DouKaku?

function test() {
  var x = "METHINKSITISAWEASEL";
  var data = sort(initialData(300, x.length), x);
  var mutate = 5;
  setTimer(x, data, mutate, 0, 100);
}
function setTimer (x, data, mutateNum, count, interval) {
  setTimeout(function() {
    print(count + " " + data[0] + " " + data[0].differenceFrom(x));
    if (data[0].differenceFrom(x) == 0) {
      return;
    }
    data = mutateAndSelect(x, data, mutateNum);
    setTimer (x, data, mutateNum, count+1, interval);
  }, interval);
}

function mutateAndSelect (x, data, mutateNum) {
  var ar = new Array(data.length * mutateNum);
  for (var i=0, n=data.length; i<n; i++) {
    for (var j=0; j<mutateNum; j++) {
      ar[i*mutateNum + j] = data[i].mutate();
    }
  }
  return sort(ar, x).slice(0, data.length);
}

function sort(data, x) {
  var cache = {};
  data.sort(function (a, b) {
    var da = cache[a] || (cache[a] = a.differenceFrom(x));
    var db = cache[b] || (cache[b] = b.differenceFrom(x));
    return da - db;
  });
  return data;
}

function initialData(num, len) {
  var ar = new Array(num);
  for (var i = 0; i<num; i++) {
    ar[i] = randomString(len);
  }
  return ar;
}
function randomString(len) {
  var chars = new Array(len);
  for (var i = 0; i<len; i++) {
   chars[i] = 65 + Math.floor(26*Math.random());
  }
  return String.fromCharCode.apply(null, chars);
}

String.prototype.differenceFrom = function (str) {
  var diff = Math.abs(this.length - str.length);
  for ( var i=0, n = Math.min(this.length, str.length); i<n; i++) {
    if( this.charAt(i) != str.charAt(i)) diff++;
  }
  return diff;
}

String.prototype.mutate = function () {
  var x = Math.floor(Math.random() * this.length);
  return this.substr(0, x) + 
    String.fromCharCode(65 + Math.floor(26*Math.random()))
         + this.substr(x+1, this.length);
}

var infoArea = document.body.appendChild(document.createElement("div"));
infoArea.innerHTML = " ";
function print(str) {
  infoArea.firstChild.nodeValue = str;
}

test();