unist-util-visit

unist utility to walk the tree

Contents

• What is this?
• When should I use this?
• Install
• Use
• API
  • visit(tree[, test], visitor|visitors[, reverse])
  • CONTINUE
  • EXIT
  • SKIP
  • Action
  • ActionTuple
  • Index
  • Test
  • TestFunction<[T][, P]>
  • VisitedAncestor<[Tree][, Check]>
  • VisitedNode<[Tree][, Check]>
  • VisitedParent<[Tree][, Check]>
  • Visitor<[Tree][, Check]>
  • VisitorResult
  • Visitors<[Tree][, Check]>
• Related
• Contribute

What is this?

This package is an essential utility for unist that lets you walk the tree.

When should I use this?

Use this utility when you want to walk the tree with ancestral information, or need to do a postorder walk.

You can use unist-util-visit-parents or syntax-tree/unist-util-visit if you only need to do a preorder traversal, or don't care about the entire stack of parents.

Install

This package is ESM only.

In Node.js (version 18+) with yarn:

yarn add @flex-development/unist-util-visit
yarn add -D @types/unist

In Deno with esm.sh:

import { CONTINUE, EXIT, SKIP, visit } from 'https://esm.sh/@flex-development/unist-util-visit'

In browsers with esm.sh:

<script type="module">
  import { CONTINUE, EXIT, SKIP, visit } from 'https://esm.sh/@flex-development/unist-util-visit'
</script>

Use

import { fromDocs } from '@flex-development/docast-util-from-docs'
import { visit } from '@flex-development/unist-util-visit'
import { directiveFromMarkdown } from 'mdast-util-directive'
import { directive } from 'micromark-extension-directive'
import { read } from 'to-vfile'

const file = await read('examples/docblock.mjs')

const tree = fromDocs(file, {
  mdastExtensions: [directiveFromMarkdown()],
  micromarkExtensions: [directive()]
})

visit(tree, (node, index, parent, ancestors) => {
  console.log(`\u001B[1m${node.type}\u001B[22m`)
  console.log(`index: ${index}`)
  console.log(`parent: ${parent?.type}`)
  console.log(`ancestors: ${JSON.stringify(ancestors.map(anc => anc.type))}\n`)
})

// enter and leave
visit(tree, {
  enter(node, index, parent, ancestors) {/* … */},
  leave(node, index, parent, ancestors) {/* … */}
})

Yields:

root
index: undefined
parent: undefined
ancestors: []

comment
index: 0
parent: root
ancestors: []

description
index: 0
parent: comment
ancestors: ["root"]

paragraph
index: 0
parent: description
ancestors: ["root","comment"]

text
index: 0
parent: paragraph
ancestors: ["root","comment","description"]

break
index: 1
parent: description
ancestors: ["root","comment"]

break
index: 2
parent: description
ancestors: ["root","comment"]

containerDirective
index: 3
parent: description
ancestors: ["root","comment"]

paragraph
index: 0
parent: containerDirective
ancestors: ["root","comment","description"]

text
index: 0
parent: paragraph
ancestors: ["root","comment","description","containerDirective"]

inlineCode
index: 1
parent: paragraph
ancestors: ["root","comment","description","containerDirective"]

text
index: 2
parent: paragraph
ancestors: ["root","comment","description","containerDirective"]

code
index: 1
parent: containerDirective
ancestors: ["root","comment","description"]

blockTag
index: 1
parent: comment
ancestors: ["root"]

typeExpression
index: 0
parent: blockTag
ancestors: ["root","comment"]

API

This package exports the identifiers CONTINUE, EXIT, SKIP, and visit. There is no default export.

visit(tree[, test], visitor|visitors[, reverse])

Visit nodes, with ancestral information.

This algorithm performs depth-first tree traversal in preorder (NLR) and/or postorder (LRN), or if reverse is given, reverse preorder (NRL) and/or reverse postorder (RLN). Nodes are handled on enter during preorder traversals and on exit during postorder traversals.

You can choose which nodes visitor functions handle by passing a test. For complex tests, you should test yourself in visitor or visitors instead, as it will be faster and also have improved type information.

Walking the tree is an intensive task. Make use of visitor return values whenever possible. Instead of walking the tree multiple times, walk it once, use unist-util-is to check if a node matches, and then perform different operations.

You can change tree. See Visitor for more info.

Parameters

• tree (Node) - tree to traverse
• test (Test, optional) - unist-util-is-compatible test
• visitor (Visitor) - handle a node on enter
• visitors (Visitors) - handle each node on enter and/or exit
• reverse (boolean, optional) - traverse in reverse

Return

Nothing (void).

CONTINUE

Continue traversing as normal (true).

const CONTINUE: Continue = true

EXIT

Stop traversing immediately (false).

const EXIT: Exit = false

SKIP

Do not traverse the children of this node ('skip').

const SKIP: Skip = 'skip'

Action

Union of action types.

type Action = Continue | Exit | Skip

ActionTuple

List with at most two (2) values, the first an Action and the second an Index.

type ActionTuple = [
  action?: Action | null | undefined | void,
  index?: Index | null | undefined
]

Index

Move to the sibling at index next (after node itself is completely traversed).

Useful if mutating the tree, such as when removing the node the Visitor is currently on, or any of its previous siblings.

Negative indices (< 0) and indices greater than or equal to parent.children.length stop traversal of the parent.

type Index = number

Test

Check for an arbitrary Node.

See unist-util-is for more details.

type Test =
  | (TestFunction | unist.Node | unist.Node['type'])[]
  | TestFunction
  | unist.Node
  | unist.Node['type']
  | null
  | undefined

TestFunction<[T][, P]>

Check if node passes a test.

• T (Node): node to check
  • default: Node
• P (Parent): parent of node T
  • default: Parent

Parameters

• node (T): node to check
• index (Index | undefined): index of node in parent.children
• parent (Parent | undefined): parent of node

Return

Test result (boolean | undefined | void).

👉 Note: For the best type-safety, test functions should return type predicates (node is Type).

VisitedAncestor<[Tree][, Check]>

Collect ancestors of visited nodes in Tree.

• Tree (Node) - tree to extract ancestors from
  • default: Node
• Check (Test) - visited node test
  • default: null | undefined

import type * as docast from '@flex-development/docast'
import type { VisitedAncestor } from '@flex-development/unist-util-visit'
import type * as unist from 'unist'

type Tree = docast.Root
type Check = (value: unist.Node) => value is docast.TypeExpression

type Visited = VisitedAncestor<Tree, Check> // docast.BlockTag | docast.Comment | docast.Root

VisitedNode<[Tree][, Check]>

Collect visited nodes in Tree.

• Tree (Node) - tree to traverse
  • default: Node
• Check (Test) - visited node test
  • default: null | undefined

import type * as docast from '@flex-development/docast'
import type { VisitedNode } from '@flex-development/unist-util-visit'

type Tree = docast.Root

type Visited = VisitedNode<Tree>
// | docast.Root
// | docast.Comment
// | docast.BlockTag
// | docast.Description
// | docast.InlineTag
// | docast.TypeExpression
// | mdast.Blockquote
// | mdast.Code
// | mdast.Definition
// | mdast.FootnoteDefinition
// | mdast.Heading
// | mdast.List
// | mdast.ListItem
// | mdast.Paragraph
// | mdast.PhrasingContent
// | mdast.Table
// | mdast.TableCell
// | mdast.TableRow
// | mdast.ThematicBreak

VisitedParent<[Tree][, Check]>

Collect parents of visited nodes in Tree.

• Tree (Node) - tree to extract parents from
  • default: Node
• Check (Test) - visited node test
  • default: null | undefined

import type * as docast from '@flex-development/docast'
import type { VisitedNode } from '@flex-development/unist-util-visit'

type Tree = docast.Root
type Check = docast.TypeExpression['type']

type Visited = VisitedNode<Tree, Check> // docast.BlockTag

Visitor<[Tree][, Check]>

Handle a node.

Visitors are free to transform node. They can also transform parent, or the grandparent of node (the last of ancestors).

👉 Note: Replacing node itself, if SKIP is not returned, still causes its descendants to be walked (which is a bug).

When adding or removing previous siblings of node, the Visitor should return a new Index to specify the sibling to traverse after node is traversed. Adding or removing next siblings of node is handled as expected without needing to return a new Index.

Removing the children of an ancestor still results in those child nodes being traversed.

• Tree (Node) - tree to traverse
  • default: Node
• Check (Test) - visited node test
  • default: null | undefined

Parameters

• node (VisitedNode<Tree, Check>) - found node
• index (Index | undefined) - index of node in parent.children
• parent (VisitedParent<Tree, Check> | undefined) - parent of node
• ancestors (VisitedAncestor<Tree, Check>[]) - ancestors of node, if any, where the last node is the grandparent of node

Return

What to do next (VisitorResult).

VisitorResult

Union of values that can be returned from a Visitor.

An Index is treated as a tuple of [CONTINUE, Index]. An Action is treated as a tuple of [Action].

Returning a tuple only makes sense if the Action is SKIP. When the Action is EXIT, that action can be returned. When the Action is CONTINUE, Index can be returned.

type VisitorResult = Action | ActionTuple | Index | null | undefined | void

Visitors<[Tree][, Check]>

Handle nodes when entering (preorder) and/or leaving (postorder).

• Tree (Node) - tree to traverse
  • default: Node
• Check (Test) - visited node test
  • default: null | undefined

Fields

• enter (Visitor<Tree, Check>, optional) - handle nodes when entering (preorder)
• leave (Visitor<Tree, Check>, optional) - handle nodes when exiting (postorder)

Related

• unist-util-filter — create a new tree with all nodes that pass a test
• unist-util-flatmap — create a new tree by mapping (to an array) with the given function
• unist-util-generated — check if a node is generated
• unist-util-is — check if a node passes a test
• unist-util-map — create a new tree with all nodes mapped by a given function
• unist-util-remove — remove nodes from a tree that pass a test
• unist-util-select — select nodes with CSS-like selectors
• unist-util-visit-parents — recursively walk over nodes, with a stack of parents

Contribute

See CONTRIBUTING.md.

This project has a code of conduct. By interacting with this repository, organization, or community you agree to abide by its terms.