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.

The has() method must return a boolean value.

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.

If the following invariants are violated, the trap throws a TypeError when invoked.

• 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.

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

• Proxy
• Proxy() constructor
• in
• Reflect.has()