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Astravel

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👟 A tiny and fast ESTree-compliant AST walker and modifier.

Key features

Installation

Install with the Node Package Manager:

npm install astravel

Alternatively, checkout this repository and install the development dependencies to build the module file:

git clone https://github.com/davidbonnet/astravel.git
cd astravel
npm install

Usage

The astravel module exports the following items:

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defaultTraveler

⬅️ traveler

⚠️ Deprecated in favor of ES6 class notation.

This object describes a basic AST traveler. It contains the following methods:

makeTraveler()

➡️ (properties) ⬅️ traveler

⚠️ Deprecated in favor of ES6 class notation.

This function is similar to astravel.defaultTraveler.makeChild: it returns a traveler that inherits from the defaultTraveler with its own provided properties and the property super that points to the defaultTraveler object. These properties should redefine the traveler's behavior by implementing the go(node, state) method and/or any node handler.

When redefining the go method, make sure its basic functionality is kept by calling the parent's go method to keep traveling through the AST:

const customTraveler = makeTraveler({
  go: function (node, state) {
    // Code before entering the node
    console.log('Entering ' + node.type)
    // Call the parent's `go` method
    this.super.go.call(this, node, state)
    // Code after leaving the node
    console.log('Leaving ' + node.type)
  },
})

To skip specific node types, the most effective way is to replace the corresponding node handlers with a function that does nothing:

import { makeTraveler } from 'astravel'

const ignore = Function.prototype
const customTraveler = makeTraveler({
  FunctionDeclaration: ignore,
  FunctionExpression: ignore,
  ArrowFunctionExpression: ignore,
})

attachComments()

➡️ (ast, comments) ⬅️ ast

This function attaches a list of comments to the corresponding nodes of a provided ast and returns that same ast. The ast is modified in-place and only the nodes getting comments are augmented with a comments and/or a trailingComments array property.

Each comment should be an object with the following properties:

The following examples show how to obtain a proper list of comments of a given source code and how to attach them on the generated ast:

Usage with Meriyah
import { parse } from 'meriyah'
import { attachComments } from 'astravel'

const comments = []
const ast = parse(code, {
  // Comments are stored in this array
  onComment: comments,
})
// Attach comments on the AST
attachComments(ast, comments)
Usage with Acorn
import { parse } from 'acorn'
import { attachComments } from 'astravel'

const comments = []
const ast = parse(code, {
  // This ensures that the `loc` property is present on comment objects
  locations: true,
  onComment: comments,
})
attachComments(ast, comments)

The algorithm assumes that comments are not put in exotic places, such as in-between function arguments, and proceeds as follows:

In this example, the comments tell to which statement they are attached:

// Attached to the variable declaration just below
const point = {
  // Attached to the property definition just below
  x: 0,
  y: 0, // Attached to the property definition on its left
}
/*
Attached to the function declaration just below.
*/
function add(a, b) {
  /*
   Attached to the function body because it is the first comment block.
   */
  return a + b // Attached to the return statement on its left
  // Trailing comment attached as such to the function body
}
// Trailing comment attached as such to the program body