The following parameter is passed to the isExtensible() method. this is bound to the handler.

target

The target object.

The isExtensible() method must return a boolean value.

This trap can intercept these operations:

• Object.isExtensible()
• Reflect.isExtensible()

Or any other operation that invokes the [[IsExtensible]] internal method.

If the following invariants are violated, the trap throws a TypeError when invoked.

• Object.isExtensible(proxy) must return the same value as Object.isExtensible(target).

The following code traps Object.isExtensible().

const p = new Proxy(
  {},
  {
    isExtensible(target) {
      console.log("called");
      return true;
    },
  },
);

console.log(Object.isExtensible(p));
// "called"
// true

The following code violates the invariant.

const p = new Proxy(
  {},
  {
    isExtensible(target) {
      return false;
    },
  },
);

Object.isExtensible(p); // TypeError is thrown

• Proxy
• Proxy() constructor
• Object.isExtensible()
• Reflect.isExtensible()
• Reflect.preventExtensions()