Few separate ways to create a levenshtein function

levenshtein.js

var levenshtein_es5 = function(a, b) {
  if(a.length == 0) return b.length; 
  if(b.length == 0) return a.length;

  // swap to save some memory O(min(a,b)) instead of O(a)
  if(a.length > b.length) {
    var tmp = a;
    a = b;
    b = tmp;
  }

  var row = [];
  // init the row
  for(var i = 0; i <= a.length; i++){
    row[i] = i;
  }

  // fill in the rest
  for(var i = 1; i <= b.length; i++){
    var prev = i;
    for(var j = 1; j <= a.length; j++){
      var val;
      if(b.charAt(i-1) == a.charAt(j-1)){
        val = row[j-1]; // match
      } else {
        val = Math.min(row[j-1] + 1, // substitution
                       prev + 1,     // insertion
                       row[j] + 1);  // deletion
      }
      row[j - 1] = prev;
      prev = val;
    }
    row[a.length] = prev;
  }

  return row[a.length];
}

// ---------------------------------------------------------------

const levenshtein_es6 = (a, b) => {
  if (a.length === 0) return b.length
  if (b.length === 0) return a.length
  let tmp, i, j, prev, val
  // swap to save some memory O(min(a,b)) instead of O(a)
  if (a.length > b.length) {
    tmp = a
    a = b
    b = tmp
  }

  row = Array(a.length + 1)
  // init the row
  for (i = 0; i <= a.length; i++) {
    row[i] = i
  }

  // fill in the rest
  for (i = 1; i <= b.length; i++) {
    prev = i
    for (j = 1; j <= a.length; j++) {
      if (b[i-1] === a[j-1]) {
        val = row[j-1] // match
      } else {
        val = Math.min(row[j-1] + 1, // substitution
              Math.min(prev + 1,     // insertion
                       row[j] + 1))  // deletion
      }
      row[j - 1] = prev
      prev = val
    }
    row[a.length] = prev
  }
  return row[a.length]
}

// ---------------------------------------------------------------

String.prototype.levenstein = function(string) {
    var a = this, b = string + "", m = [], i, j, min = Math.min;

    if (!(a && b)) return (b || a).length;

    for (i = 0; i <= b.length; m[i] = [i++]);
    for (j = 0; j <= a.length; m[0][j] = j++);

    for (i = 1; i <= b.length; i++) {
        for (j = 1; j <= a.length; j++) {
            m[i][j] = b.charAt(i - 1) == a.charAt(j - 1)
                ? m[i - 1][j - 1]
                : m[i][j] = min(
                    m[i - 1][j - 1] + 1, 
                    min(m[i][j - 1] + 1, m[i - 1 ][j] + 1))
        }
    }

    return m[b.length][a.length];
}