The following example calls the valueOf() method on an ObjectId():

ObjectId("507c7f79bcf86cd7994f6c0e").valueOf()

This will return the following string:

507c7f79bcf86cd7994f6c0e

You can confirm the type of this object using the following operation:

typeof ObjectId("507c7f79bcf86cd7994f6c0e").valueOf()