Get decades from year Typescript, JavaScript

decades-from-year.ts

// Set of utility functions
export const CommonUtils = {
  /**
   * Get the decades from year
   * =========================================================================
   * This utility will get the nearest decades of a year.<br/>
   * example: getDecadesFromYear(1965) -> { start: 1960, end: 1970 }
   * <br/>
   *
   * @return {[type]} [description]
   */
  getDecadesFromYear: function(year: number): any {
      if (Number.isNaN(year) || (year.toString().length < 4) || (year.toString().length > 4)) {
          throw new Error('Date must be valid and have a 4-digit year attribute');
      }
      let start       = Number(`${year.toString()[2]}0`);
      let startIdx    = year.toString().substring(0, 2);
      let end         = 0;
      start           = (start === 0) ? Number(`${startIdx}00`) : Number(`${startIdx}${start}`);
      end             = start + 10;
      return { start: start, end: end };
  }
};