handler.has()

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

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