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Proxy.proxy.has

The handler.has() method is a trap for the in operator.

Syntax

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

Parameters

The following parameters are passed to has() method. this is bound to the handler.

target
The target object.
prop
The name or Symbol of the property to check for existence.

Return value

The has() method must return a boolean value.

Description

The handler.has() method is a trap for the in operator.

Interceptions

This trap can intercept these operations:

  • Property query: foo in proxy
  • Inherited property query: foo in Object.create(proxy)
  • with check: with(proxy) { (foo); }
  • Reflect.has()

Invariants

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

  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.

Examples

Trapping the in operator

The following code traps the in operator.

const p = new Proxy({}, {
  has: function(target, prop) {
    console.log('called: ' + prop);
    return true;
  }
});

console.log('a' in p); // "called: a"
                       // true

The following code violates an invariant.

const obj = { a: 10 };
Object.preventExtensions(obj);

const p = new Proxy(obj, {
  has: function(target, prop) {
    return false;
  }
});

'a' in p; // TypeError is thrown

Specifications

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
has 49 12 18 No 36 10
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
has 49 49 18 36 10 5.0
Server
Node.js
has 6.0.0

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/has