The Jaro-Winkler distance metric in JavaScript. See also: https://github.com/jordanthomas/jaro-winkler

jaro-winkler.js

var distance = function(s1, s2) {
  var m = 0;

  // Exit early if either are empty.
  if ( s1.length === 0 || s2.length === 0 ) {
    return 0;
  }

  // Exit early if they're an exact match.
  if ( s1 === s2 ) {
    return 1;
  }

  var range     = (Math.floor(Math.max(s1.length, s2.length) / 2)) - 1,
      s1Matches = new Array(s1.length),
      s2Matches = new Array(s2.length);

  for ( i = 0; i < s1.length; i++ ) {
    var low  = (i >= range) ? i - range : 0,
        high = (i + range <= s2.length) ? (i + range) : (s2.length - 1);

    for ( j = low; j <= high; j++ ) {
      if ( s1Matches[i] !== true && s2Matches[j] !== true && s1[i] === s2[j] ) {
        ++m;
        s1Matches[i] = s2Matches[j] = true;
        break;
      }
    }
  }

  // Exit early if no matches were found.
  if ( m === 0 ) {
    return 0;
  }

  // Count the transpositions.
  var k = n_trans = 0;

  for ( i = 0; i < s1.length; i++ ) {
    if ( s1Matches[i] === true ) {
      for ( j = k; j < s2.length; j++ ) {
        if ( s2Matches[j] === true ) {
          k = j + 1;
          break;
        }
      }

      if ( s1[i] !== s2[j] ) {
        ++n_trans;
      }
    }
  }

  var weight = (m / s1.length + m / s2.length + (m - (n_trans / 2)) / m) / 3,
      l      = 0,
      p      = 0.1;

  if ( weight > 0.7 ) {
    while ( s1[l] === s2[l] && l < 4 ) {
      ++l;
    }

    weight = weight + l * p * (1 - weight);
  }

  return weight;
};