unekointhehousenow.js

var unekointhehousenow = function(input) {
  var copy = function(source, buffer, pointer) {
    for (var i = 0, l = source.length; i < l; ++i) {
      buffer[pointer + i] = source[i];
    }
  };

  var tokens = input.match(/うーねこいんざおうちなうよー|うーねこ|いんざ|おうち|なうよー/g) || [];
  var buffer = [0];
  var pointer = 0;

  for (var i = 0, l = tokens.length; i < l; ++i) {
    var token = tokens[i];

    if (token == 'うーねこいんざおうちなうよー') {
      var fanon = (function(text) {
        var buffer = [];

        for (var i = 0, l = text.length; i < l; ++i) {
          buffer.push(text.charCodeAt(i));
        }

        return buffer;
      })('Hello world');

      copy(fanon, buffer, pointer);
      pointer += fanon.length - 1;
    }
    else if (token == 'うーねこ') {
      ++pointer;
      if (pointer >= buffer.length) buffer[pointer] = 0;
    }
    else if (token == 'いんざ') {
      --pointer;
    }
    else if (token == 'おうち') {
      ++buffer[pointer];
    }
    else if (token == 'なうよー') {
      --buffer[pointer];
    }
  }

  return String.fromCharCode.apply(undefined, buffer);
};