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handler.has()

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

Try it

Syntax

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 compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
has
49
12
18
No
36
10
49
49
18
36
10
5.0
1.0
6.0.0

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/has