Callback function `cb` is called for each schema object (not including draft-06 boolean schemas), including the root schema, in pre-order traversal. Schema references ($ref) are not resolved, they are passed as is. Alternatively, you can pass a `{pre, post}` object as `cb`, and then `pre` will be called before traversing child elements, and `post` will be called after all child elements have been traversed.
- _JSON pointer_: from the root schema to the current schema object
- _root schema_: the schema passed to `traverse` object
- _parent JSON pointer_: from the root schema to the parent schema object (see below)
- _parent keyword_: the keyword inside which this schema appears (e.g. `properties`, `anyOf`, etc.)
- _parent schema_: not necessarily parent object/array; in the example above the parent schema for `{type: 'string'}` is the root schema
- _index/property_: index or property name in the array/object containing multiple schemas; in the example above for `{type: 'string'}` the property name is `'foo'`