search results. This is called a ["race condition"](https://en.wikipedia.org/wiki/Race_condition): two different requests "raced" against each other and came in a different order than you expected.
+
+**To fix the race condition, you need to [add a cleanup function](learn/synchronizing-with-effects#fetching-data) to ignore stale responses:**
+
+```js {5,7,9,11-13}
+function SearchResults({ query }) {
+ const [results, setResults] = useState([]);
+ const [page, setPage] = useState(1);
+ useEffect(() => {
+ let ignore = false;
+ fetchResults(query, page).then(json => {
+ if (!ignore) {
+ setResults(json);
+ }
+ });
+ return () => {
+ ignore = true;
+ };
+ }, [query, page]);
+
+ function handleNextPageClick() {
+ setPage(page + 1);
+ }
+ // ...
+}
+```
+
+This ensures that when you Effect fetches data, all responses except the last requested one will be ignored.
+
+Handling race conditions is not the only difficulty with implementing data fetching. You might also want to think about how to cache the responses (so that the user can click Back and see the previous screen instantly instead of a spinner), how to fetch them on the server (so that the initial server-rendered HTML contains the fetched content instead of a spinner), and how to avoid network waterfalls (so that a child component that needs to fetch data doesn't have to wait for every parent above it to finish fetching their data before it can start). **These issues apply to any UI library, not just React. Solving them is not trivial, which is why modern [frameworks](/learn/start-a-new-react-project#building-with-a-full-featured-framework) provide more efficient built-in data fetching mechanisms than writing Effects directly in your components.**
+
+If you don't use a framework (and don't want to build your own) but would like to make data fetching from Effects more ergonomic, consider extracting your fetching logic into a custom Hook like in this example:
+
+```js {4}
+function SearchResults({ query }) {
+ const [page, setPage] = useState(1);
+ const params = new URLSearchParams({ query, page });
+ const results = useData(`/api/search?${params}`);
+
+ function handleNextPageClick() {
+ setPage(page + 1);
+ }
+ // ...
+}
+
+function useData(url) {
+ const [result, setResult] = useState(null);
+ useEffect(() => {
+ let ignore = false;
+ fetch(url)
+ .then(response => response.json())
+ .then(json => {
+ if (!ignore) {
+ setResult(json);
+ }
+ });
+ return () => {
+ ignore = true;
+ };
+ }, [url]);
+ return result;
+}
+```
+
+You'll likely also want to add some logic for error handling and to track whether the content is loading. You can build a Hook like this yourself or use one of the many solutions already available in the React ecosystem. **Although this alone won't be as efficient as using a framework's built-in data fetching mechanism, moving the data fetching logic into a custom Hook will make it easier to adopt an efficient data fetching strategy later.**
+
+In general, whenever you have to resort to writing Effects, keep an eye out for when you can extract a piece of functionality into a custom Hook with a more declarative and purpose-built API like `useData` above. The fewer raw `useEffect` calls you have in your components, the easier you will find to maintain your application.
+
+
+
+- If you can calculate something during render, you don't need an Effect.
+- To cache expensive calculations, add `useMemo` instead of `useEffect`.
+- To reset the state of an entire component tree, pass a different `key` to it.
+- To reset a particular bit of state in response to a prop change, set it during rendering.
+- Code that needs to run because a component was *displayed* should be in Effects, the rest should be in events.
+- If you need to update the state of several components, it's better to do it during a single event.
+- Whenever you try to synchronize state variables in different components, consider lifting state up.
+- You can fetch data with Effects, but you need to implement cleanup to avoid race conditions.
+
+
+
+
+
+### Transform data without Effects {/*transform-data-without-effects*/}
+
+The `TodoList` below displays a list of todos. When the "Show only active todos" checkbox is ticked, completed todos are not displayed in the list. Regardless of which todos are visible, the footer displays the count of todos that are not yet completed.
+
+Simplify this component by removing all the unnecessary state and Effects.
+
+
+
+```js
+import { useState, useEffect } from 'react';
+import { initialTodos, createTodo } from './todos.js';
+
+export default function TodoList() {
+ const [todos, setTodos] = useState(initialTodos);
+ const [showActive, setShowActive] = useState(false);
+ const [activeTodos, setActiveTodos] = useState([]);
+ const [visibleTodos, setVisibleTodos] = useState([]);
+ const [footer, setFooter] = useState(null);
+
+ useEffect(() => {
+ setActiveTodos(todos.filter(todo => !todo.completed));
+ }, [todos]);
+
+ useEffect(() => {
+ setVisibleTodos(showActive ? activeTodos : todos);
+ }, [showActive, todos, activeTodos]);
+
+ useEffect(() => {
+ setFooter(
+
+ );
+ }, [activeTodos]);
+
+ return (
+ <>
+
+ setTodos([...todos, newTodo])} />
+
+ {visibleTodos.map(todo => (
+
+ {todo.completed ? {todo.text} : todo.text}
+
+ ))}
+
+ {footer}
+
+ );
+}
+
+function NewTodo({ onAdd }) {
+ const [text, setText] = useState('');
+
+ function handleAddClick() {
+ setText('');
+ onAdd(createTodo(text));
+ }
+
+ return (
+ <>
+ setText(e.target.value)} />
+
+
+ );
+}
+```
+
+```js todos.js
+let nextId = 0;
+
+export function createTodo(text, completed = false) {
+ return {
+ id: nextId++,
+ text,
+ completed
+ };
+}
+
+export const initialTodos = [
+ createTodo('Get apples', true),
+ createTodo('Get oranges', true),
+ createTodo('Get carrots'),
+];
+```
+
+```css
+label { display: block; }
+input { margin-top: 10px; }
+```
+
+
+
+
+
+If you can calculate something during rendering, you don't need state or an Effect that updates it.
+
+
+
+
+
+There are only two essential pieces of state in this example: the list of `todos` and the `showActive` state variable which represents whether the checkbox is ticked. All of the other state variables are [redundant](/learn/choosing-the-state-structure#avoid-redundant-state) and can be calculated during rendering instead. This includes the `footer` which you can move directly into the surrounding JSX.
+
+Your result should end up looking like this:
+
+
+
+```js
+import { useState } from 'react';
+import { initialTodos, createTodo } from './todos.js';
+
+export default function TodoList() {
+ const [todos, setTodos] = useState(initialTodos);
+ const [showActive, setShowActive] = useState(false);
+ const activeTodos = todos.filter(todo => !todo.completed);
+ const visibleTodos = showActive ? activeTodos : todos;
+
+ return (
+ <>
+
+ setTodos([...todos, newTodo])} />
+
+ {visibleTodos.map(todo => (
+
+ {todo.completed ? {todo.text} : todo.text}
+
+ ))}
+
+
+
+ );
+}
+
+function NewTodo({ onAdd }) {
+ const [text, setText] = useState('');
+
+ function handleAddClick() {
+ setText('');
+ onAdd(createTodo(text));
+ }
+
+ return (
+ <>
+ setText(e.target.value)} />
+
+
+ );
+}
+```
+
+```js todos.js
+let nextId = 0;
+
+export function createTodo(text, completed = false) {
+ return {
+ id: nextId++,
+ text,
+ completed
+ };
+}
+
+export const initialTodos = [
+ createTodo('Get apples', true),
+ createTodo('Get oranges', true),
+ createTodo('Get carrots'),
+];
+```
+
+```css
+label { display: block; }
+input { margin-top: 10px; }
+```
+
+
+
+
+
+### Cache a calculation without Effects {/*cache-a-calculation-without-effects*/}
+
+In this example, filtering the todos was extracted into a separate function called `getVisibleTodos()`. This function contains a `console.log()` call inside of it which helps you notice when it's being called. Toggle "Show only active todos" and notice that it causes `getVisibleTodos()` to re-run. This is expected because visible todos change when you toggle which ones to display.
+
+Your task is to remove the Effect that recomputes the `visibleTodos` list in the `TodoList` component. However, you need to make sure that `getVisibleTodos()` does *not* re-run (and so does not print any logs) when you type into the input.
+
+
+
+One solution is to add a `useMemo` call to cache the visible todos. There is also another, less obvious solution.
+
+
+
+
+
+```js
+import { useState, useEffect } from 'react';
+import { initialTodos, createTodo, getVisibleTodos } from './todos.js';
+
+export default function TodoList() {
+ const [todos, setTodos] = useState(initialTodos);
+ const [showActive, setShowActive] = useState(false);
+ const [text, setText] = useState('');
+ const [visibleTodos, setVisibleTodos] = useState([]);
+
+ useEffect(() => {
+ setVisibleTodos(getVisibleTodos(todos, showActive));
+ }, [todos, showActive]);
+
+ function handleAddClick() {
+ setText('');
+ setTodos([...todos, createTodo(text)]);
+ }
+
+ return (
+ <>
+
+ setText(e.target.value)} />
+
+
+
+ );
+}
+```
+
+```js todos.js
+let nextId = 0;
+let calls = 0;
+
+export function getVisibleTodos(todos, showActive) {
+ console.log(`getVisibleTodos() was called ${++calls} times`);
+ const activeTodos = todos.filter(todo => !todo.completed);
+ const visibleTodos = showActive ? activeTodos : todos;
+ return visibleTodos;
+}
+
+export function createTodo(text, completed = false) {
+ return {
+ id: nextId++,
+ text,
+ completed
+ };
+}
+
+export const initialTodos = [
+ createTodo('Get apples', true),
+ createTodo('Get oranges', true),
+ createTodo('Get carrots'),
+];
+```
+
+```css
+label { display: block; }
+input { margin-top: 10px; }
+```
+
+
+
+With this change, `getVisibleTodos()` will be called only if `todos` or `showActive` change. Typing into the input only changes the `text` state variable, so it does not trigger a call to `getVisibleTodos()`.
+
+There is also another solution which does not need `useMemo`. Since the `text` state variable can't possibly affect the list of todos, you can extract the `NewTodo` form into a separate component, and move the `text` state variable inside of it:
+
+
+
+```js
+import { useState, useMemo } from 'react';
+import { initialTodos, createTodo, getVisibleTodos } from './todos.js';
+
+export default function TodoList() {
+ const [todos, setTodos] = useState(initialTodos);
+ const [showActive, setShowActive] = useState(false);
+ const visibleTodos = getVisibleTodos(todos, showActive);
+
+ return (
+ <>
+
+ setTodos([...todos, newTodo])} />
+
+ {visibleTodos.map(todo => (
+
+ {todo.completed ? {todo.text} : todo.text}
+
+ ))}
+
+
+ );
+}
+
+function NewTodo({ onAdd }) {
+ const [text, setText] = useState('');
+
+ function handleAddClick() {
+ setText('');
+ onAdd(createTodo(text));
+ }
+
+ return (
+ <>
+ setText(e.target.value)} />
+
+
+ );
+}
+```
+
+```js todos.js
+let nextId = 0;
+let calls = 0;
+
+export function getVisibleTodos(todos, showActive) {
+ console.log(`getVisibleTodos() was called ${++calls} times`);
+ const activeTodos = todos.filter(todo => !todo.completed);
+ const visibleTodos = showActive ? activeTodos : todos;
+ return visibleTodos;
+}
+
+export function createTodo(text, completed = false) {
+ return {
+ id: nextId++,
+ text,
+ completed
+ };
+}
+
+export const initialTodos = [
+ createTodo('Get apples', true),
+ createTodo('Get oranges', true),
+ createTodo('Get carrots'),
+];
+```
+
+```css
+label { display: block; }
+input { margin-top: 10px; }
+```
+
+
+
+This approach satisfies the requirements too. When you type into the input, only the `text` state variable updates. Since the `text` state variable is in the child `NewTodo` component, the parent `TodoList` component won't get re-rendered. This is why `getVisibleTodos()` doesn't get called when you type. (It would still be called if the `TodoList` re-renders for another reason.)
+
+
+
+### Reset state without Effects {/*reset-state-without-effects*/}
+
+This `EditContact` component receives a contact object with shaped like `{ id, name, email }` as the `savedContact` prop. Try editing the name and email input fields. When you press Save, the contact's button above the form updates to the edited name. When you press Reset, any pending changes in the form are discarded. Play around with this UI to get a feel for it.
+
+When you select a contact with the buttons at the top, the form resets to reflect that contact's details. This is done with an Effect inside `EditContact.js`. Remove this Effect. Find another way to reset the form when `savedContact.id` changes.
+
+
+
+```js App.js hidden
+import { useState } from 'react';
+import ContactList from './ContactList.js';
+import EditContact from './EditContact.js';
+
+export default function ContactManager() {
+ const [
+ contacts,
+ setContacts
+ ] = useState(initialContacts);
+ const [
+ selectedId,
+ setSelectedId
+ ] = useState(0);
+ const selectedContact = contacts.find(c =>
+ c.id === selectedId
+ );
+
+ function handleSave(updatedData) {
+ const nextContacts = contacts.map(c => {
+ if (c.id === updatedData.id) {
+ return updatedData;
+ } else {
+ return c;
+ }
+ });
+ setContacts(nextContacts);
+ }
+
+ return (
+
+
+ );
+}
+```
+
+```js EditContact.js active
+import { useState } from 'react';
+
+export default function EditContact(props) {
+ return (
+
+ );
+}
+
+function EditForm({ savedContact, onSave }) {
+ const [name, setName] = useState(savedContact.name);
+ const [email, setEmail] = useState(savedContact.email);
+
+ return (
+
+
+
+
+
+
+ );
+}
+```
+
+```css
+ul, li {
+ list-style: none;
+ margin: 0;
+ padding: 0;
+}
+li { display: inline-block; }
+li button {
+ padding: 10px;
+}
+label {
+ display: block;
+ margin: 10px 0;
+}
+button {
+ margin-right: 10px;
+ margin-bottom: 10px;
+}
+```
+
+
+
+
+
+### Submit a form without Effects {/*submit-a-form-without-effects*/}
+
+This `Form` component lets you send a message to a friend. When you submit the form, the `showForm` state variable is set to `false`. This triggers an Effect calling `sendMessage(message)`, which sends the message (you can see it in the console). After the message is sent, you see a "Thank you" dialog with an "Open chat" button that lets you get back to the form.
+
+Your app's users are sending way too many messages. To make chatting a little bit more difficult, you've decided to show the "Thank you" dialog *first* rather than the form. Change the `showForm` state variable to initialize to `false` instead of `true`. As soon as you make that change, the console will show that an empty message was sent twice. Something in this logic is wrong!
+
+What's the root cause of this problem? And how can you fix it?
+
+
+
+Should the message be sent _because_ the user saw the "Thank you" dialog? Or is it the other way around?
+
+
+
+
+
+```js
+import { useState, useEffect } from 'react';
+
+export default function Form() {
+ const [showForm, setShowForm] = useState(true);
+ const [message, setMessage] = useState('');
+
+ useEffect(() => {
+ if (!showForm) {
+ sendMessage(message);
+ }
+ }, [showForm, message]);
+
+ function handleSubmit(e) {
+ e.preventDefault();
+ setShowForm(false);
+ }
+
+ if (!showForm) {
+ return (
+ <>
+