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


var p = new Proxy(target, {
  has: function(target, prop) {


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

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

Return value

The has method must return a boolean value.


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


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()


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.


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

'a' in p; // TypeError is thrown


ECMAScript Latest Draft (ECMA-262)
The definition of '[[HasProperty]]' in that specification.

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
hasChrome 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 49Firefox Android Full support 18Opera Android Full support 36Safari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support 6.0.0


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See also