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Object.defineProperty()

Object.defineProperty()

Introduction

Object.defineProperty() lets you define or modify a property with fine-grained control over its descriptor attributes.

Syntax

Object.defineProperty(obj, propertyName, descriptor);

Example

const user = {};

Object.defineProperty(user, 'id', {
  value: 12345,
  writable: false,    // cannot change
  enumerable: false,  // hidden from iteration
  configurable: false // cannot be deleted
});

console.log(user.id); // 12345
user.id = 999;        // ❌ Fails silently (strict: throws)
console.log(Object.keys(user)); // [] — not enumerable

Getters and Setters

const user = {
  _name: 'Alice'
};

Object.defineProperty(user, 'name', {
  get() { return this._name.toUpperCase(); },
  set(value) { this._name = value.trim(); },
  enumerable: true
});

user.name = '  bob  ';
console.log(user.name); // 'BOB'

Use Cases

Hiding internal properties (enumerable: false)
Creating read-only properties (writable: false)
Implementing computed properties (getters/setters)
Library/Framework internal APIs

Summary

Object.defineProperty() gives full control over property behavior
Use writable: false for constants
Use enumerable: false to hide properties
Use getters/setters for computed or validated properties

Further Reading

MDN: Object.defineProperty()