its-thursday/src/index.js

A simple utility to check if a given date is Thursday

const THURSDAY_DAY_INDEX = 4;
const MONTH_INDEX_OFFSET = 1;
const DATE_FORMAT_REGEX = /^\d{4}-\d{2}-\d{2}$/;
const ISO_TIME_SUFFIX = 'T00:00:00';

/**
 * Validates and parses a date string in Y-m-d format
 * @param {string} dateString - Date string in Y-m-d format
 * @returns {Date} - Parsed and validated Date object
 * @throws {Error} - If date format is invalid or date is not valid
 */
function validateAndParseDate(dateString) {
  if (!DATE_FORMAT_REGEX.test(dateString)) {
    throw new Error('Date must be in Y-m-d format (e.g., 2023-12-07)');
  }
  
  const [year, month, day] = dateString.split('-').map(Number);
  const parsedDate = new Date(dateString + ISO_TIME_SUFFIX);
  
  if (isNaN(parsedDate.getTime())) {
    throw new Error('Invalid date provided');
  }
  
  if (parsedDate.getFullYear() !== year || 
      parsedDate.getMonth() !== month - MONTH_INDEX_OFFSET || 
      parsedDate.getDate() !== day) {
    throw new Error('Invalid date provided');
  }
  
  return parsedDate;
}

/**
 * Check if a given date is Thursday
 * @param {string} [dateString] - Optional date string in Y-m-d format
 * @returns {boolean} - True if the date is Thursday, false otherwise
 */
function itsThursday(dateString) {
  const targetDate = dateString ? validateAndParseDate(dateString) : new Date();
  return targetDate.getDay() === THURSDAY_DAY_INDEX;
}

module.exports = itsThursday;