Object.defineProperty()

Object.defineProperty(obj, prop, descriptor)

Object.defineProperty() lets you add or modify a property on an object with fine-grained control over its behavior. Unlike simple assignment, it lets you define whether the property is writable, enumerable, or configurable — the three attribute flags that govern property behavior.

Syntax

Object.defineProperty(obj, prop, descriptor)

Parameters

Parameter	Type	Default	Description
obj	Object	—	The object on which to define the property
prop	String	—	The property name (string or symbol)
descriptor	Object	—	The property descriptor object

The descriptor object can include:

Attribute	Type	Default	Description
value	any	undefined	The property’s value
writable	Boolean	false	If true, the value can be changed
enumerable	Boolean	false	If true, the property appears in for...in loops
configurable	Boolean	false	If true, the property can be deleted or modified

Examples

Adding a read-only property

const user = {};

Object.defineProperty(user, 'name', {
  value: 'Alice',
  writable: false,
  enumerable: true,
  configurable: false
});

console.log(user.name);
// 'Alice'

user.name = 'Bob';
console.log(user.name);
// 'Alice' — unchanged because writable is false

Creating a getter-only property

const product = {
  _price: 99
};

Object.defineProperty(product, 'price', {
  get() {
    return this._price;
  },
  enumerable: true,
  configurable: true
});

console.log(product.price);
// 99

product.price = 50;
console.log(product.price);
// 99 — setter not defined, assignment ignored

Making a property invisible to loops

const config = {
  apiKey: 'secret123'
};

Object.defineProperty(config, 'apiKey', {
  enumerable: false
});

console.log(Object.keys(config));
// [Global_Objects::eval] — apiKey is hidden

for (const key in config) {
  console.log(key);
}
// (nothing) — property is not enumerable

Common Patterns

Factory pattern for private data: Use closures with Object.defineProperty() to create true private fields before ES6 classes existed.

Immutability: Combine Object.defineProperty() with Object.freeze() to create fully immutable objects.

Reactive data: Define getters and setters to trigger updates when a property changes — the basis for many reactivity systems.

See Also

• Object.freeze() — make an object entirely immutable
• Object.keys() — Get array of object keys