JS Built-ins
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ObjectObject.preventExtensionsbeginner

Object.preventExtensions() — verify with isExtensible

📖 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() — checking with isExtensible

After calling Object.preventExtensions(), you can verify the result with Object.isExtensible().

Challenge: Implement makeAndCheckNonExtensible(obj) that locks the object and returns whether it is now non-extensible.

💡 Hints (1)
  • • preventExtensions is the weakest of the three object locking methods