What is the difference between Object.freeze, Object.seal, and Object.preventExtensions?

The short answer

All three restrict what you can do to an object, but at different levels. Object.preventExtensions stops you from adding new properties. Object.seal does that plus prevents deleting properties. Object.freeze does all of that plus prevents changing existing property values. Freeze is the most restrictive.

Object.preventExtensions

You cannot add new properties, but you can modify and delete existing ones:

const obj = { name: 'John', age: 30 };
Object.preventExtensions(obj);
obj.name = 'Jane'; // works: modifies an existing property
delete obj.age; // works: deletes an existing property
obj.email = 'john@example.com'; // silently fails (throws in strict mode)

Object.seal

You cannot add or delete properties, but you can still change existing values:

const obj = { name: 'John', age: 30 };
Object.seal(obj);
obj.name = 'Jane'; // works: modifies an existing property
delete obj.age; // ignored (TypeError in strict mode)
obj.email = 'john@example.com'; // ignored (TypeError in strict mode)

Object.freeze

You cannot add, delete, or change properties. The object becomes completely immutable:

const obj = { name: 'John', age: 30 };
Object.freeze(obj);
obj.name = 'Jane'; // ignored (TypeError in strict mode)
delete obj.age; // ignored (TypeError in strict mode)
obj.email = 'john@example.com'; // ignored (TypeError in strict mode)

Comparison table

Important: shallow only

All three methods are shallow. They do not affect nested objects:

const obj = { name: 'John', address: { city: 'NYC' } };
Object.freeze(obj);
obj.name = 'Jane'; // fails: obj is frozen
obj.address.city = 'Boston'; // works: the nested object is not frozen

To deep freeze, you would need to recursively freeze all nested objects.

Checking the state

Object.isExtensible(obj); // false if preventExtensions/seal/freeze
Object.isSealed(obj); // true if sealed or frozen
Object.isFrozen(obj); // true if frozen

Why interviewers ask this

This tests your knowledge of JavaScript's object immutability tools. Interviewers want to see if you know the differences and when you would use each one. Understanding these methods shows you think about data integrity in your applications.