Understanding Object.preventExtensions, Object.seal, and Object.freeze in JavaScript

In JavaScript, objects are mutable by default, meaning properties can be added, modified, or deleted. However, there are ways to control an object's mutability using Object.preventExtensions, Object.seal, and Object.freeze. These methods help in different levels of restricting modifications to objects.

Comparison Table

Method	Prevent Adding Properties	Prevent Deleting Properties	Prevent Modifying Properties
Object.preventExtensions	✅ Yes	❌ No	❌ No
Object.seal	✅ Yes	✅ Yes	❌ No
Object.freeze	✅ Yes	✅ Yes	✅ Yes

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1. Object.preventExtensions(obj)

• Prevents new properties from being added.
• Existing properties can be modified or deleted.
• Returns the same object.

const obj = { name: "Alice" };
Object.preventExtensions(obj);

obj.age = 25; // ❌ Fails silently in non-strict mode, throws an error in strict mode
delete obj.name; // ✅ Allowed
obj.name = "Bob"; // ✅ Allowed (modification)

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2. Object.seal(obj)

• Prevents adding or deleting properties.
• Existing properties can still be modified.
• Returns the same object.

const obj = { name: "Alice" };
Object.seal(obj);

obj.age = 25; // ❌ Fails silently in non-strict mode, throws an error in strict mode
delete obj.name; // ❌ Not allowed
obj.name = "Bob"; // ✅ Allowed (modification)

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3. Object.freeze(obj)

• Prevents adding, deleting, or modifying properties.
• Makes the object completely immutable.
• Returns the same object.

const obj = { name: "Alice" };
Object.freeze(obj);

obj.age = 25; // ❌ Fails silently in non-strict mode, throws an error in strict mode
delete obj.name; // ❌ Not allowed
obj.name = "Bob"; // ❌ Not allowed (modification)

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Key Takeaways

• Use Object.preventExtensions() if you want to prevent new properties from being added.
• Use Object.seal() if you want to prevent new properties and deletion but allow modification.
• Use Object.freeze() if you want to make the object completely immutable.