typo-check.js

function checkForCloseMatch(longString, shortString) {
  // too many false positives with very short strings
  if (shortString.length < 3) return '';

  // test if the shortString is in the string (so everything is fine)
  if (longString.includes(shortString)) return '';

  // split the shortString string into two at each postion e.g. g|mail gm|ail gma|il gmai|l
  for (let i = 1; i < shortString.length; i++) {
    const firstPart = shortString.substring(0, i);
    const secondPart = shortString.substring(i);

    // test for wrong letter
    const wrongLetterRegEx = new RegExp(`${firstPart}.${secondPart.substring(1)}`);
    if (wrongLetterRegEx.test(longString)) {
      return longString.replace(wrongLetterRegEx, shortString);
    }

    // test for extra letter
    const extraLetterRegEx = new RegExp(`${firstPart}.${secondPart}`);
    if (extraLetterRegEx.test(longString)) {
      return longString.replace(extraLetterRegEx, shortString);
    }

    // test for missing letter
    if (secondPart !== 'mail') {
      const missingLetterRegEx = new RegExp(`${firstPart}{0}${secondPart}`);
      if (missingLetterRegEx.test(longString)) {
        return longString.replace(missingLetterRegEx, shortString);
      }
    }

    // test for switched letters
    const switchedLetters = [
     shortString.substring(0, i - 1),
     shortString.charAt(i),
     shortString.charAt(i - 1),
     shortString.substring(i + 1),
    ].join('');

    if (longString.includes(switchedLetters)) {
      return longString.replace(switchedLetters, shortString);
    }
  }

  // if nothing was close, then there wasn't a typo
  return '';
}