In general, you can't undo a defineProperty call, since there's no undo stack or something. The JS engine does not keep track of previous attribute descriptors.

For example,

Object.defineProperty(Object.prototype, 'foo', {

    configurable: true,

    value: 1,

    enumerable: false

});

Object.defineProperty(Object.prototype, 'foo', {

    get: function () {

        alert('You cannot revert me');

        return 2;

    },

    enumerable: true

});

What you can do is remove or reconfigure an attribute, or overwrite its value. As mentioned in the other answer, the configurable flag is required to be true if you want to remove or reconfigure.
Once a property is defined with configurable:false, you cannot change the configurable flag.

To remove an attribute (this is supposedly what you want to do), use delete:

Object.defineProperty(Object.prototype, 'foo', {

    configurable: true, // defaults to false

    writable: false,

    value: 1

});

delete Object.prototype.foo;

console.log(Object.prototype.hasOwnProperty('foo')); // false

To reconfigure, use defineProperty again and pass a different descriptor:

Object.defineProperty(Object.prototype, 'foo', {

    configurable: true,

    get: ...

    set: ...

});

Object.defineProperty(Object.prototype, 'foo', {

    value: undefined

});

console.log({}.foo); // undefined

console.log(Object.prototype.hasOwnProperty('foo')); // true

As shown in this sample, you can use defineProperty to switch between accessor (get/set) and data (value) properties.

To overwrite, use simple assignment. In this case, you need the writable flag to be true. Obviously this does not work with accessor properties. It even throws an exception:

Object.defineProperty(Object.prototype, 'foo', {

    configurable: true,

    value: 1,

    writable: true // defaults to false

});

Object.prototype.foo = undefined;

console.log(Object.prototype.foo); // undefined

console.log(Object.prototype.hasOwnProperty('foo')); // true



Object.defineProperty(Object.prototype, 'foo', {

    get: function () {

        return 1;

    },

    writable: true // JS error!

});

Note that writable defaults to false when you use defineProperty, but true when you use the simple syntax o.attr = val; to define a (previously not existing) property.