index.js

module.exports = { isValid };

/**
 * Validate phone number
 * @param  {string}  n
 * @return {Boolean} 
 */
function isValid(n) {

  var firstChar;
  var number;
  var pattern = /^([0]{1})([7-9]{1})([0|1]{1})([\d]{1})([\d]{7,8})$/g;

  if (!n || n.length < 5) return false;

  if (typeof n === 'number') {

    // numbers never begin with 0, force this to become a string
    number = '0' + n;

  } else if (typeof n === 'string') {

    firstChar = n.substring(0, 1);

    // user may supply 0 before the number or not
    // e.g 0703 or 703 (two types of people ¯\_(ツ)_/¯)
    // either way supply missing leading 0
    number = (firstChar === '0') ? n : '0' + n;

  } else {

    return false;

  }

  // remove all whitespace(s) before running test 
  return pattern.test(number.replace(/\s+/g, ''));

};