Acorn AST walker

An abstract syntax tree walker for the ESTree format.

Community

Acorn is open source software released under an MIT license.

You are welcome to report bugs or create pull requests on github. For questions and discussion, please use the Tern discussion forum.

Installation

The easiest way to install acorn is from npm:

npm install acorn-walk

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

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. 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.)

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.

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.

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).