Proxy.handler.has

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 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 2017 Draft (ECMA-262) The definition of '[[HasProperty]]' in that specification.	Draft

Browser compatibility

See also

• Proxy
• handler
• in operator
• Reflect.has()