@rbxts/jsnatives

Object.isFrozen

Determines if an object is frozen.

Signature

function isFrozen<T>(obj: T): obj is Readonly<T>

Description

The Object.isFrozen() method determines whether an object is frozen. An object is frozen if and only if it is not extensible, all its properties are non-configurable, and all its data properties are non-writable.

The method returns a type predicate that narrows the type to Readonly<T> when the object is frozen.

Examples

Checking frozen objects

const regularObj = {a: 1, b: 2};
const frozenObj = Object.freeze({c: 3, d: 4});

print(Object.isFrozen(regularObj)); // false
print(Object.isFrozen(frozenObj)); // true

Using with type narrowing

function processObject<T>(obj: T) {
  if (Object.isFrozen(obj)) {
    // In this block, obj is typed as Readonly<T>
    console.log("Object is frozen and immutable");
    // obj.someProp = 5; // TypeScript error: Cannot assign to property of readonly object
  } else {
    // In this block, obj is still typed as T
    console.log("Object is not frozen");
    // We can potentially modify it
  }
}

const normalObj = {x: 10};
const immutableObj = Object.freeze({y: 20});

processObject(normalObj);     // "Object is not frozen"
processObject(immutableObj);  // "Object is frozen and immutable"

Working with collections

const array = [1, 2, 3];
print(Object.isFrozen(array)); // false

const frozenArray = Object.freeze([4, 5, 6]);
print(Object.isFrozen(frozenArray)); // true

const map = new Map([["a", 1]]);
print(Object.isFrozen(map)); // false

const frozenMap = Object.freeze(new Map([["b", 2]]));
print(Object.isFrozen(frozenMap)); // true