# yyyy-mm-dd

A date library dealing only with days in the `YYYY-MM-DD` format.

* No dependencies.
* All functions with multiple arguments are curried.
* Types are included.

## Usage

Install

```bash
npm install @idris-maps/yyyy-mm-dd
```

Example usage

```ts
import { add } from '@idris-maps/yyyy-mm-dd'

add('days', 3, '2021-01-01') // 2021-01-04

// or

const addThreeDays = add('days', 3)
addThreeDays('2021-01-01') // 2021-01-04

// or

const addDays = add('days')
const add3Days = addDays(3)
add3Days('2021-01-01') // 2021-01-04
```

## Functions

### `add(unit: Unit, n: number, day: string) => string`

where `Unit` is:

```ts
type Unit = 'day'
  | 'days'
  | 'week'
  | 'weeks'
  | 'month'
  | 'months'
  | 'year'
  | 'years'
```

and `n` is an integer.

### `addDays(day, number) => day`

A shorthand for `add('day')` or `add('days')`

### `addMonths(day, number) => day`

A shorthand for `add('month')` or `add('months')`

### `addWeeks(day, number) => day`

A shorthand for `add('week')` or `add('weeks')`

### `addYears(day, number) => day`

A shorthand for `add('year')` or `add('years')`

### `daysInMonth(day) => day[]`

Takes a day and returns an array of all the days in that month.

```ts
daysInMonth('2021-01-01') // ['2021-01-01', '2021-01-02', ..., '2021-01-31']
```

### `details(day) => DayDetails`

Takes a day and returns `DayDetails` as:

```ts
interface DayDetails {
  day: string
  index: {
    week: number
    weekInMonth: number
    weekday: number
    month: number
  },
  iso: {
    week: number
    weekday: number
  }
  string: {
    DD: string
    MM: string
    YYYY: string
    YY: string
  }
  number: {
    day: number
    month: number
    year: number
  }
}
```

### `firstDayOfMonth(day) => day`

Takes a day and returns the first day of that month.

### `fromJsDate(Date) => day`

Takes a javascript `Date` and returns a day (YYYY-MM-DD string).

### `isoWeek(day) => number`

Takes a day and returns the [ISO-8601 week](https://en.wikipedia.org/wiki/ISO_week_date).

### `isoWeekday(day) => number`

Takes a day and returns the [ISO-8601 weekday](https://en.wikipedia.org/wiki/Names_of_the_days_of_the_week#Numbered_days_of_the_week).

### `lastDayOfMonth(day) => day`

Takes a day and returns the last day of that month.

### `monthsInRange(Range) => month[]`

Get all month within a range. See `range()` for how to get a range.

```ts
interface Range {
  start: string // yyyy-mm-dd
  end: string // yyyy-mm-dd
}
```

`month` is a string in the `yyyy-mm` format.

### `range(day[]) => Range`

Takes an array of days and returns a `Range`

```ts
interface Range {
  start: string // yyyy-mm-dd
  end: string // yyyy-mm-dd
}
```

### `subtract(unit: Unit, n: number, day: string) => string`

where `Unit` is:

```ts
type Unit = 'day'
  | 'days'
  | 'week'
  | 'weeks'
  | 'month'
  | 'months'
  | 'year'
  | 'years'
```

and `n` is an integer.

### `subtractDays(day, number) => day`

A shorthand for `subtract('day')` or `subtract('days')`

### `subtractMonths(day, number) => day`

A shorthand for `subtract('month')` or `subtract('months')`

### `subtractWeeks(day, number) => day`

A shorthand for `subtract('week')` or `subtract('weeks')`

### `subtractYears(day, number) => day`

A shorthand for `subtract('year')` or `subtract('years')`

### `today() => day`

Returns today as a `yyyy-mm-dd` string.

### `validate(any) => DayValidation`

Checks if a value is a day. Returns `DayValidation`

```ts
interface DayValidation {
  valid: boolean
  error?: string
}
```

### `weekdayIndex(day) => number`

Takes a day and returns the index of the weekday. `0` is Monday.