How to Seal Object in JavaScript: Syntax and Examples

To seal an object in JavaScript, use Object.seal(object). This method prevents adding or removing properties but allows modifying existing ones.

Syntax

The syntax to seal an object is simple:

• Object.seal(obj): Seals the object obj.
• After sealing, you cannot add or delete properties.
• You can still change the values of existing properties.

javascript

Object.seal(obj);

Example

This example shows sealing an object and what changes are allowed or blocked:

javascript

const car = { brand: 'Toyota', year: 2020 };

Object.seal(car);

car.year = 2021; // Allowed: modifying existing property
car.color = 'red'; // Ignored: cannot add new property

delete car.brand; // Fails: cannot delete property

console.log(car);

Output

{"brand":"Toyota","year":2021}

Common Pitfalls

Common mistakes when sealing objects include:

• Expecting sealing to make properties read-only (it does not).
• Trying to add or delete properties after sealing (these operations fail silently or throw errors in strict mode).
• Confusing Object.seal() with Object.freeze(), which also prevents modifying existing properties.

javascript

const obj = { a: 1 };
Object.seal(obj);

// Wrong: expecting this to fail
obj.a = 2; // This works because sealing allows modification

// Right: adding new property fails
obj.b = 3; // Does not add property

// Right: deleting property fails
delete obj.a; // Does not delete property

console.log(obj);

Output

{"a":2}

Quick Reference

Method	Effect
Object.seal(obj)	Prevents adding/removing properties, allows modifying existing ones
Object.freeze(obj)	Prevents adding/removing/modifying properties
Object.preventExtensions(obj)	Prevents adding new properties only

Key Takeaways

Use Object.seal(obj) to prevent adding or deleting properties on an object.

Sealed objects still allow changing values of existing properties.

Sealing is different from freezing; freezing also blocks changes to property values.

Adding or deleting properties after sealing fails silently or throws errors in strict mode.

Use Object.freeze() if you want to make an object completely immutable.