handler.preventExtensions()

The handler.preventExtensions() method is a trap for the [[PreventExtensions]] object internal method, which is used by operations such as Object.preventExtensions().

Try it

Syntax

new Proxy(target, {
  preventExtensions(target) {
  }
});

Parameters

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

target

The target object.

Return value

The preventExtensions() method must return a boolean value.

Description

Interceptions

This trap can intercept these operations:

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

Invariants

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.

Examples

Trapping of preventExtensions

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

Specifications

Specification
ECMAScript Language Specification
# sec-proxy-object-internal-methods-and-internal-slots-preventextensions

Browser compatibility

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See also