@rbxts/jsnatives

Object.seal

Seals an object, preventing new properties from being added and marking all existing properties as non-configurable.

Signature

function seal<T>(obj: T): Readonly<T>

Description

The Object.seal() method seals an object, preventing new properties from being added to it and marking all existing properties as non-configurable. Values of existing properties can still be changed as long as they are writable.

Unlike Object.freeze(), which makes properties non-writable as well, Object.seal() only prevents adding and removing properties.

Examples

Basic sealing

const obj = {a: 1, b: 2, c: 3};
const sealedObj = Object.seal(obj);

// Modifying existing properties still works
sealedObj.a = 5;
print(sealedObj.a); // 5

// But adding new properties does not work
sealedObj.d = 4; // This has no effect in JavaScript
// In TypeScript, this would be a compilation error:
// Property 'd' does not exist on type 'Readonly<{ a: number; b: number; c: number; }>'

// Deleting properties also does not work
delete sealedObj.b; // This has no effect
print(sealedObj.b); // 2

Comparing seal and freeze

const regularObj = {x: 1, y: 2};
const sealedObj = Object.seal({x: 1, y: 2});
const frozenObj = Object.freeze({x: 1, y: 2});

// Regular object - can add, modify, delete
regularObj.z = 3;
regularObj.x = 10;
delete regularObj.y;
print(regularObj); // {x: 10, z: 3}

// Sealed object - can only modify existing properties
sealedObj.x = 10; // Can modify
// sealedObj.z = 3; // Cannot add (no effect)
// delete sealedObj.y; // Cannot delete (no effect)
print(sealedObj); // {x: 10, y: 2}

// Frozen object - cannot modify, add, or delete
// frozenObj.x = 10; // Cannot modify (no effect)
// frozenObj.z = 3; // Cannot add (no effect)
// delete frozenObj.y; // Cannot delete (no effect)
print(frozenObj); // {x: 1, y: 2}

Sealing arrays and collections

const array = [1, 2, 3];
const sealedArray = Object.seal(array);

// Can modify existing elements
sealedArray[0] = 10;
print(sealedArray); // [10, 2, 3]

// But cannot add new elements or change the length
// sealedArray.push(4); // Error: Property 'push' does not exist on type 'Readonly<number[]>'.