estree-util-visit

estree (and esast) utility to visit nodes.

Contents

• What is this?
• When should I use this?
• Install
• Use
• API
  • visit(tree, visitor|visitors)
  • CONTINUE
  • EXIT
  • SKIP
  • Action
  • ActionTuple
  • Index
  • Visitor
  • Visitors
• Types
• Compatibility
• Related
• Contribute
• License

What is this?

This package is a utility that helps you walk the tree.

When should I use this?

This package helps when dealing with JavaScript ASTs. Use unist-util-visit for other unist ASTs.

Install

This package is ESM only. In Node.js (version 16+), install with npm:

npm install estree-util-visit

In Deno with esm.sh:

import {visit} from 'https://esm.sh/estree-util-visit@2'

In browsers with esm.sh:

<script type="module">
  import {visit} from 'https://esm.sh/estree-util-visit@2?bundle'
</script>

Use

import {parse} from 'acorn'
import {visit} from 'estree-util-visit'

const tree = parse(
  'export function x() { console.log(1 + "2"); process.exit(3) }',
  {sourceType: 'module', ecmaVersion: 2020}
)

visit(tree, function (node) {
  if (node.type === 'Literal' && 'value' in node) console.log(node.value)
})

// Both enter and leave:
walk(tree, {
  enter(node, field, index, parents) { /* … */ },
  leave(node, field, index, parents) { /* … */ }
})

Yields:

1
"2"
3

API

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

visit(tree, visitor|visitors)

Visit nodes, with ancestral information.

This algorithm performs depth-first tree traversal in preorder (NLR) and/or postorder (LRN).

Compared to other estree walkers, this does not need a dictionary of which fields are nodes, because it ducktypes instead.

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

Parameters

• tree (Node) — tree to traverse
• visitor (Visitor) — same as passing {enter: visitor}
• visitors (Visitors) — handle each node

Returns

Nothing (undefined).

CONTINUE

Continue traversing as normal (symbol).

EXIT

Stop traversing immediately (symbol).

SKIP

Do not traverse this node’s children (symbol).

Action

Union of the action types (TypeScript type).

Type

type Action = typeof CONTINUE | typeof EXIT | typeof SKIP

ActionTuple

List with one or two values, the first an action, the second an index (TypeScript type).

Type

type ActionTuple = [
  (Action | null | undefined | void)?,
  (Index | null | undefined)?
]

Index

Move to the sibling at index next (after node itself is completely traversed), when moving in an array (TypeScript type).

Useful if mutating the tree, such as removing the node the visitor is currently on, or any of its previous siblings. Results less than 0 or greater than or equal to children.length stop traversing the parent.

Type

type Index = number

Visitor

Handle a node (TypeScript type).

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

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.

Parameters

• node (Node) — found node
• key (string or undefined) — field at which node lives in its parent (or where a list of nodes lives)
• index (number or undefined) — index where node lives if parent[key] is an array
• ancestors (Array<Node>) — ancestors of node

Returns

What to do next (Action, Index, or ActionTuple, optional).

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

Passing a tuple back 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.

Visitors

Handle nodes when entering (preorder) and leaving (postorder) (TypeScript type).

Fields

• enter (Visitor, optional) — handle nodes when entering (preorder)
• leave (Visitor, optional) — handle nodes when leaving (postorder)

Types

This package is fully typed with TypeScript. It exports the additional types Action, ActionTuple, Index, Visitor, and Visitors.

Compatibility

Projects maintained by the unified collective are compatible with maintained versions of Node.js.

When we cut a new major release, we drop support for unmaintained versions of Node. This means we try to keep the current release line, estree-util-visit@^2, compatible with Node.js 16.

Related

• unist-util-visit — walk any unist tree

Contribute

See contributing.md in syntax-tree/.github for ways to get started. See support.md for ways to get help.

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

License

MIT © Titus Wormer