unified

Project: syntax-tree/estree-util-visit

Package: estree-util-visit@1.2.0

  1. Dependents: 0
  2. esast (and estree) utility to visit nodes
  1. util 145
  2. utility 141
  3. tree 44
  4. ast 37
  5. javascript 25
  6. node 25
  7. ecmascript 10
  8. estree 9
  9. walk 8
  10. visit 6
  11. esast 5
  12. esast-util 4
  13. recma 3

estree-util-visit

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estree (and esast) utility to visit nodes.

Contents

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 14.14+ and 16.0+), install with npm:

npm install estree-util-visit

In Deno with esm.sh:

import {visit} from 'https://esm.sh/estree-util-visit@1'

In browsers with esm.sh:

<script type="module">
  import {visit} from 'https://esm.sh/estree-util-visit@1?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, (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
Returns

Nothing (void).

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 SKIP | typeof EXIT

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
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

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 all maintained versions of Node.js. As of now, that is Node.js 12.20+, 14.14+, 16.0+, and 18.0+. Our projects sometimes work with older versions, but this is not guaranteed.

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