detect differences between two strings with Javascript

I don't have a Javascript implementation on hand per se, but you're doing something for which well-established algorithms exist. Specifically, I believe you're looking for the "Levenshtein distance" between two strings -- i.e. the number of insertions, substitutions and deletions (assuming you are treating a deletion as a change).

The wikipedia page for Levenshtein distance has various pseudo-code implementations from which you could start, and references which may also help you.

Alternative implemenation:

/**
 * Computes the Levenshtein edit distance between two strings.
 * @param {string} a
 * @param {string} b
 * @return {number} The edit distance between the two strings.
 */
goog.string.editDistance = function(a, b) {
  var v0 = [];
  var v1 = [];

  if (a == b) {
    return 0;
  }

  if (!a.length || !b.length) {
    return Math.max(a.length, b.length);
  }

  for (var i = 0; i < b.length + 1; i++) {
    v0[i] = i;
  }

  for (var i = 0; i < a.length; i++) {
    v1[0] = i + 1;

    for (var j = 0; j < b.length; j++) {
      var cost = Number(a[i] != b[j]);
      // Cost for the substring is the minimum of adding one character, removing
      // one character, or a swap.
      v1[j + 1] = Math.min(v1[j] + 1, v0[j + 1] + 1, v0[j] + cost);
    }

    for (var j = 0; j < v0.length; j++) {
      v0[j] = v1[j];
    }
  }

  return v1[b.length];
};