Object.getOwnPropertyDescriptor()

Introduction

The Object.getOwnPropertyDescriptor() method is used to get the descriptor of a property of an object. The descriptor provides information about the property, such as whether it is writable, enumerable, and configurable. This method is useful when you need to access information about an object property.

How it works ?

The Object.getOwnPropertyDescriptor() method takes two arguments: the object that contains the property and the name of the property. It returns an object that contains the following properties:

• configurable: A boolean value that indicates whether the property can be deleted or modified.
• enumerable: A boolean value that indicates whether the property can be enumerated using a for...in loop or Object.keys().
• value: The value of the property. This property is only present if the property is not an accessor property.
• writable: A boolean value that indicates whether the property can be changed.

IF the specified property does not exist on the object, undefined is returned.

Examples

Example 1: Get the descriptor of a property

const obj = {
  name: "John",
  age: 25,
};

const descriptor = Object.getOwnPropertyDescriptor(obj, "name");

console.log(descriptor);
// { value: 'John', writable: true, enumerable: true, configurable: true }

Example 2: Get the descriptor of an accessor property

const obj = {
  _name: "John",
  get name() {
    return this._name;
  },
  set name(value) {
    this._name = value;
  },
};

const descriptor = Object.getOwnPropertyDescriptor(obj, "name");

console.log(descriptor);
// { get: [Function: get name], set: [Function: set name], enumerable: true, configurable: true }

Detailed explanation

Syntax

Object.getOwnPropertyDescriptor(obj, prop);

Parameter

• obj: The object that contains the property.
• prop: The name of the property whose descriptor is to be retrieved.

Return Value

An object that contains the descriptor of the property.

The descriptor object contains the following properties:

• configurable: A boolean value that indicates whether the property can be deleted or modified.
• enumerable: A boolean value that indicates whether the property can be enumerated using a for...in loop or Object.keys().
• value: The value of the property. This property is only present if the property is not an accessor property.
• writable: A boolean value that indicates whether the property can be changed.
• get: A function that gets the value of the property. This property is only present if the property is an accessor property.
• set: A function that sets the value of the property. This property is only present if the property is an accessor property.

Exceptions

• If the obj parameter is not an object, a TypeError is thrown.
• If the prop parameter is not a string, a TypeError is thrown.

Limitations

The Object.getOwnPropertyDescriptor() method only retrieves the descriptor of the specified property. To retrieve the descriptors of all properties of an object, use Object.getOwnPropertyDescriptors() instead.

The descriptors of properties inherited from the object's prototype chain are not retrieved by this method.

Common use cases:

• Retrieving property descriptors for specific properties of an object
• Checking if a property is writable, enumerable or configurable
• Copying property descriptors from one object to another

Tips and tricks:

• You can use Object.getOwnPropertyDescriptor() on built-in objects such as arrays and functions as well
• If a property is inherited, Object.getOwnPropertyDescriptor() only returns its own descriptor, not the one from the prototype chain
• Use Object.getOwnPropertyDescriptor() when you need to retrieve information about a single property
• When you need to retrieve descriptors for multiple properties, use Object.getOwnPropertyDescriptors() instead
• Use Object.defineProperty() or Object.defineProperties() in combination with Object.getOwnPropertyDescriptor() to modify the descriptors of properties

Ecmascript specification

The Object.getOwnPropertyDescriptor() method was introduced in ECMAScript 5 and is supported in all modern web browsers.

See ECMAScript specification

Exercises

Here are a few exercises that can be used to practice using Object.getOwnPropertyDescriptor():

1. Write a function that takes an object and returns an array of all the property descriptors for that object.

function getAllPropertyDescriptors(obj) {
   const props = Object.getOwnPropertyNames(obj);
   const descriptors = [];
   for (let i = 0; i < props.length; i++) {
      descriptors.push(Object.getOwnPropertyDescriptor(obj, props[i]));
   }
   return descriptors;
}

2. Write a function that takes an object and a property name, and checks if that property is writable.

function isPropertyWritable(obj, propName) {
   const descriptor = Object.getOwnPropertyDescriptor(obj, propName);
   if (descriptor) {
      return descriptor.writable;
   }
   return false;
}

3. Write a function that takes an object and an array of property names, and sets the configurable property to true for all those properties.

function makePropertiesConfigurable(obj, props) {
   for (let i = 0; i < props.length; i++) {
      const descriptor = Object.getOwnPropertyDescriptor(obj, props[i]);
      if (descriptor) {
         descriptor.configurable = true;
         Object.defineProperty(obj, props[i], descriptor);
      }
   }
}