ES proposal: Object.getOwnPropertyDescriptors()

[2016-02-04] dev, javascript, esnext, es proposal
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The ECMAScript proposal “Object.getOwnPropertyDescriptors()” by Jordan Harband and Andrea Giammarchi is part of ECMAScript 2017. This blog post explains it.


Object.getOwnPropertyDescriptors(obj) accepts an object obj and returns an object result:

  • For each own (non-inherited) property of obj, it adds a property to result whose key is the same and whose value is the the former property’s descriptor.

Property descriptors describe the attributes of a property (its value, whether it is writable, etc.). For more information, consult Sect. “Property Attributes and Property Descriptors” in “Speaking JavaScript”.

This is an example of using Object.getOwnPropertyDescriptors():

const obj = {
    [Symbol('foo')]: 123,
    get bar() { return 'abc' },

// Output:
// { [Symbol('foo')]:
//    { value: 123,
//      writable: true,
//      enumerable: true,
//      configurable: true },
//   bar:
//    { get: [Function: bar],
//      set: undefined,
//      enumerable: true,
//      configurable: true } }

This is how you would implement Object.getOwnPropertyDescriptors():

function getOwnPropertyDescriptors(obj) {
    const result = {};
    for (let key of Reflect.ownKeys(obj)) {
        result[key] = Object.getOwnPropertyDescriptor(obj, key);
    return result;

Use cases for Object.getOwnPropertyDescriptors()  

Use case: copying properties into an object  

Since ES6, JavaScript already has a tool method for copying properties: Object.assign(). However, this method uses simple get and set operations to copy a property whose key is key:

const value = source[key]; // get
target[key] = value; // set

That means that it doesn’t properly copy properties with non-default attributes (getters, setters, non-writable properties, etc.). The following example illustrates this limitation. The object source has a getter whose key is foo:

const source = {
    set foo(value) {
console.log(Object.getOwnPropertyDescriptor(source, 'foo'));
// { get: undefined,
//   set: [Function: foo],
//   enumerable: true,
//   configurable: true }

Using Object.assign() to copy property foo to object target fails:

const target1 = {};
Object.assign(target1, source);
console.log(Object.getOwnPropertyDescriptor(target1, 'foo'));
// { value: undefined,
//   writable: true,
//   enumerable: true,
//   configurable: true }

Fortunately, using Object.getOwnPropertyDescriptors() together with Object.defineProperties() works:

const target2 = {};
Object.defineProperties(target2, Object.getOwnPropertyDescriptors(source));
console.log(Object.getOwnPropertyDescriptor(target2, 'foo'));
// { get: undefined,
//   set: [Function: foo],
//   enumerable: true,
//   configurable: true }

Use case: cloning objects  

Shallow cloning is similar to copying properties, which is why Object.getOwnPropertyDescriptors() is a good choice here, too.

This time, we use Object.create() that has two parameters:

  • The first parameter specifies the prototype of the object it returns.
  • The optional second parameter is a property descriptor collection like the ones returned by Object.getOwnPropertyDescriptors().
const clone = Object.create(Object.getPrototypeOf(obj),

Use case: cross-platform object literals with arbitrary prototypes  

The syntactically nicest way of using an object literal to create an object with an arbitrary prototype prot is to use the special property __proto__:

const obj = {
    __proto__: prot,
    foo: 123,

Alas, that feature is only guaranteed to be there in browsers. The common work-around is Object.create() and assignment:

const obj = Object.create(prot); = 123;

But you can also use Object.getOwnPropertyDescriptors():

const obj = Object.create(
        foo: 123,

Another alternative is Object.assign():

const obj = Object.assign(
        foo: 123,

Pitfall: copying methods that use super  

A method that uses super is firmly connected with its home object (the object it is stored in). There is currently no way to copy or move such a method to a different object.

Further reading  

JavaScript design process:

Object.getOwnPropertyDescriptors() and property descriptors: