Object.preventExtensions(), Object.seal(), and Object.freeze

Property Descriptor

Each data property has these important attributes:

• value -- the property's value.
• writable -- whether the value can be changed.
• enumerable -- whether the property appears in Object.keys(), for...in, etc.
• configurable -- whether the property can be deleted or its descriptor changed.

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Object.preventExtensions()

Prevents adding new properties.

const obj = { name: "Chandan" };

Object.preventExtensions(obj);

obj.age = 20; // Ignored (or TypeError in strict mode)
obj.name = "Rahul"; // Works
delete obj.name; // Works

Effects:

• Cannot add new properties.
• Can modify existing writable properties.
• Can delete configurable properties.
• Does not change property descriptors.

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Object.seal()

Locks the object's shape.

const obj = { name: "Chandan" };

Object.seal(obj);

obj.name = "Rahul"; // Works
obj.age = 20;       // Not allowed
delete obj.name;    // Not allowed

Effects:

• Cannot add new properties.
• Cannot delete properties.
• Existing writable properties can still be modified.
• Sets configurable: false for all own properties.
• Leaves writable unchanged.
• Leaves enumerable unchanged.

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Object.freeze()

Makes the object effectively immutable (shallow).

const obj = { name: "Chandan" };

Object.freeze(obj);

obj.name = "Rahul"; // Not allowed
obj.age = 20;       // Not allowed
delete obj.name;    // Not allowed

Effects:

• Cannot add new properties.
• Cannot delete properties.
• Cannot modify existing data properties.
• Sets configurable: false for all own properties.
• Sets writable: false for all own data properties.
• Leaves enumerable unchanged.
• Shallow only.

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Shallow Freeze Example

const obj = {
  user: { name: "Chandan" }
};

Object.freeze(obj);

obj.user.name = "Rahul"; // Works

Only the top-level object is frozen.

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Understanding configurable

configurable controls whether a property can be:

• Deleted.
• Redefined with Object.defineProperty().
• Have attributes like enumerable changed.

Once configurable is set to false, it can never become true again.

Exception:

• If writable is true, it may be changed to false.
• writable: false cannot later become true when configurable is false.

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Comparison

Feature	preventExtensions	seal	freeze
Add new properties	No	No	No
Delete properties	Yes	No	No
Modify existing values	Yes	Yes (if writable)	No
configurable	Unchanged	false	false
writable	Unchanged	Unchanged	false (data properties)
enumerable	Unchanged	Unchanged	Unchanged
Shallow	Yes	Yes	Yes

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Summary

• preventExtensions() → Prevents adding new properties.
• seal() → Prevents adding and deleting properties. Existing writable values can still change.
• freeze() → Prevents adding, deleting, and changing existing data property values.
• freeze() and seal() are shallow.
• configurable: false is irreversible.