Convert post code

postcode.js

/**
 * @param {string} postCode post code
 * @return {string|null}
 *      post code after an attempt converting to canonical format, 
 *      in case conversion cannot be made, null will be returned.
 */
function convertPostCode(postCode) {
  if (!postCode || typeof postCode !== 'string') {
    // postCode is null, undefined, or non string
    return null;
  }

  var leadingChars = ['K', 'L', 'M', 'N', 'P'];
  var invalidChars = ['D', 'F', 'I', 'O', 'Q', 'U'];

  // convert to uppercase & remove white spaces and hyphen
  postCode = postCode.toUpperCase().replace(/\s|-/g, '');
  if (postCode.length !== 6) {
    // # of non-whitespace characters is not 6
    return null;
  }

  var firstChar = postCode[0];
  if (leadingChars.indexOf(firstChar) < 0) {
    // first character is illegal
    return null;
  }
  for (var i = 0; i < postCode.length; i++) {
    if (invalidChars.indexOf(postCode[i]) >= 0) {
      // contains invalid character
      return null;
    }
  }

  // insert space in between
  postCode = postCode.slice(0, 3) + ' ' + postCode.slice(3, 6);
  if (postCode.match(/^[A-Z]\d[A-Z] \d[A-Z]\d$/g)) {
    return postCode;
  }
  return null;
}