JavaScript .seal()

The Object.seal() static method seals an object, preventing new properties from being added and making all existing properties non-configurable. In other words:

• New properties cannot be added.
• Existing properties cannot be removed.
• Property enumerability and configurability cannot be changed.

Unlike Object.freeze(), objects sealed with Object.seal() may still have their existing properties updated if they are writable (writable: true).

Note: For debugging purposes, it is good practice to use strict mode when working with sealed objects, as it helps detect unauthorized modifications and prevents silent errors.

Syntax

Object.seal(obj)

Parameters:

obj: The object to seal.

Return value:

The Object.seal() method returns the sealed object.

Example 1: Basic Object Sealing

In this example, the person object is sealed. In non-strict mode, attempts to add or remove properties will fail silently (no error is thrown), while modifying writable properties will still work:

const person = {
  name: 'Joe',
  age: 30,
};

// Sealing the person object
Object.seal(person);

// Modifying the age property will work
person.age = 31;

// Attempting to add or delete a property will not work
person.city = 'Madrid';
delete person.name;

console.log(person);

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The code will produce this output:

{ name: 'Joe', age: 31 }

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Example 2: Check Sealed Object

The following example uses the Object.isSealed() to check if the car object is sealed. This method returns true if the object is sealed, regardless of how it was sealed (e.g., via Object.seal() or Object.freeze():

const car = {
  brand: 'Audi',
  model: 'Sedan',
};

// Sealing the car object
Object.seal(car);

// This will print 'true' because the car object is sealed
console.log(Object.isSealed(car));

car.brand = 'Honda';
car.color = 'red';

console.log(car);

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The code will produce this output:

true
{ brand: 'Honda', model: 'Sedan' }

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Codebyte Example: Strict Mode Behavior

This example demonstrates the core functionality of Object.seal() by showing what operations are allowed versus prohibited on a sealed object. The sealed object allows modification of existing properties but prevents structural changes like adding new properties or deleting existing ones:

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