Difference between Object.preventExtensions/seal/freeze

All tested with node --use_strict (to prevent silent errors).

const obj = { a: 1 };
Object.preventExtensions(obj);
obj.b = 2; // TypeError: Cannot add property b, object is not extensible ❌
obj.a = -1; // ✅
delete obj.a; // ✅

Object.getOwnPropertyDescriptors(obj); // { a: { value: 1, writable: true, enumerable: true, configurable: true } }

const obj = { a: 1 };
Object.seal(obj);
obj.b = 2; // TypeError: Cannot add property b, object is not extensible ❌
obj.a = -1; // ✅
delete obj.a; // TypeError: Cannot delete property 'a' of #<Object> ❌

Object.getOwnPropertyDescriptors(obj); // { a: { value: 1, writable: true, enumerable: true, configurable: false } }

const obj = { a: 1 };
Object.freeze(obj);
obj.b = 2; // TypeError: Cannot add property b, object is not extensible ❌
obj.a = -1; // TypeError: Cannot assign to read only property 'a' of object '#<Object>' ❌
delete obj.a; // TypeError: Cannot delete property 'a' of #<Object> ❌

Object.getOwnPropertyDescriptors(obj); // { a: { value: 1, writable: false, enumerable: true, configurable: false } }

// You can basically only read:
console.log(obj.a);

Note: Flow tracks these flags on objects: frozen, sealed and exact.