# Custom attributes
Define additional attributes specific to your business.
In addition to the [standard attributes](/reference/user/standard-attributes), the user profile supports custom attributes. Use them to track information specific to your business needs, such as preferences, categories, or internal classifications.
## Implementation
Here is an example of how to set a custom attribute:
**JavaScript**
```javascript
import croct from '@croct/plug';
await croct.user.edit()
.set('custom.expertise', "Engineering")
.save();
```
**HTML**
```html
My awesome application
```
**React — JavaScript**
```jsx
import {useCroct} from '@croct/plug-react';
export function Example() {
const croct = useCroct();
const update = () => croct.user.edit()
.set('custom.expertise', "Engineering")
.save();
return ();
}
```
**React — TypeScript**
```typescript
import {useCroct} from '@croct/plug-react';
import type {ReactElement} from 'react';
export function Example(): ReactElement {
const croct = useCroct();
const update = () => croct.user.edit()
.set('custom.expertise', "Engineering")
.save();
return ();
}
```
**Next — JavaScript**
```jsx
import {useCroct} from '@croct/plug-next';
export function Example() {
const croct = useCroct();
const update = () => croct.user.edit()
.set('custom.expertise', "Engineering")
.save();
return ();
}
```
**Next — TypeScript**
```tsx
import {useCroct} from '@croct/plug-next';
import type {ReactElement} from 'react';
export function Example(): ReactElement {
const croct = useCroct();
const update = () => croct.user.edit()
.set('custom.expertise', "Engineering")
.save();
return ();
}
```
**Vue — JavaScript**
```vue
```
**Vue — TypeScript**
```vue
```
**Nuxt — JavaScript**
```vue
```
**Nuxt — TypeScript**
```vue
```
## Attributes
- `custom.*`: `string|number|boolean|array|object|null`
A custom attribute, where `*` is the name of the attribute, such as `custom.pets` or `custom.loyaltyNumber`.
> **Best practice**
>
> Use descriptive camel case names like `pets`, `favoriteColor`, and `loyaltyNumber` for custom attributes to improve readability and consistency with standard attributes.
## Restrictions
The following restrictions apply to custom attributes:
- Profiles support up to 10 custom attributes.
- Attribute names must be strings up to 20 characters, starting with a letter or underscore, followed by letters, digits, or underscores.
- Attributes can be primitives (strings, numbers, booleans, null) or collections (lists, maps).
- Strings can be up to 100 characters long.
- Lists can contain up to 10 elements, including primitives, lists of primitives, or maps of primitives.
- Maps can contain up to 10 elements, including primitives, lists of primitives, or maps of primitives.
- Map keys must be strings up to 50 characters long.
Attribute names are case-insensitive, so `loyaltyNumber` and `loyaltynumber` are treated as the same attribute and will override each other.
## Explore
- [Standard attributes](/reference/user/standard-attributes): Browse the predefined attributes available on every user profile.
- [Audiences](/explanation/audience): Understand how to deliver content to specific groups of users.