Telephone formatter

telFormatter.js

// eg (123) 456-7890
formatter(s) {
    // strip out all formatting that exists on the string
    const tel = s.replace(/[(\-)\s]/g, '');
    // if there is nothing after sanitizing or non-numeric characters
    // were entered then simply return the current state
    if (!tel) {
      return { tel: '' };
    } else if (tel.match(/[^.\d]/g)) {
      return this.state;
    }

    const { length } = tel;
    const areaCode = tel.substring(0, 3);
    const firstSet = tel.substring(3, 6);
    const lastSet = tel.substring(6, 10);

    // the below could probably be better included in another regex
    // or even better the rules for formatting would come as a schema 
    // to this function so that formatting can be dynamic, eg international country codes
    const openingParen = length && '(';
    const closingParen = length > 3 ? ') ' : '';
    const dash = firstSet.length === 3 && lastSet ? '-' : '';

    return {
      tel: `${openingParen}${areaCode}${closingParen}${firstSet}${dash}${lastSet}`
    };
  }