**Let's walk through an example to see when this is useful.**
[HTML accessibility attributes](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA) like [`aria-describedby`](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-describedby) let you specify that two tags are related to each other. For example, you can specify that a certain element (like a button) is described by another element (like a paragraph).
`useId` is a hook for generating unique IDs that are stable across the server and client, while avoiding hydration mismatches.
In regular HTML, you would write it like this:
```html {5,8}
<label>
Password:
<input
type="password"
aria-describedby="password-hint"
/>
</label>
<pid="password-hint">
The password should contain at least 18 characters
</p>
```
However, hardcoding IDs like this is not a good practice in React. A component may be rendered more than once on the page--but IDs have to be unique! Instead of hardcoding an ID, you can generate a unique ID with `useId`:
```js {4,11,14}
import { useId } from 'react';
function PasswordField() {
const passwordHintId = useId();
return (
<>
<label>
Password:
<input
type="password"
aria-describedby={passwordHintId}
/>
</label>
<pid={passwordHintId}>
The password should contain at least 18 characters
</p>
</>
);
}
```
Now, even if `PasswordField` appears multiple times on the screen, the generated IDs won't clash.
<Sandpack>
```js
const id = useId();
import { useId } from 'react';
function PasswordField() {
const passwordHintId = useId();
return (
<>
<label>
Password:
<input
type="password"
aria-describedby={passwordHintId}
/>
</label>
<pid={passwordHintId}>
The password should contain at least 18 characters
</p>
</>
);
}
export default function App() {
return (
<>
<h2>Choose password</h2>
<PasswordField/>
<h2>Confirm password</h2>
<PasswordField/>
</>
);
}
```
</Intro>
```css
input { margin: 5px; }
```
<InlineToc/>
</Sandpack>
[Watch this video](https://www.youtube.com/watch?v=0dNzNcuEuOo) to see the difference in the user experience with assistive technologies.
<Gotcha>
`useId` is not for generating keys in a list. Keys should be generated from your data.
If you use [server rendering,](/apis/react-dom/server) you must render an identical component tree on the server and the client. If the trees you render on the server and the client don't match exactly, the generated IDs won't match up.
</Gotcha>
<DeepDivetitle="Why is useId better than an incrementing counter?">
You might be wondering why `useId` is better than incrementing a global variable like `nextId++`.
The primary benefit of `useId` is that React ensures that it works with [server rendering.](/apis/react-dom/server) During server rendering, your components generate HTML output. Later, on the client, [hydration](/apis/react-dom/client/hydrateRoot) attaches your event handlers to the generated HTML. For hydration to work, the client output must match the server HTML.
This is very difficult to guarantee with an incrementing counter because the order in which the client components are hydrated may not match the order in which the server HTML was emitted. By calling `useId`, you ensure that hydration will work, and the output will match between the server and the client.
</DeepDive>
---
### Generating IDs for several related elements {/*generating-ids-for-several-related-elements*/}
If you need to give IDs to multiple related elements, you can call `useId` to generate a shared prefix for them:
This lets you avoid calling `useId` for every single element that needs a unique ID.
---
### Specifying a shared prefix for all generated IDs {/*specifying-a-shared-prefix-for-all-generated-ids*/}
If you render multiple independent React applications on a single page, you may pass `identifierPrefix` as an option to your [`createRoot`](/apis/react-dom/client/createRoot#parameters) or [`hydrateRoot`](/apis/react-dom/client/hydrateRoot) calls. This ensures that the IDs generated by the two different apps never clash because every identifier generated with `useId` will start with the distinct prefix you've specified.
Call `useId` at the top level of your component to generate a unique ID:
```js
import { useId } from 'react';
function PasswordField() {
const passwordHintId = useId();
// ...
```
[See more examples above.](#usage)
#### Parameters {/*parameters*/}
`useId` does not take any parameters.
#### Returns {/*returns*/}
`useId` returns a unique ID string associated with this particular `useId` call in this particular component.
#### Caveats {/*caveats*/}
* `useId` 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.
* `useId`**should not be used to generate keys** in a list. [Keys should be generated from your data.](/learn/rendering-lists#where-to-get-your-key)
dan
2 years ago
GitHub