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

Syntax

var 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

The following code traps the in operator.

var 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.

var obj = { a: 10 };
Object.preventExtensions(obj);
var p = new Proxy(obj, {
  has: function(target, prop) {
    return false;
  }
});

'a' in p; // TypeError is thrown

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of '[[HasProperty]]' in that specification.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
The definition of '[[HasProperty]]' in that specification.
Draft  

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung InternetNode.js
Basic supportChrome Full support 49Edge Full support 12Firefox Full support 18IE No support NoOpera Full support 36Safari Full support 10WebView Android Full support 49Chrome Android Full support 49Edge Mobile Full support YesFirefox Android Full support 18Opera Android Full support 36Safari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support 6.0.0

Legend

Full support  
Full support
No support  
No support

See also

Document Tags and Contributors

Contributors to this page: alshakero, wbamberg, fscholz, nmve, kdex, leobalter, getify, arai
Last updated by: alshakero,