# JS Tip of the Day: Freezing Objects

**URL:** <https://forum.kirupa.com/t/js-tip-of-the-day-freezing-objects/643092>\
**Category:** web dev\
**Created:** [May 14, 2020, 12:29pm UTC](https://forum.kirupa.com/t/js-tip-of-the-day-freezing-objects/643092 "2020-05-14T12:29:47Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![senocular](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/senocular/32/7217_2.png) [@senocular](https://forum.kirupa.com/u/senocular)\
**Post date:** [May 14, 2020, 12:29pm UTC](https://forum.kirupa.com/t/js-tip-of-the-day-freezing-objects/643092/1 "2020-05-14T12:29:47Z")

</div>

**Freezing Objects**  
Level: Beginner

Just like you can prevent extensions in and seal in objects, you can also freeze them. When an object is frozen, extensions are prevented, existing properties are set to non-configurable and set to non-writable. Freezing is achieved with `Object.freeze()`.

```javascript
let obj = { existing: true };
Object.freeze(obj);
obj.prop = 'a new property'; // TypeError in strict mode
console.log(obj.prop); // undefined
delete obj.existing; // TypeError in strict mode
console.log(obj.existing); // true
obj.existing = false; // TypeError in strict mode
console.log(obj.existing); // true

```

Freezing an object locks the properties of an object completely so that no changes are allowed. As with preventing extensions and sealing, it doesn’t apply to inheritance so a new property could be added, removed, or changed if inherited.

Once you freeze an object, you can’t go back. But you can check to see if an object has been sealed using `Object.isFrozen()`.

```javascript
let obj = {};
console.log(Object.isFrozen(obj)); // false
Object.freeze(obj);
console.log(Object.isFrozen(obj)); // true

```

Of the different levels of object lockdown available (preventing extensions, sealing, freezing), freezing is likely going to be the most common as it helps enforce immutability in objects, a characteristic popular with the functional style of programming which has gained a lot of traction recently. Instead of modifying existing objects, you’d instead create new objects with the changed properties. By freezing an object you can ensure that it is immutable and a new object is required if changes are needed.

```javascript
let user = Object.freeze({ first: 'Jane', last: 'Doe' });
// user.last = 'Smith' // not allowed
let marriedUser = {
  ...user, // copy all existing properties
  last: 'Smith' // specify changes
};

```

More info:

- [freeze on MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze)

---

<div class="post-metadata">

**Author:** ![REB\_412](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/reb_412/32/9233_2.png) [@REB\_412](https://forum.kirupa.com/u/REB_412)\
**Post date:** [May 14, 2020, 11:20pm UTC](https://forum.kirupa.com/t/js-tip-of-the-day-freezing-objects/643092/2 "2020-05-14T23:20:46Z")

</div>

The example that you give:  
let obj = {};  
console.log(Object.isFrozen(obj)); // true  
Object.freeze(obj);  
console.log(Object.isFrozen(obj)); // false  
does not seem right. I tried this in the console of the latest version of Chrome it reports completely different results. The first one is false and the second one is true which is what you would expect.

---

<div class="post-metadata">

**Author:** ![senocular](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/senocular/32/7217_2.png) [@senocular](https://forum.kirupa.com/u/senocular)\
**Post date:** [May 15, 2020, 4:28am UTC](https://forum.kirupa.com/t/js-tip-of-the-day-freezing-objects/643092/3 "2020-05-15T04:28:19Z")

</div>

@REB_412 Thanks for that! I thought I fixed it but must have forgotten (this was a copy-paste error from `isExtensible()` which has the opposite results with `preventExtensions()`). It should be fixed now.