normalized levenshtein to get % of match between 2 strings. 1 = 100%, 0 = 0%

gist.js

let levenshtein = function (str1, str2) {
  let current = [], prev, value;

  for (let i = 0; i <= str2.length; i++)
    for (let j = 0; j <= str1.length; j++) {
      if (i && j)
        if (str1.charAt(j - 1) === str2.charAt(i - 1))
          value = prev;
        else
          value = Math.min(current[j], current[j - 1], prev) + 1;
      else
        value = i + j;

      prev = current[j];
      current[j] = value;
    }

  return current.pop();
};

// 0 = 0% 1 = 100% match
export default function ({a, b}) {
  let str1 = a;
  let str2 = b;
  if (str1 === null && str2 === null) return 0;
  if (str1 === null || str2 === null) return 0;
  str1 = String(str1);
  str2 = String(str2);

  let distance = levenshtein(str1, str2);
  if (str1.length > str2.length) {
    return 1 - distance / str1.length;
  } else {
    return 1 - distance / str2.length;
  }
}

extracted most of the code from https://github.com/Glench/fuzzyset.js/blob/master/lib/fuzzyset.js lib