Brainf*ckのJavaScript実装です。Cのプリプロセッサで実装したアレみたいな感じです。やはり雑。

bf.js

var bfeval = function (source) {
  var
    code = '',

    index = 0,
    length = source.length,

    result;

  code += '(function () {';
  code += 'var mem = [], ptr = 0, result = "";';

  for (; index < length; index++) {
    switch (source[index]) {
      case '>': code += 'ptr = (ptr + 1) % 30000;'; break;
      case '<': code += 'ptr = ptr - 1 < 0 ? 30000 - 1 : ptr - 1 ;'; break;
      case '+': code += 'mem[ptr] = typeof mem[ptr] !== "undefined" ? (mem[ptr] + 1) % 256 : mem[ptr] = 1;'; break;
      case '-': code += 'mem[ptr] = typeof mem[ptr] !== "undefined" ? (mem[ptr] - 1 < 0 ? 256 - 1 : mem[ptr] - 1) : mem[ptr] = 256 - 1;'; break;
      case '.': code += 'result += String.fromCharCode(mem[ptr]);'; break;
      case ',': code += 'mem[ptr] = prompt("").charCodeAt(0) % 256;'; break;
      case '[': code += 'while (mem[ptr] !== 0) {'; break;
      case ']': code += '}'; break;
    }
  }

  code += 'return result;';
  code += '})();';

  result = eval(code);

  return result;
};