What does Object.defineProperty() do?

JavaScriptJunior~1 min read

Object.defineProperty() is a powerful built-in JavaScript method that lets you create or modify object properties with fine-grained control - for example, making them read-only, hidden, computed, and so on.

Syntax

javascript

Object.defineProperty(obj, propName, descriptor);

Argument	Description
`obj`	The object in which the property is created or changed
`propName`	The property name (a string or a `Symbol`)
`descriptor`	A descriptor object - specifies how the property behaves

Example - a regular property

javascript

const user = {};

Object.defineProperty(user, 'name', {
  value: 'Tim',
  writable: true,      // can be changed
  enumerable: true,    // visible when iterated (for...in / Object.keys)
  configurable: true   // can be deleted or changed
});

console.log(user.name); // "Tim"

This is the same as if you wrote:

javascript

const user = { name: 'Tim' };

But with defineProperty you can precisely control the property's behavior.

1. `writable` - whether it can be changed

javascript

const user = {};
Object.defineProperty(user, 'name', {
  value: 'Tim',
  writable: false
});

user.name = 'Oleh'; // will not change
console.log(user.name); // "Tim"

writable: false makes the property read-only.

2. `enumerable` - whether the property is visible when iterated

javascript

const user = {};
Object.defineProperty(user, 'secret', {
  value: '12345',
  enumerable: false
});

console.log(Object.keys(user)); // [] not visible
console.log(user.secret);       // can be read directly

Useful for "internal" data.

3. `configurable` - whether it can be deleted or redefined

javascript

const user = {};
Object.defineProperty(user, 'name', {
  value: 'Tim',
  configurable: false
});

delete user.name; // will not work
Object.defineProperty(user, 'name', { value: 'Oleh' }); // error

configurable: false makes the property permanently "locked".

4. Getter and setter via a descriptor

Object.defineProperty() can be used to create computed properties:

javascript

const user = { firstName: 'Tim', lastName: 'Cooper' };

Object.defineProperty(user, 'fullName', {
  get() {
    return `${this.firstName} ${this.lastName}`;
  },
  set(value) {
    const [first, last] = value.split(' ');
    this.firstName = first;
    this.lastName = last;
  }
});

console.log(user.fullName); // "Tim Cooper"
user.fullName = 'Oleh Shevchenko';
console.log(user.firstName); // "Oleh"

If get or set are specified, value and writable cannot be used.

5. Inspect the current property settings

javascript

console.log(Object.getOwnPropertyDescriptor(user, 'fullName'));

Example output:

javascript

{
  get: [Function: get],
  set: [Function: set],
  enumerable: false,
  configurable: false
}

6. Create several properties at once

javascript

Object.defineProperties(user, {
  age: { value: 25, writable: true },
  city: { value: 'Kyiv', enumerable: true }
});

defineProperties() is analogous, but for a group of properties.

Summary

Attribute	Default	What it does
`value`	`undefined`	The property's value
`writable`	`false`	Whether the value can be changed
`enumerable`	`false`	Whether the property is visible when iterated
`configurable`	`false`	Whether the descriptor can be deleted / changed
`get`	`undefined`	The getter function
`set`	`undefined`	The setter function

Short Answer

Interview ready

Premium

A concise answer to help you respond confidently on this topic during an interview.