Astravel

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

Key features

• Works on ESTree-compliant ASTs such as the ones produced by Acorn.
• Out-of-the-box functions such as source code comments insertion for Astring.
• No dependencies and small footprint.
• Now also includes a TypeScript example ;)

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

A browser-ready minified version of Astravel is available at dist/astravel.min.js.

Usage

The astravel module exports the following items:

• defaultTraveler
• makeTraveler(properties) ➞ traveler
• attachComments(ast, comments) ➞ ast

astravel.defaultTraveler

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

• go(node, state, opts): Travels through the provided AST node with a given state (an object that can be of any type) by recursively calling this method.
• find(predicate, node, state, opts) ➞ { node, state }?: Returns { node, state, opts } for which predicate(node, state, opts) returns truthy, starting at the specified AST node and with the provided state. Otherwise, returns undefined.
• [NodeType](node, state, opts): Method handler for a specific NodeType.
• makeChild(properties) ➞ traveler: Returns a custom AST traveler that inherits from this traveler with its own provided properties and the property super that points to this traveler.

astravel.makeTraveler(properties) ➞ traveler

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, opts) 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:

var customTraveler = astravel.makeTraveler({
  go: function(node, state, opts) {
    // 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:

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

astravel.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:

• type: "Line" or "Block"
• value: Comment string value
• start: Comment starting character offset number
• end: Comment ending character offset number
• loc: Location object with start and end properties containing one-based line number and zero-based column number properties.

This example shows how to obtain a proper list of comments of a given source code with Acorn and how to attach them on the generated ast:

var comments = [];
var ast = acorn.parse(code, {
   // This ensures that the `loc` property is present on comment objects
   locations: true,
   // Acorn will store the comment objects in this array
   onComment: comments
});
// Attach comments on the ast
astravel.attachComments(ast, comments);

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

• For a given statement, it stores all comments right above it and on the same line to it's right side in a comments property.
• If a comment block is at the beginning of a code block, it is attached to that code block.
• Comments not followed by any statement in a code block are attached as trailingComments to that code block.

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

// Attached to the variable declaration just below
var 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

Building

All building scripts are defined in the package.json file and rely on the Node Package Manager. All commands must be run from within the root repository folder.

Production

Production code can be obtained from the dist folder by running:

npm run build

If you are already using a JavaScript 6 to 5 compiler for your project, or a JavaScript 6 compliant interpreter, you can include the src/astravel.js file directly.

A minified version of Astravel with source maps can be obtained at dist/astravel.min.js by running:

npm run build:minified

Development

If you are working on Astring, you can enable Babel's watch mode to automatically transpile to the dist folder at each update by running:

npm start

Tests

While making changes to Astravel, make sure it passes the tests (it checks code formatting and unit tests):

npm test

Examples

The examples demonstrate using the parser with both typescript-eslint-parser and acorn

Roadmap

Planned features and releases are outlined on the milestones page.