Object.isSealed()

Introduction

The Object.isSealed() method determines if an object is sealed. An object is sealed if it is not extensible and all its properties are non-configurable. This method returns a boolean value indicating whether the object is sealed.

How it works ?

The Object.isSealed() method checks if the provided object is sealed or not. If the object is sealed, it returns true, otherwise it returns false. An object is considered sealed if:

• It is not extensible using Object.seal().
• All of its properties are non-configurable (i.e., their configurable attribute is set to false) and non-writable (i.e., their writable attribute is set to false).
• All of its properties are non-configurable using Object.defineProperty() with configurable: false.

Examples

const obj1 = { a: 1 };
Object.seal(obj1);
console.log(Object.isSealed(obj1)); // true

const obj2 = { b: 2 };
Object.defineProperty(obj2, "b", { configurable: false });
console.log(Object.isSealed(obj2)); // true

const obj3 = { c: 3 };
console.log(Object.isSealed(obj3)); // false

Detailed explanation

Syntax

Object.isSealed(obj);

Parameter

• obj: The object to be checked.

Return Value

A boolean value indicating whether the object is sealed or not.

Limitations

The Object.isSealed() method only checks if an object is sealed or not. It does not provide any information about which properties are non-configurable.

Notes

• Sealing an object also prevents any new properties from being added to it, but the existing properties can still be modified.

• An object can be sealed using the Object.seal() method.

• An object cannot be unsealed once it has been sealed.

Common use cases

• Checking if an object is sealed before attempting to modify it.

• Ensuring that an object is not accidentally modified by making it non-extensible and non-configurable.

Tips and tricks

• Avoid sealing objects unless absolutely necessary, as it can make debugging and maintenance more difficult.

• If sealing an object, make sure it is thoroughly tested to ensure that it behaves as expected.

• Use the Object.isFrozen() method to check if an object is frozen (i.e., both non-extensible and all properties are non-writable).

Ecmascript specification

The Object.isSealed() method was introduced in ECMAScript 5 (ES5) and is included in all modern web browsers.

See ECMAScript specification

Exercises

1. Write a function that takes an object as an argument and returns true if the object is sealed, otherwise false.

function isObjectSealed(obj) {
  return Object.isSealed(obj);
}

2. Write a function that takes an array of objects and returns an array containing only the sealed objects.

function getSealedObjects(objArr) {
  return objArr.filter((obj) => Object.isSealed(obj));
}

3. Write a program that creates an object, seals it, and then tries to add a new property to it. Use Object.isSealed() to check if the object is sealed before and after attempting to add the property.

const obj = {
  name: "John",
  age: 30,
};

console.log(Object.isSealed(obj)); // false

Object.seal(obj);

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

obj.newProp = "test";

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