JS Built-ins
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ObjectObject.preventExtensionsintermediate

Object.preventExtensions() — existing props modifiable

📖 Usage Example
// Prevent new properties from being added
const obj = { a: 1 }
Object.preventExtensions(obj)
obj.b = 2    // silently ignored
Object.keys(obj)   // → ['a']

Use Object.preventExtensions() to lock an object so no new properties can be added, while still allowing modifications to existing ones.

Object.preventExtensions() — mutability preserved

Object.preventExtensions() only blocks new property additions. Existing properties remain fully mutable.

Challenge: Show that existing properties are still modifiable after calling Object.preventExtensions().

💡 Hints (1)
  • • Use isExtensible to confirm the object is non-extensible