visit() will walk through an AST using a depth first traversal, calling the visitor's enter function at each node in the traversal, and calling the leave function after visiting that node and all of its child nodes.

By returning different values from the enter and leave functions, the behavior of the visitor can be altered, including skipping over a sub-tree of the AST (by returning false), editing the AST by returning a value or null to remove the value, or to stop the whole traversal by returning BREAK.

When using visit() to edit an AST, the original AST will not be modified, and a new version of the AST with the changes applied will be returned from the visit function.

const editedAST = visit(ast, {
  enter(node, key, parent, path, ancestors) {
    // @return
    //   undefined: no action
    //   false: skip visiting this node
    //   visitor.BREAK: stop visiting altogether
    //   null: delete this node
    //   any value: replace this node with the returned value
  },
  leave(node, key, parent, path, ancestors) {
    // @return
    //   undefined: no action
    //   false: no action
    //   visitor.BREAK: stop visiting altogether
    //   null: delete this node
    //   any value: replace this node with the returned value
  }
});

Alternatively to providing enter() and leave() functions, a visitor can instead provide functions named the same as the kinds of AST nodes, or enter/leave visitors at a named key, leading to four permutations of visitor API:

1. Named visitors triggered when entering a node a specific kind.

   visit(ast, { Kind(node) { // enter the "Kind" node } })

2. Named visitors that trigger upon entering and leaving a node of a specific kind.

   visit(ast, { Kind: { enter(node) { // enter the "Kind" node } leave(node) { // leave the "Kind" node } } })

3. Generic visitors that trigger upon entering and leaving any node.

   visit(ast, { enter(node) { // enter any node }, leave(node) { // leave any node } })

4. Parallel visitors for entering and leaving nodes of a specific kind.

   visit(ast, { enter: { Kind(node) { // enter the "Kind" node } }, leave: { Kind(node) { // leave the "Kind" node } } })