JavaScript API

Powered by napi.rs, ast-grep's JavaScript API enables you to write JavaScript to programmatically inspect and change syntax trees.

ast-grep's JavaScript API design is pretty stable now. No major breaking changes are expected in the future.

To try out the JavaScript API, you can use the code sandbox here.

Installation

First, install ast-grep's napi package.

npm

npm install --save @ast-grep/napi

pnpm

pnpm add @ast-grep/napi

Now let's explore ast-grep's API!

Core Concepts

The core concepts in ast-grep's JavaScript API are:

• SgRoot: a class representing the whole syntax tree
• SgNode: a node in the syntax tree

Make AST like a DOM tree!

Using ast-grep's API is like using jQuery. You can use SgNode to traverse the syntax tree and collect information from the nodes.

Remember your old time web programming?

A common workflow to use ast-grep's JavaScript API is:

1. Get a syntax tree object SgRoot from string by calling a language's parse method
2. Get the root node of the syntax tree by calling ast.root()
3. find relevant nodes by using patterns or rules
4. Collect information from the nodes

Example:

import { parse, Lang } from '@ast-grep/napi';

let source = `console.log("hello world")`
const ast = parse(Lang.JavaScript, source)  // 1. parse the source
const root = ast.root()                     // 2. get the root
const node = root.find('console.log($A)')   // 3. find the node
node.getMatch('A').text()                   // 4. collect the info
// "hello world"

SgRoot

SgRoot represents the syntax tree of a source string.

We can import the Lang enum from the @ast-grep/napi package and call the parse function to transform string.

import { Lang, parse } from '@ast-grep/napi';

const source = `console.log("hello world")`
const ast = parse(Lang.JavaScript, source)

The SgRoot object has a root method that returns the root SgNode of the AST.

const root = ast.root() // root is an instance of SgNode

SgNode

SgNode is the main interface to view and manipulate the syntax tree.

It has several jQuery like methods for us to search, filter and inspect the AST nodes we are interested in.

const log = root.find('console.log($A)') // search node
const arg = log.getMatch('A') // get matched variable
log.text() // "hello world"

Let's see its details in the following sections!

Search

You can use find and findAll to search for nodes in the syntax tree.

• find returns the first node that matches the pattern or rule.
• findAll returns an array of nodes that match the pattern or rule.

// search
class SgNode {
  find(matcher: string): SgNode | null
  find(matcher: number): SgNode | null
  find(matcher: NapiConfig): SgNode | null
  findAll(matcher: string): Array<SgNode>
  findAll(matcher: number): Array<SgNode>
  findAll(matcher: NapiConfig): Array<SgNode>
}

Both find and findAll are overloaded functions. They can accept either string, number or a config object. The argument is called Matcher in ast-grep JS.

Matcher

A Matcher can be one of the three types: string, number or object.

• string is parsed as a pattern. e.g. 'console.log($A)'

• number is interpreted as the node's kind. In tree-sitter, an AST node's type is represented by a number called kind id. Different syntax node has different kind ids. You can convert a kind name like function to the numeric representation by calling the kind function. e.g. kind('function', Lang.JavaScript).

• A NapiConfig has a similar type of config object. See details below.

// basic find example
root.find('console.log($A)')    // returns SgNode of call_expression
let l = Lang.JavaScript         // calling kind function requires Lang
const kind = kind(l, 'string')  // convert kind name to kind id number
root.find(kind)                 // returns SgNode of string
root.find('notExist')           // returns null if not found

// basic find all example
const nodes = root.findAll('function $A($$$) {$$$}')
Array.isArray(nodes)     // true, findAll returns SgNode
nodes.map(n => n.text()) // string array of function source
const empty = root.findAll('not exist') // returns []
empty.length === 0 // true

Note, find returns null if no node is found. findAll returns an empty array if nothing matches.

Match

Once we find a node, we can use the following methods to get meta variables from the search.

The getMatch method returns the single node that matches the single meta variable.

And the getMultipleMatches returns an array of nodes that match the multi meta variable.

// search
export class SgNode {
  getMatch(m: string): SgNode | null
  getMultipleMatches(m: string): Array<SgNode>
}

Example:

const src = `
console.log('hello')
logger('hello', 'world', '!')
`
const root = parse(Lang.JavaScript, src).root()
const node = root.find('console.log($A)')
const arg = node.getMatch("A") // returns SgNode('hello')
arg !== null // true, node is found
arg.text() // returns 'hello'
// returns [] because $A and $$$A are different
node.getMultipleMatches('A')

const logs = root.find('logger($$$ARGS)')
// returns [SgNode('hello'), SgNode(','), SgNode('world'), SgNode(','), SgNode('!')]
logs.getMultipleMatches("ARGS")
logs.getMatch("A") // returns null

Inspection

The following methods are used to inspect the node.

// node inspection
export class SgNode {
  range(): Range
  isLeaf(): boolean
  kind(): string
  text(): string
}

Example:

const ast = parse(Lang.JavaScript, "console.log('hello world')")
root = ast.root()
root.text() // will return "console.log('hello world')"

Another important method is range, which returns two Pos object representing the start and end of the node.

One Pos contains the line, column, and offset of that position. All of them are 0-indexed.

You can use the range information to locate the source and modify the source code.

const rng = node.range()
const pos = rng.start // or rng.end, both are `Pos` objects
pos.line // 0, line starts with 0
pos.column // 0, column starts with 0
rng.end.index // 17, index starts with 0

Refinement

You can also filter nodes after matching by using the following methods.

This is dubbed as "refinement" in the documentation. Note these refinement methods only support using pattern at the moment.

export class SgNode {
  matches(m: string): boolean
  inside(m: string): boolean
  has(m: string): boolean
  precedes(m: string): boolean
  follows(m: string): boolean
}

Example:

const node = root.find('console.log($A)')
node.matches('console.$METHOD($B)') // true

Traversal

You can traverse the tree using the following methods, like using jQuery.

export class SgNode {
  children(): Array<SgNode>
  field(name: string): SgNode | null
  parent(): SgNode | null
  child(nth: number): SgNode | null
  ancestors(): Array<SgNode>
  next(): SgNode | null
  nextAll(): Array<SgNode>
  prev(): SgNode | null
  prevAll(): Array<SgNode>
}

Fix code

SgNode is immutable so it is impossible to change the code directly.

However, SgNode has a replace method to generate an Edit object. You can then use the commitEdits method to apply the changes and generate new source string.

interface Edit {
  /** The start position of the edit */
  startPos: number
  /** The end position of the edit */
  endPos: number
  /** The text to be inserted */
  insertedText: string
}

class SgNode {
  replace(text: string): Edit
  commitEdits(edits: Edit[]): string
}

Example

const root = parse(Lang.JavaScript, "console.log('hello world')").root()
const node = root.find('console.log($A)')
const edit = node.replace("console.error('bye world')")
const newSource = node.commitEdits([edit])
// "console.error('bye world')"

Note, console.error($A) will not generate console.error('hello world') in JavaScript API unlike the CLI. This is because using the host language to generate the replacement string is more flexible.

WARNING

Metavariable will not be replaced in the replace method. You need to create a string using getMatch(var_name) by using JavaScript.

See also ast-grep#1172

Use Other Language

To access other languages, you can use the parse/parseAsync function and the Lang enum.

Example

import { parse, Lang } from '@ast-grep/napi'

const sg = parse(Lang.Python, 'def test(): pass')

console.log(sg.root().has('function_definition'))