---
title: isoWeek
description: Creates an ISO week validation action.
source: /actions/isoWeek/isoWeek.ts
contributors:
  - fabian-hiller
---

import { ApiList, Property } from '~/components';
import { properties } from './properties';

# isoWeek

Creates an [ISO week](https://en.wikipedia.org/wiki/ISO_8601) validation action.

Format: `yyyy-Www`

> The regex used cannot validate the maximum number of weeks based on the year. For example, "2021W53" is valid although 2021 has only 52 weeks.

```ts
const Action = v.isoWeek<TInput, TMessage>(message);
```

## Generics

- `TInput` <Property {...properties.TInput} />
- `TMessage` <Property {...properties.TMessage} />

## Parameters

- `message` <Property {...properties.message} />

### Explanation

With `isoWeek` you can validate the formatting of a string. If the input is not an ISO week, you can use `message` to customize the error message.

## Returns

- `Action` <Property {...properties.Action} />

## Examples

The following examples show how `isoWeek` can be used.

### ISO week schema

Schema to validate an ISO week.

```ts
const IsoWeekSchema = v.pipe(
  v.string(),
  v.isoWeek('The week is badly formatted.')
);
```

## Related

The following APIs can be combined with `isoWeek`.

### Schemas

<ApiList items={['any', 'custom', 'string']} />

### Methods

<ApiList items={['pipe']} />

### Utils

<ApiList items={['isOfKind', 'isOfType']} />
