What does Object.seal() do?

JavaScriptJunior~2 min read

Object.seal() is a built-in JavaScript method that "seals" an object, forbidding adding or removing properties, but allowing the values of existing properties to be changed.

It is similar to Object.freeze(), but slightly softer.

In short

Syntax:

javascript

Object.seal(obj)

Argument	Description
`obj`	The object to "seal"

Returns the same object, but with restrictions.

Example

javascript

const user = {
  name: 'Tim',
  age: 25
};

Object.seal(user);

user.age = 30;        // the value can be changed
user.city = 'Berlin'; // cannot add a new one
delete user.name;     // cannot delete

console.log(user); // { name: 'Tim', age: 30 }

After Object.seal():

new properties cannot be added;
existing ones cannot be deleted;
but the values of existing properties can be changed.

Checking whether an object is sealed

javascript

console.log(Object.isSealed(user)); // true

What happens "under the hood"

Object.seal(obj):

Prevents adding new properties (Object.preventExtensions(obj));
Sets configurable: false for all existing properties - they cannot be deleted or have their descriptor changed;
Leaves writable unchanged - so the values can still be changed.

Example with a descriptor

javascript

const user = { name: 'Tim' };

Object.seal(user);

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

Will print:

javascript

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

You can see that configurable: false, while writable stays true.

Comparison with `Object.freeze()`

Method	Can change values	Can add	Can delete	Can change descriptors
`Object.preventExtensions()`	Yes	No	Yes	Yes
`Object.seal()`	Yes	No	No	No
`Object.freeze()`	No	No	No	No

So seal() is an intermediate option between "flexible" and "rigid".

Shallowness (shallow seal)

Object.seal() does not protect nested objects:

javascript

const user = {
  name: 'Tim',
  address: { city: 'Berlin' }
};

Object.seal(user);

user.address.city = 'Lviv'; // allowed (the nested object isn't sealed)
user.city = 'Warsaw';       // not allowed (a new property)

console.log(user.address.city); // "Lviv"

To "seal" the whole tree, you need to do a deep seal recursively.

Example of a deep "seal"

javascript

function deepSeal(obj) {
  Object.seal(obj);

  for (const key in obj) {
    if (typeof obj[key] === 'object' && obj[key] !== null && !Object.isSealed(obj[key])) {
      deepSeal(obj[key]);
    }
  }

  return obj;
}

Now everything can be "sealed", including nested structures.

SUMMARY

What it does	Example	Behavior
"Seals" an object	`Object.seal(obj)`	Forbids adding and removing properties
Can change values	Yes
Can add new properties	No
Can delete properties	No
Check	`Object.isSealed(obj)`
Depth	Shallow (top level only)

Short Answer

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