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

target

The target object.

The preventExtensions() method must return a boolean value.

This trap can intercept these operations:

• Object.preventExtensions()
• Reflect.preventExtensions()
• Object.seal()
• Object.freeze()

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

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

• Object.preventExtensions(proxy) only returns true if Object.isExtensible(proxy) is false.

The following code traps Object.preventExtensions().

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

console.log(Object.preventExtensions(p));
// "called"
// false

The following code violates the invariant.

const p = new Proxy(
  {},
  {
    preventExtensions(target) {
      return true;
    },
  },
);

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

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