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Showing content from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/has below:

handler.has() - JavaScript | MDN

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The handler.has() method is a trap for the [[HasProperty]] object internal method, which is used by operations such as the in operator.

const handler = {
  has(target, key) {
    if (key[0] === "_") {
      return false;
    }
    return key in target;
  },
};

const monster = {
  _secret: "easily scared",
  eyeCount: 4,
};

const proxy = new Proxy(monster, handler);
console.log("eyeCount" in proxy);
// Expected output: true

console.log("_secret" in proxy);
// Expected output: false

console.log("_secret" in monster);
// Expected output: true

new Proxy(target, {
  has(target, property) {
  }
})

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

target

The target object.

property

A string or Symbol representing the property name.

The has() method must return a Boolean indicating whether or not the property exists. Other values are coerced to booleans.

This trap can intercept these operations:

• The in operator: foo in proxy
• with check: with(proxy) { (foo); }
• Reflect.has()

Or any other operation that invokes the [[HasProperty]] internal method.

The proxy's [[HasProperty]] internal method throws a TypeError if the handler definition violates one of the following invariants:

• A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object. That is, if Reflect.getOwnPropertyDescriptor() returns configurable: false for the property on target, the trap must return true.
• 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. That is, if Reflect.isExtensible() returns false on target, and Reflect.getOwnPropertyDescriptor() returns a property descriptor for the property on target, the trap must return true.

The following code traps the in operator.

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

"a" in p; // TypeError is thrown

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