What does the `instanceof` operator do?

The instanceof operator is used in JavaScript to determine whether an object is an instance of a particular constructor or class. This is particularly useful in object-oriented programming when dealing with inheritance and prototypes. When used, it checks the prototype chain of the object to see whether it has been constructed by the specified constructor or if it inherits from it.

For example, if you have a class Animal and create an object dog using new Animal(), using dog instanceof Animal will return true, indicating that dog is indeed an instance of Animal. This capability allows developers to perform type-checking within their code, ensuring that objects are of the expected type before executing type-specific operations.

The other options do not accurately describe the functionality of the instanceof operator. The operator does not check for the presence of properties, return constructors, or perform equality comparisons between two objects.