gistfile1.js

// More convenient modification of ES5 property descriptors (by @kangax)

Object.changeDesc = function(obj, prop, newDesc) {
  var oldDesc = Object.getOwnPropertyDescriptor(obj, prop);
  for (var p in newDesc) {
    if (p === 'get' || p === 'set') {
      delete oldDesc.value;
      delete oldDesc.writable;
    }
    else if (p === 'value' || p === 'writable') {
      delete oldDesc.get;
      delete oldDesc.set;
    }
    oldDesc[p] = newDesc[p];
  }
  Object.defineProperty(obj, prop, oldDesc);
};

// Examples:

var o = { x: 1 };

Object.changeDesc(o, 'x', { get: function(){ } }); // o.x is now an accessor with no setter
Object.changeDesc(o, 'x', { value: 2 }); // o.x is back to a data property with another value
Object.changeDesc(o, 'x', { enumerable: false }); // o.x's enumerable attribute is now false
Object.changeDesc(o, 'x', { set: function(){ return 'boo' }, enumerable: true }); // o.x is accessor again, with a setter and enumerable attribute set to `true`

// etc.