count-days-in-month/index.jsnext.js

Get the number of days in a given month

/*!
 * count-days-in-month | MIT (c) Shinnosuke Watanabe
 * https://github.com/shinnn/count-days-in-month
*/
import isNaturalNumber from 'is-natural-number';

var EXPECTED_YEAR_ARG = ' Expected a number of full year, for example `2016`.';
var EXPECTED_MONTH_ARG = ' Expected a zero-based number of month (0-11).';

export default function countDaysInMonth(fullYear, zeroBasedMonth) {
  if (typeof fullYear !== 'number') {
    throw new TypeError(String(fullYear) + ' is not a number.' + EXPECTED_YEAR_ARG);
  }

  if (!isNaturalNumber(fullYear, {includeZero: true})) {
    throw new RangeError(String(fullYear) + ' is a number, but not a natural number.' + EXPECTED_YEAR_ARG);
  }

  if (typeof zeroBasedMonth !== 'number') {
    throw new TypeError(String(zeroBasedMonth) + ' is not a number.' + EXPECTED_MONTH_ARG);
  }

  if (!isNaturalNumber(zeroBasedMonth, {includeZero: true})) {
    throw new RangeError(String(zeroBasedMonth) + ' is a number, but not a natural number.' + EXPECTED_MONTH_ARG);
  }

  if (zeroBasedMonth > 11) {
    throw new RangeError(String(zeroBasedMonth) + ' is larger than 11.' + EXPECTED_MONTH_ARG);
  }

  var date = new Date();
  date.setFullYear(fullYear, zeroBasedMonth + 1, 0);

  if (isNaN(date.getTime())) {
    throw new RangeError(
      String(fullYear) +
      ' is too large for JavaScript year. Expected a valid number of full year.'
    );
  }

  return date.getDate();
}