162 lines
3.1 KiB
Markdown
162 lines
3.1 KiB
Markdown
<!--
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@license Apache-2.0
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Copyright (c) 2018 The Stdlib Authors.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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# isLeapYear
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> Test if a value corresponds to a [leap year][leap-year] in the [Gregorian calendar][gregorian-calendar].
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A [leap year][leap-year] is defined as any year which is exactly divisible by `4`, except for years which are exactly divisible by `100` and not by `400`. In this definition, `100` corresponds to years marking a new century, and `400` corresponds to the length of the _leap cycle_.
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<section class="usage">
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## Usage
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```javascript
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var isLeapYear = require( '@stdlib/assert/is-leap-year' );
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```
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#### isLeapYear( \[value] )
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Tests if a `value` corresponds to a [leap year][leap-year] in the [Gregorian calendar][gregorian-calendar].
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```javascript
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var bool = isLeapYear();
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// returns <boolean>
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bool = isLeapYear( new Date() );
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// returns <boolean>
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bool = isLeapYear( 2000 );
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// returns true
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bool = isLeapYear( 2017 );
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// returns false
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```
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</section>
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<!-- /.usage -->
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<section class="notes">
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## Notes
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- If not provided any `arguments`, the function returns a `boolean` indicating if the current year (according to local time) is a [leap year][leap-year].
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- If provided a `value` which is neither an integer value nor a [`Date`][date-object], the function **always** returns `false`.
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```javascript
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var bool = isLeapYear( '2016' );
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// returns false
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bool = isLeapYear( null );
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// returns false
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```
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</section>
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<!-- /.notes -->
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<section class="examples">
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## Examples
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<!-- eslint no-undef: "error" -->
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```javascript
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var isLeapYear = require( '@stdlib/assert/is-leap-year' );
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var bool;
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var i;
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for ( i = 0; i < 2021; i++ ) {
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bool = isLeapYear( i );
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console.log( 'The year %d %s a leap year.', i, ( bool ) ? 'is' : 'is not' );
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}
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```
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</section>
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<!-- /.examples -->
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* * *
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<section class="cli">
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## CLI
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<section class="usage">
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### Usage
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```text
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Usage: is-leap-year [options] [<year>]
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Options:
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-h, --help Print this message.
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-V, --version Print the package version.
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```
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</section>
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<!-- /.usage -->
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<section class="examples">
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### Examples
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```bash
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$ is-leap-year
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<boolean>
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$ is-leap-year 1993
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false
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```
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To use as a [standard stream][standard-streams],
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```bash
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$ echo -n 1992 | is-leap-year
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true
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```
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</section>
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<!-- /.examples -->
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</section>
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<!-- /.cli -->
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<section class="links">
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[leap-year]: https://en.wikipedia.org/wiki/Leap_year
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[gregorian-calendar]: https://en.wikipedia.org/wiki/Gregorian_calendar
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[date-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date
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[standard-streams]: https://en.wikipedia.org/wiki/Standard_streams
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</section>
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<!-- /.links -->
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