handler.preventExtensions()

The handler.preventExtensions() method is a trap for Object.preventExtensions().

const p = new Proxy(target, {
  preventExtensions: function(target) {
  }
});

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.

The handler.preventExtensions() method is a trap for Object.preventExtensions().

This trap can intercept these operations:

• Object.preventExtensions()
• Reflect.preventExtensions()

If the following invariants are violated, the proxy will throw a TypeError:

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

The following code traps Object.preventExtensions().

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

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

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