An example of using decorators in ES5.

decorators.js

function memoize(object, name, descriptor) {
  var fn = descriptor.value;
  var memoized = function() {
    memoized.cache = memoized.cache || {};
    var key = JSON.stringify(arguments);
    return memoized.cache[key] = memoized.cache[key] 
      ? memoized.cache[key]
      : fn.apply(this, arguments);
  } 
  descriptor.value = memoized;
  return descriptor;
}

function decorate(object, name, descriptor, decorator) {
  Object.defineProperty(
    object, name, (decorator(object, name, descriptor) || descriptor)
  );
}

var descriptor = {
  value: function(n) {
    if (n === 0 || n === 1) return n;
    return this.fibonacci(n-1) + this.fibonacci(n-2);
  },
  enumerable: true,
  configurable: true,
  writable: true,
};

decorate(Math, 'fibonacci', descriptor, memoize);

// first call
Math.fibonacci(3688) // Infinity

// first call (MacBook Pro)
Math.fibonacci(3689) // RangeError: Maximum call stack size exceeded