handler.isExtensible()

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

Try it

Syntax

js

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

Parameters

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

target

The target object.

Return value

The isExtensible() method must return a boolean value.

Description

Interceptions

This trap can intercept these operations:

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

Invariants

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).

Examples

Trapping of isExtensible

The following code traps Object.isExtensible().

js

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

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

The following code violates the invariant.

js

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

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

Specifications

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

Browser compatibility

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See also