The visitor pattern has two components. The visitor processes the subject objects and the subjects tell other related subjects about the visitor, and when the current subject should be visited.

The accept method is required for each subject to receive a notification that there is a visitor. That method then makes two types of method call. The first is the accept method on its related subjects. The second is the visitor method on the visitor. In this way, the visitor traverses a structure by being passed around by the subjects.

The visitor methods are used to process different types of node. In some languages, this is handled by language-level polymorphism. In JavaScript, we can use run-time type checks to do this.

The visitor pattern is a good option for processing hierarchical structures of objects, where the structure is not known ahead of time, but the types of subjects are known.