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# Acorn AST walker

An abstract syntax tree walker for the
[ESTree](https://github.com/estree/estree) format.

## Community

Acorn is open source software released under an
[MIT license](https://github.com/acornjs/acorn/blob/master/acorn-walk/LICENSE).

You are welcome to
[report bugs](https://github.com/acornjs/acorn/issues) or create pull
requests on [github](https://github.com/acornjs/acorn). For questions
and discussion, please use the
[Tern discussion forum](https://discuss.ternjs.net).

## Installation

The easiest way to install acorn is from [`npm`](https://www.npmjs.com/):

```sh
npm install acorn-walk
```

Alternately, you can download the source and build acorn yourself:

```sh
git clone https://github.com/acornjs/acorn.git
cd acorn
npm install
```

## Interface

An algorithm for recursing through a syntax tree is stored as an
object, with a property for each tree node type holding a function
that will recurse through such a node. There are several ways to run
such a walker.

**simple**`(node, visitors, base, state)` does a 'simple' walk over a
tree. `node` should be the AST node to walk, and `visitors` an object
with properties whose names correspond to node types in the [ESTree
spec](https://github.com/estree/estree). The properties should contain
functions that will be called with the node object and, if applicable
the state at that point. The last two arguments are optional. `base`
is a walker algorithm, and `state` is a start state. The default
walker will simply visit all statements and expressions and not
produce a meaningful state. (An example of a use of state is to track
scope at each point in the tree.)

```js
const acorn = require("acorn")
const walk = require("acorn-walk")

walk.simple(acorn.parse("let x = 10"), {
  Literal(node) {
    console.log(`Found a literal: ${node.value}`)
  }
})
```

**ancestor**`(node, visitors, base, state)` does a 'simple' walk over
a tree, building up an array of ancestor nodes (including the current node)
and passing the array to the callbacks as a third parameter.

```js
const acorn = require("acorn")
const walk = require("acorn-walk")

walk.ancestor(acorn.parse("foo('hi')"), {
  Literal(_, ancestors) {
    console.log("This literal's ancestors are:", ancestors.map(n => n.type))
  }
})
```

**recursive**`(node, state, functions, base)` does a 'recursive'
walk, where the walker functions are responsible for continuing the
walk on the child nodes of their target node. `state` is the start
state, and `functions` should contain an object that maps node types
to walker functions. Such functions are called with `(node, state, c)`
arguments, and can cause the walk to continue on a sub-node by calling
the `c` argument on it with `(node, state)` arguments. The optional
`base` argument provides the fallback walker functions for node types
that aren't handled in the `functions` object. If not given, the
default walkers will be used.

**make**`(functions, base)` builds a new walker object by using the
walker functions in `functions` and filling in the missing ones by
taking defaults from `base`.

**full**`(node, callback, base, state)` does a 'full' walk over a
tree, calling the callback with the arguments (node, state, type) for
each node

**fullAncestor**`(node, callback, base, state)` does a 'full' walk
over a tree, building up an array of ancestor nodes (including the
current node) and passing the array to the callbacks as a third
parameter.

```js
const acorn = require("acorn")
const walk = require("acorn-walk")

walk.full(acorn.parse("1 + 1"), node => {
  console.log(`There's a ${node.type} node at ${node.ch}`)
})
```

**findNodeAt**`(node, start, end, test, base, state)` tries to locate
a node in a tree at the given start and/or end offsets, which
satisfies the predicate `test`. `start` and `end` can be either `null`
(as wildcard) or a number. `test` may be a string (indicating a node
type) or a function that takes `(nodeType, node)` arguments and
returns a boolean indicating whether this node is interesting. `base`
and `state` are optional, and can be used to specify a custom walker.
Nodes are tested from inner to outer, so if two nodes match the
boundaries, the inner one will be preferred.

**findNodeAround**`(node, pos, test, base, state)` is a lot like
`findNodeAt`, but will match any node that exists 'around' (spanning)
the given position.

**findNodeAfter**`(node, pos, test, base, state)` is similar to
`findNodeAround`, but will match all nodes *after* the given position
(testing outer nodes before inner nodes).