The writable, enumerable, configurable flags?

JavaScriptMiddle~1 min read

The writable, enumerable and configurable flags are three service parameters (flags) in a property descriptor that control the behavior of object properties in JavaScript.

These flags are not visible during regular use, but they determine whether a property can be changed, deleted or enumerated.

Summary

Flag	What it does	Default
`writable`	Allows changing the property's value	`false` (if the property was created via `defineProperty`)
`enumerable`	Makes the property visible during enumeration (`for...in`, `Object.keys`)	`false`
`configurable`	Allows deleting the property and changing its flags	`false`

Example for all three

javascript

const user = {};

Object.defineProperty(user, 'name', {
  value: 'Tim',
  writable: false,     // cannot be changed
  enumerable: false,   // does not appear in loops
  configurable: false  // cannot be deleted or have its flags changed
});

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

user.name = 'Oleh';     // will not change
delete user.name;       // will not be deleted
console.log(Object.keys(user)); // [] - not enumerated

1. writable - "can the value be changed"

javascript

const user = {};

Object.defineProperty(user, 'age', {
  value: 25,
  writable: false
});

user.age = 30; // has no effect
console.log(user.age); // 25

writable: true -> the value can be changed
writable: false -> the property is read-only

2. enumerable - "should the property show up during enumeration"

javascript

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

console.log(user.secret);         // accessible directly
console.log(Object.keys(user));   // [] (not visible)
for (const key in user) console.log(key); // nothing

enumerable: true -> the property is visible in for...in, Object.keys(), JSON.stringify()
enumerable: false -> a "hidden" property

A real-world example: built-in methods (toString, valueOf) have enumerable: false so they do not get in the way during enumeration.

3. configurable - "can it be deleted or have its flags changed"

javascript

const user = {};
Object.defineProperty(user, 'id', {
  value: 10,
  configurable: false
});

delete user.id; // will not be deleted
console.log(user.id); // 10

// Try to change writable:
Object.defineProperty(user, 'id', { writable: true });
// TypeError: Cannot redefine property: id

configurable: true -> the property can be deleted or its flags changed
configurable: false -> it cannot be deleted, and cannot be changed for enumerable / configurable / writable: false -> true

The only thing that can be changed is writable: true -> false, but not the other way around.

How to get the flag values

javascript

const obj = { name: 'Tim' };
const descriptor = Object.getOwnPropertyDescriptor(obj, 'name');
console.log(descriptor);

Outputs:

javascript

{
  value: 'Tim',
  writable: true,
  enumerable: true,
  configurable: true
}

How to set flags

javascript

Object.defineProperty(obj, 'prop', {
  value: 42,
  writable: false,
  enumerable: true,
  configurable: false
});

Summary

Flag	Purpose	Example
`writable`	Whether the property's value can be changed	`user.age = 30`
`enumerable`	Whether the property is visible during enumeration	`Object.keys(user)`
`configurable`	Whether the property can be deleted or redefined	`delete user.prop`

Short Answer

Interview ready

Premium

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