an idea for a safe eval function

safe_eval.js

function safe_eval(code, ctx) {
 return (new Function (["window", "undefined"], "try { " + code + " } catch (e){ return e; }"))(ctx); 
}

function restrict (base, perm) {
  var out = {}, k = null,
      READ = 'read',
      WRITE = 'write',
      RW = 'readwrite';
  
  for (k in perm) {
    (function (key, val) {
      if (!val) return;
      
      if (perm[key] === READ)
        out.__defineGetter__(key, function(){ return val; });
      else if (perm[key] === WRITE)
        out.__defineSetter__(key, function (v){ base[key] = v; });
      else if (perm[key] === RW)
        out[key] = val;
    })(k, base[k]);
  }
  
  return out;
} 

var READ = 'read', WRITE = 'write', RW = 'readwrite';

// save eval with some window properties
safe_eval("console.log(window.document);", restrict(window, { document: READ }));

Hey. This seems really cool. I'd like to have a tool like this.