handler.ownKeys()

The handler.ownKeys() method is a trap for Reflect.ownKeys().

Syntax

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

Parameters

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

target
The target object.

Return value

The ownKeys() method must return an enumerable object.

Description

The handler.ownKeys() method is a trap for Reflect.ownKeys().

Interceptions

This trap can intercept these operations:

Invariants

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

  • The result of ownKeys() must be an array.
  • The type of each array element is either a String or a Symbol.
  • The result List must contain the keys of all non-configurable own properties of the target object.
  • If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.

Examples

Trapping of getOwnPropertyNames

The following code traps Object.getOwnPropertyNames().

const p = new Proxy({}, {
  ownKeys: function(target) {
    console.log('called');
    return ['a', 'b', 'c'];
  }
});

console.log(Object.getOwnPropertyNames(p)); // "called"
                                            // [ 'a', 'b', 'c' ]

The following code violates an invariant.

const obj = {};
Object.defineProperty(obj, 'a', {
  configurable: false,
  enumerable: true,
  value: 10 }
);

const p = new Proxy(obj, {
  ownKeys: function(target) {
    return [123, 12.5, true, false, undefined, null, {}, []];
  }
});

console.log(Object.getOwnPropertyNames(p));

// TypeError: proxy [[OwnPropertyKeys]] must return an array
// with only string and symbol elements

Specifications

Specification
ECMAScript (ECMA-262)
The definition of '[[OwnPropertyKeys]]' in that specification.

Browser compatibility

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See also