---
title: Sharing State Between Components
---

<Intro>

Sometimes, you want the state of two components to always change together. To do it, remove state from both of them, move it to their closest common parent, and then pass it down to them via props. This is known as *lifting state up,* and it's one of the most common things you will do writing React code.

</Intro>

<YouWillLearn>

- How to share state between components by lifting it up
- What are controlled and uncontrolled components

</YouWillLearn>

## Lifting state up by example {/*lifting-state-up-by-example*/}

In this example, a parent `Accordion` component renders two separate `Panel`s:

* `Accordion`
  - `Panel`
  - `Panel`

Each `Panel` component has a boolean `isActive` state that determines whether its content is visible.

Press the Show button for both panels:

<Sandpack>

```js
import { useState } from 'react';

function Panel({ title, children }) {
  const [isActive, setIsActive] = useState(false);
  return (
    <section className="panel">
      <h3>{title}</h3>
      {isActive ? (
        <p>{children}</p>
      ) : (
        <button onClick={() => setIsActive(true)}>
          Show
        </button>
      )}
    </section>
  );
}

export default function Accordion() {
  return (
    <>
      <h2>Almaty, Kazakhstan</h2>
      <Panel title="About">
        With a population of about 2 million, Almaty is Kazakhstan's largest city. From 1929 to 1997, it was its capital city.
      </Panel>
      <Panel title="Etymology">
        The name comes from <span lang="kk-KZ">алма</span>, the Kazakh word for "apple" and is often translated as "full of apples". In fact, the region surrounding Almaty is thought to be the ancestral home of the apple, and the wild <i lang="la">Malus sieversii</i> is considered a likely candidate for the ancestor of the modern domestic apple.
      </Panel>
    </>
  );
}
```

```css
h3, p { margin: 5px 0px; }
.panel {
  padding: 10px;
  border: 1px solid #aaa;
}
```

</Sandpack>

Notice how pressing one panel's button does not affect the other panel--they are independent.

<DiagramGroup>

<Diagram name="sharing_state_child" height={367} width={477} alt="Diagram showing a tree of three components, one parent labeled Accordion and two children labeled Panel. Both Panel components contain isActive with value false.">

Initially, each `Panel`'s `isActive` state is `false`, so they both appear collapsed

</Diagram>

<Diagram name="sharing_state_child_clicked" height={367} width={480} alt="The same diagram as the previous, with the isActive of the first child Panel component highlighted indicating a click with the isActive value set to true. The second Panel component still contains value false." >

Clicking either `Panel`'s button will only update that `Panel`'s `isActive` state alone

</Diagram>

</DiagramGroup>

**But now let's say you want to change it so that only one panel is expanded at any given time.** With that design, expanding the second panel should collapse the first one. How would you do that?

To coordinate these two panels, you need to "lift their state up" to a parent component in three steps:

1. **Remove** state from the child components.
2. **Pass** hardcoded data from the common parent.
3. **Add** state to the common parent and pass it down together with the event handlers.

This will allow the `Accordion` component to coordinate both `Panel`s and only expand one at a time.

### Step 1: Remove state from the child components {/*step-1-remove-state-from-the-child-components*/}

You will give control of the `Panel`'s `isActive` to its parent component. This means that the parent component will pass `isActive` to `Panel` as a prop instead. Start by **removing this line** from the `Panel` component:

```js
const [isActive, setIsActive] = useState(false);
```

And instead, add `isActive` to the `Panel`'s list of props:

```js
function Panel({ title, children, isActive }) {
```

Now the `Panel`'s parent component can *control* `isActive` by [passing it down as a prop.](/learn/passing-props-to-a-component) Conversely, the `Panel` component now has *no control* over the value of `isActive`--it's now up to the parent component!

### Step 2: Pass hardcoded data from the common parent {/*step-2-pass-hardcoded-data-from-the-common-parent*/}

To lift state up, you must locate the closest common parent component of *both* of the child components that you want to coordinate:

* `Accordion` *(closest common parent)*
  - `Panel`
  - `Panel`

In this example, it's the `Accordion` component. Since it's above both panels and can control their props, it will become the "source of truth" for which panel is currently active. Make the `Accordion` component pass a hardcoded value of `isActive` (for example, `true`) to both panels:

<Sandpack>

```js
import { useState } from 'react';

export default function Accordion() {
  return (
    <>
      <h2>Almaty, Kazakhstan</h2>
      <Panel title="About" isActive={true}>
        With a population of about 2 million, Almaty is Kazakhstan's largest city. From 1929 to 1997, it was its capital city.
      </Panel>
      <Panel title="Etymology" isActive={true}>
        The name comes from <span lang="kk-KZ">алма</span>, the Kazakh word for "apple" and is often translated as "full of apples". In fact, the region surrounding Almaty is thought to be the ancestral home of the apple, and the wild <i lang="la">Malus sieversii</i> is considered a likely candidate for the ancestor of the modern domestic apple.
      </Panel>
    </>
  );
}

function Panel({ title, children, isActive }) {
  return (
    <section className="panel">
      <h3>{title}</h3>
      {isActive ? (
        <p>{children}</p>
      ) : (
        <button onClick={() => setIsActive(true)}>
          Show
        </button>
      )}
    </section>
  );
}
```

```css
h3, p { margin: 5px 0px; }
.panel {
  padding: 10px;
  border: 1px solid #aaa;
}
```

</Sandpack>

Try editing the hardcoded `isActive` values in the `Accordion` component and see the result on the screen.

### Step 3: Add state to the common parent {/*step-3-add-state-to-the-common-parent*/}

Lifting state up often changes the nature of what you're storing as state.

In this case, only one panel should be active at a time. This means that the `Accordion` common parent component needs to keep track of *which* panel is the active one. Instead of a `boolean` value, it could use a number as the index of the active `Panel` for the state variable:

```js
const [activeIndex, setActiveIndex] = useState(0);
```

When the `activeIndex` is `0`, the first panel is active, and when it's `1`, it's the second one.

Clicking the "Show" button in either `Panel` needs to change the active index in `Accordion`. A `Panel` can't set the `activeIndex` state directly because it's defined inside the `Accordion`. The `Accordion` component needs to *explicitly allow* the `Panel` component to change its state by [passing an event handler down as a prop](/learn/responding-to-events#passing-event-handlers-as-props):

```js
<>
  <Panel
    isActive={activeIndex === 0}
    onShow={() => setActiveIndex(0)}
  >
    ...
  </Panel>
  <Panel
    isActive={activeIndex === 1}
    onShow={() => setActiveIndex(1)}
  >
    ...
  </Panel>
</>
```

The `<button>` inside the `Panel` will now use the `onShow` prop as its click event handler:

<Sandpack>

```js
import { useState } from 'react';

export default function Accordion() {
  const [activeIndex, setActiveIndex] = useState(0);
  return (
    <>
      <h2>Almaty, Kazakhstan</h2>
      <Panel
        title="About"
        isActive={activeIndex === 0}
        onShow={() => setActiveIndex(0)}
      >
        With a population of about 2 million, Almaty is Kazakhstan's largest city. From 1929 to 1997, it was its capital city.
      </Panel>
      <Panel
        title="Etymology"
        isActive={activeIndex === 1}
        onShow={() => setActiveIndex(1)}
      >
        The name comes from <span lang="kk-KZ">алма</span>, the Kazakh word for "apple" and is often translated as "full of apples". In fact, the region surrounding Almaty is thought to be the ancestral home of the apple, and the wild <i lang="la">Malus sieversii</i> is considered a likely candidate for the ancestor of the modern domestic apple.
      </Panel>
    </>
  );
}

function Panel({
  title,
  children,
  isActive,
  onShow
}) {
  return (
    <section className="panel">
      <h3>{title}</h3>
      {isActive ? (
        <p>{children}</p>
      ) : (
        <button onClick={onShow}>
          Show
        </button>
      )}
    </section>
  );
}
```

```css
h3, p { margin: 5px 0px; }
.panel {
  padding: 10px;
  border: 1px solid #aaa;
}
```

</Sandpack>

This completes lifting state up! Moving state into the common parent component allowed you to coordinate the two panels. Using the active index instead of two "is shown" flags ensured that only one panel is active at a given time. And passing down the event handler to the child allowed the child to change the parent's state.

<DiagramGroup>

<Diagram name="sharing_state_parent" height={385} width={487} alt="Diagram showing a tree of three components, one parent labeled Accordion and two children labeled Panel. Accordion contains an activeIndex value of zero which turns into isActive value of true passed to the first Panel, and isActive value of false passed to the second Panel." >

Initially, `Accordion`'s `activeIndex` is `0`, so the first `Panel` receives `isActive = true`

</Diagram>

<Diagram name="sharing_state_parent_clicked" height={385} width={521} alt="The same diagram as the previous, with the activeIndex value of the parent Accordion component highlighted indicating a click with the value changed to one. The flow to both of the children Panel components is also highlighted, and the isActive value passed to each child is set to the opposite: false for the first Panel and true for the second one." >

When `Accordion`'s `activeIndex` state changes to `1`, the second `Panel` receives `isActive = true` instead

</Diagram>

</DiagramGroup>

<DeepDive>

#### Controlled and uncontrolled components {/*controlled-and-uncontrolled-components*/}

It is common to call a component with some local state "uncontrolled". For example, the original `Panel` component with an `isActive` state variable is uncontrolled because its parent cannot influence whether the panel is active or not.

In contrast, you might say a component is "controlled" when the important information in it is driven by props rather than its own local state. This lets the parent component fully specify its behavior. The final `Panel` component with the `isActive` prop is controlled by the `Accordion` component.

Uncontrolled components are easier to use within their parents because they require less configuration. But they're less flexible when you want to coordinate them together. Controlled components are maximally flexible, but they require the parent components to fully configure them with props.

In practice, "controlled" and "uncontrolled" aren't strict technical terms--each component usually has some mix of both local state and props. However, this is a useful way to talk about how components are designed and what capabilities they offer.

When writing a component, consider which information in it should be controlled (via props), and which information should be uncontrolled (via state). But you can always change your mind and refactor later.

</DeepDive>

## A single source of truth for each state {/*a-single-source-of-truth-for-each-state*/}

In a React application, many components will have their own state. Some state may "live" close to the leaf components (components at the bottom of the tree) like inputs. Other state may "live" closer to the top of the app. For example, even client-side routing libraries are usually implemented by storing the current route in the React state, and passing it down by props!

**For each unique piece of state, you will choose the component that "owns" it.** This principle is also known as having a ["single source of truth".](https://en.wikipedia.org/wiki/Single_source_of_truth) It doesn't mean that all state lives in one place--but that for _each_ piece of state, there is a _specific_ component that holds that piece of information. Instead of duplicating shared state between components, *lift it up* to their common shared parent, and *pass it down* to the children that need it.

Your app will change as you work on it. It is common that you will move state down or back up while you're still figuring out where each piece of the state "lives". This is all part of the process!

To see what this feels like in practice with a few more components, read [Thinking in React.](/learn/thinking-in-react)

<Recap>

* When you want to coordinate two components, move their state to their common parent.
* Then pass the information down through props from their common parent.
* Finally, pass the event handlers down so that the children can change the parent's state.
* It's useful to consider components as "controlled" (driven by props) or "uncontrolled" (driven by state).

</Recap>

<Challenges>

#### Synced inputs {/*synced-inputs*/}

These two inputs are independent. Make them stay in sync: editing one input should update the other input with the same text, and vice versa. 

<Hint>

You'll need to lift their state up into the parent component.

</Hint>

<Sandpack>

```js
import { useState } from 'react';

export default function SyncedInputs() {
  return (
    <>
      <Input label="First input" />
      <Input label="Second input" />
    </>
  );
}

function Input({ label }) {
  const [text, setText] = useState('');

  function handleChange(e) {
    setText(e.target.value);
  }

  return (
    <label>
      {label}
      {' '}
      <input
        value={text}
        onChange={handleChange}
      />
    </label>
  );
}
```

```css
input { margin: 5px; }
label { display: block; }
```

</Sandpack>

<Solution>

Move the `text` state variable into the parent component along with the `handleChange` handler. Then pass them down as props to both of the `Input` components. This will keep them in sync.

<Sandpack>

```js
import { useState } from 'react';

export default function SyncedInputs() {
  const [text, setText] = useState('');

  function handleChange(e) {
    setText(e.target.value);
  }

  return (
    <>
      <Input
        label="First input"
        value={text}
        onChange={handleChange}
      />
      <Input
        label="Second input"
        value={text}
        onChange={handleChange}
      />
    </>
  );
}

function Input({ label, value, onChange }) {
  return (
    <label>
      {label}
      {' '}
      <input
        value={value}
        onChange={onChange}
      />
    </label>
  );
}
```

```css
input { margin: 5px; }
label { display: block; }
```

</Sandpack>

</Solution>

#### Filtering a list {/*filtering-a-list*/}

In this example, the `SearchBar` has its own `query` state that controls the text input. Its parent `FilterableList` component displays a `List` of items, but it doesn't take the search query into account.

Use the `filterItems(foods, query)` function to filter the list according to the search query. To test your changes, verify that typing "s" into the input filters down the list to "Sushi", "Shish kebab", and "Dim sum".

Note that `filterItems` is already implemented and imported so you don't need to write it yourself!

<Hint>

You will want to remove the `query` state and the `handleChange` handler from the `SearchBar`, and move them to the `FilterableList`. Then pass them down to `SearchBar` as `query` and `onChange` props.

</Hint>

<Sandpack>

```js
import { useState } from 'react';
import { foods, filterItems } from './data.js';

export default function FilterableList() {
  return (
    <>
      <SearchBar />
      <hr />
      <List items={foods} />
    </>
  );
}

function SearchBar() {
  const [query, setQuery] = useState('');

  function handleChange(e) {
    setQuery(e.target.value);
  }

  return (
    <label>
      Search:{' '}
      <input
        value={query}
        onChange={handleChange}
      />
    </label>
  );
}

function List({ items }) {
  return (
    <table>
      <tbody>
        {items.map(food => (
          <tr key={food.id}>
            <td>{food.name}</td>
            <td>{food.description}</td>
          </tr>
        ))}
      </tbody>
    </table>
  );
}
```

```js data.js
export function filterItems(items, query) {
  query = query.toLowerCase();
  return items.filter(item =>
    item.name.split(' ').some(word =>
      word.toLowerCase().startsWith(query)
    )
  );
}

export const foods = [{
  id: 0,
  name: 'Sushi',
  description: 'Sushi is a traditional Japanese dish of prepared vinegared rice'
}, {
  id: 1,
  name: 'Dal',
  description: 'The most common way of preparing dal is in the form of a soup to which onions, tomatoes and various spices may be added'
}, {
  id: 2,
  name: 'Pierogi',
  description: 'Pierogi are filled dumplings made by wrapping unleavened dough around a savoury or sweet filling and cooking in boiling water'
}, {
  id: 3,
  name: 'Shish kebab',
  description: 'Shish kebab is a popular meal of skewered and grilled cubes of meat.'
}, {
  id: 4,
  name: 'Dim sum',
  description: 'Dim sum is a large range of small dishes that Cantonese people traditionally enjoy in restaurants for breakfast and lunch'
}];
```

</Sandpack>

<Solution>

Lift the `query` state up into the `FilterableList` component. Call `filterItems(foods, query)` to get the filtered list and pass it down to the `List`. Now changing the query input is reflected in the list:

<Sandpack>

```js
import { useState } from 'react';
import { foods, filterItems } from './data.js';

export default function FilterableList() {
  const [query, setQuery] = useState('');
  const results = filterItems(foods, query);

  function handleChange(e) {
    setQuery(e.target.value);
  }

  return (
    <>
      <SearchBar
        query={query}
        onChange={handleChange}
      />
      <hr />
      <List items={results} />
    </>
  );
}

function SearchBar({ query, onChange }) {
  return (
    <label>
      Search:{' '}
      <input
        value={query}
        onChange={onChange}
      />
    </label>
  );
}

function List({ items }) {
  return (
    <table>
      <tbody> 
        {items.map(food => (
          <tr key={food.id}>
            <td>{food.name}</td>
            <td>{food.description}</td>
          </tr>
        ))}
      </tbody>
    </table>
  );
}
```

```js data.js
export function filterItems(items, query) {
  query = query.toLowerCase();
  return items.filter(item =>
    item.name.split(' ').some(word =>
      word.toLowerCase().startsWith(query)
    )
  );
}

export const foods = [{
  id: 0,
  name: 'Sushi',
  description: 'Sushi is a traditional Japanese dish of prepared vinegared rice'
}, {
  id: 1,
  name: 'Dal',
  description: 'The most common way of preparing dal is in the form of a soup to which onions, tomatoes and various spices may be added'
}, {
  id: 2,
  name: 'Pierogi',
  description: 'Pierogi are filled dumplings made by wrapping unleavened dough around a savoury or sweet filling and cooking in boiling water'
}, {
  id: 3,
  name: 'Shish kebab',
  description: 'Shish kebab is a popular meal of skewered and grilled cubes of meat.'
}, {
  id: 4,
  name: 'Dim sum',
  description: 'Dim sum is a large range of small dishes that Cantonese people traditionally enjoy in restaurants for breakfast and lunch'
}];
```

</Sandpack>

</Solution>

</Challenges>
