Acorn
A tiny, fast JavaScript parser, written completely in JavaScript.
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.
Packages
This repository holds three packages:
- acorn: The
main parser - acorn-loose: The
error-tolerant parser - acorn-walk: The
syntax tree walker
To build the content of the repository, run npm install.
git clone https://github.com/acornjs/acorn.git
cd acorn
npm install
Plugin developments
Acorn is designed to support plugins which can, within reasonable
bounds, redefine the way the parser works. Plugins can add new token
types and new tokenizer contexts (if necessary), and extend methods in
the parser object. This is not a clean, elegant API—using it requires
an understanding of Acorn's internals, and plugins are likely to break
whenever those internals are significantly changed. But still, it is
possible, in this way, to create parsers for JavaScript dialects
without forking all of Acorn. And in principle it is even possible to
combine such plugins, so that if you have, for example, a plugin for
parsing types and a plugin for parsing JSX-style XML literals, you
could load them both and parse code with both JSX tags and types.
A plugin is a function from a parser class to an extended parser
class. Plugins can be used by simply applying them to the Parser
class (or a version of that already extended by another plugin). But
because that gets a little awkward, syntactically, when you are using
multiple plugins, the static method Parser.extend can be called with
any number of plugin values as arguments to create a Parser class
extended by all those plugins. You'll usually want to create such an
extended class only once, and then repeatedly call parse on it, to
avoid needlessly confusing the JavaScript engine's optimizer.
const {Parser} = require("acorn")
const MyParser = Parser.extend(
require("acorn-jsx")(),
require("acorn-bigint")
)
console.log(MyParser.parse("// Some bigint + JSX code"))
Plugins override methods in their new parser class to implement
additional functionality. It is recommended for a plugin package to
export its plugin function as its default value or, if it takes
configuration parameters, to export a constructor function that
creates the plugin function.
This is what a trivial plugin, which adds a bit of code to the
readToken method, might look like:
module.exports = function noisyReadToken(Parser) {
return class extends Parser {
readToken(code) {
console.log("Reading a token!")
super.readToken(code)
}
}
}