target

The target object in which to look for the property.

propertyKey

The name of the property to check.

A Boolean indicating whether or not the target has the property.

TypeError

Thrown if target is not an object.

Reflect.has() provides the reflective semantic of checking if a property is in an object. That is, Reflect.has(target, propertyKey) is semantically equivalent to:

propertyKey in target;

Reflect.has() invokes the [[HasProperty]] object internal method of target.

Reflect.has({ x: 0 }, "x"); // true
Reflect.has({ x: 0 }, "y"); // false

// returns true for properties in the prototype chain
Reflect.has({ x: 0 }, "toString");

// Proxy with .has() handler method
obj = new Proxy(
  {},
  {
    has(t, k) {
      return k.startsWith("door");
    },
  },
);
Reflect.has(obj, "doorbell"); // true
Reflect.has(obj, "dormitory"); // false

Reflect.has returns true for any inherited properties, like the in operator:

const a = { foo: 123 };
const b = { __proto__: a };
const c = { __proto__: b };
// The prototype chain is: c -> b -> a
Reflect.has(c, "foo"); // true

• Polyfill of Reflect.has in core-js
• Reflect
• in
• handler.has()