5.6.10 • Public • Published

Parse a function

parse-function npm version License Libera Manifesto

Parse a function into an object using espree, acorn or babylon parsers. Extensible through Smart Plugins

Please consider following this project's author, Charlike Mike Reagent, and ⭐️ the project to show your ❤️ and support.

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If you have any how-to kind of questions, please read the Contributing Guide and Code of Conduct documents. For bugs reports and feature requests, please create an issue or ping @tunnckoCore at Twitter.

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Project is semantically versioned & automatically released from GitHub Actions with Lerna.

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  • Always up-to-date: auto-publish new version when new version of dependency is out, Renovate
  • Standard: using StandardJS, Prettier, SemVer, Semantic Release and conventional commits
  • Smart Plugins: for extending the core API or the end Result, see .use method and Plugins Architecture
  • Extensible: using plugins for working directly on AST nodes, see the Plugins Architecture
  • ES2020+ Ready: by using .parseExpression method of the Babel v7.x parser
  • Customization: allows switching the parser, through options.parse
  • Support for: arrow functions, default parameters, generators and async/await functions
  • Stable: battle-tested in production and against all parsers - espree, acorn, @babel/parser
  • Tested: with 450+ tests for 200% coverage

Table of Contents

(TOC generated by verb using markdown-toc)


This project requires Node.js >=8.11 (see Support & Release Policy). Install it using yarn or npm.
We highly recommend to use Yarn when you think to contribute to this project.

$ yarn add parse-function

Which version to use?

There's no breaking changes between the v2.x version. The only breaking is v2.1 which also is not working properly, so no use it.

Use v2.0.x

When you don't need support for arrow functions and es6 default params. This version uses a RegExp expression to work.

Use v2.2.x

Only when you need a basic support for es6 features like arrow functions. This version uses a RegExp expression to work.

Use v2.3.x

When you want full* support for arrow functions and es6 default params. Where this "full", means "almost full", because it has bugs. This version also uses (acorn.parse) real parser to do the parsing.

Use v3.x

When you want to use different parser instead of the default babylon.parse, by passing custom parse function to the options.parse option. From this version we require node >= 4.

Use v4.x

When you want full customization and most stable support for old and modern features. This version uses babylon.parseExpression for parsing and provides a Plugins API. See the Features section for more info.

Use v5.x

It is basically the same as v4, but requires Node 6 & npm 5. Another is boilerplate stuff.

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Throws in one specific case

see: issue #3 and test/index.js#L229-L235

It may throw in one specific case, otherwise it won't throw, so you should relay on the result.isValid for sure.

Function named "anonymous"

see: test/index.js#L319-L324 and Result section

If you pass a function which is named "anonymous" the will be 'anonymous', but the result.isAnonymous will be false and result.isNamed will be true, because in fact it's a named function.

Real anonymous function

see: test/index.js#L326-L331 and Result section

Only if you pass really an anonymous function you will get equal to null, result.isAnonymous equal to true and result.isNamed equal to false.

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Plugins Architecture

see: the .use method, test/index.js#L305-L317 and test/index.js#L396-L414

A more human description of the plugin mechanism. Plugins are synchronous - no support and no need for async plugins here, but notice that you can do that manually, because that exact architecture.

The first function that is passed to the .use method is used for extending the core API, for example adding a new method to the app instance. That function is immediately invoked.

const parseFunction = require('parse-function');
const app = parseFunction();
app.use((self) => {
  // self is same as `app`
  self.define(self, 'foo', (bar) => bar + 1);
console.log(; // => 3

On the other side, if you want to access the AST of the parser, you should return a function from that plugin, which function is passed with (node, result) signature.

This function is lazy plugin, it is called only when the .parse method is called.

const parseFunction = require('parse-function');
const app = parseFunction();
app.use((self) => {
  console.log('immediately called');
  return (node, result) => {
    console.log('called only when .parse is invoked');

Where 1) the node argument is an object - actual and real AST Node coming from the parser and 2) the result is an object too - the end Result, on which you can add more properties if you want.

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Generated using jest-runner-docs.


Initializes with optional opts object which is passed directly to the desired parser and returns an object with .use and .parse methods. The default parse which is used is babylon's .parseExpression method from v7.


function(opts = {})


  • opts {object} - optional, merged with options passed to .parse method
  • returns {object} - app object with .use and .parse methods


const parseFunction = require('parse-function');
const app = parseFunction({
  ecmaVersion: 2017,
const fixtureFn = (a, b, c) => {
  a = b + c;
  return a + 2;
const result = app.parse(fixtureFn);
// see more
console.log(; // => null
console.log(result.isNamed); // => false
console.log(result.isArrow); // => true
console.log(result.isAnonymous); // => true
// array of names of the arguments
console.log(result.args); // => ['a', 'b', 'c']
// comma-separated names of the arguments
console.log(result.params); // => 'a, b, c'


Parse a given code and returns a result object with useful properties - such as name, body and args. By default it uses Babylon parser, but you can switch it by passing options.parse - for example options.parse: acorn.parse. In the below example will show how to use acorn parser, instead of the default one.


  • code {Function|string} - any kind of function or string to be parsed
  • options {object} - directly passed to the parser babylon, acorn, espree
  • options.parse {Function} - by default babylon.parseExpression, all options are passed as second argument
  • returns {object} - result see result section for more info


const acorn = require('acorn');
const parseFn = require('parse-function');
const app = parseFn();
const fn = function foo(bar, baz) {
  return bar * baz;
const result = app.parse(fn, {
  parse: acorn.parse,
  ecmaVersion: 2017,
console.log(; // => 'foo'
console.log(result.args); // => ['bar', 'baz']
console.log(result.body); // => ' return bar * baz '
console.log(result.isNamed); // => true
console.log(result.isArrow); // => false
console.log(result.isAnonymous); // => false
console.log(result.isGenerator); // => false


Add a plugin fn function for extending the API or working on the AST nodes. The fn is immediately invoked and passed with app argument which is instance of parseFunction() call. That fn may return another function that accepts (node, result) signature, where node is an AST node and result is an object which will be returned result from the .parse method. This retuned function is called on each node only when .parse method is called.


  • fn {Function} - plugin to be called
  • returns {object} - app instance for chaining

See Plugins Architecture section.


// plugin extending the `app`
app.use((app) => {
  app.define(app, 'hello', (place) => `Hello ${place}!`);
const hi = app.hello('World');
console.log(hi); // => 'Hello World!'
// or plugin that works on AST nodes
app.use((app) => (node, result) => {
  if (node.type === 'ArrowFunctionExpression') {
    result.thatIsArrow = true;
  return result;
const result = app.parse((a, b) => a + b + 123);
console.log(; // => null
console.log(result.isArrow); // => true
console.log(result.thatIsArrow); // => true
const result = app.parse(function foo() {
  return 123;
console.log(; // => 'foo'
console.log(result.isArrow); // => false
console.log(result.thatIsArrow); // => undefined


Define a non-enumerable property on an object. Just a convenience mirror of the define-property library, so check out its docs. Useful to be used in plugins.


  • obj {object} - the object on which to define the property
  • prop {string} - the name of the property to be defined or modified
  • val {any} - the descriptor for the property being defined or modified
  • returns {object} - obj the passed object, but modified


const parseFunction = require('parse-function');
const app = parseFunction();
// use it like `define-property` lib
const obj = {};
app.define(obj, 'hi', 'world');
console.log(obj); // => { hi: 'world' }
// or define a custom plugin that adds `.foo` property
// to the end result, returned from `app.parse`
app.use((app) => {
  return (node, result) => {
    // this function is called
    // only when `.parse` is called
    app.define(result, 'foo', 123);
    return result;
// fixture function to be parsed
const asyncFn = async (qux) => {
  const bar = await Promise.resolve(qux);
  return bar;
const result = app.parse(asyncFn);
console.log(; // => null
console.log(; // => 123
console.log(result.args); // => ['qux']
console.log(result.isAsync); // => true
console.log(result.isArrow); // => true
console.log(result.isNamed); // => false
console.log(result.isAnonymous); // => true

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Guides and Community

Please read the Contributing Guide and Code of Conduct documents for advices.

For bug reports and feature requests, please join our community forum and open a thread there with prefixing the title of the thread with the name of the project if there's no separate channel for it.

Consider reading the Support and Release Policy guide if you are interested in what are the supported Node.js versions and how we proceed. In short, we support latest two even-numbered Node.js release lines.

Support the project

Become a Partner or Sponsor? 💵 Check the OpenSource Commision (tier). 🎉 You can get your company logo, link & name on this file. It's also rendered on package's page in and sites too! 🚀

Not financial support? Okey! Pull requests, stars and all kind of contributions are always welcome. ✨


This project follows the all-contributors specification. Contributions of any kind are welcome!

Thanks goes to these wonderful people (emoji key), consider showing your support to them:

Charlike Mike Reagent

🚇 💻 📖 🤔 🚧 ⚠️

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Copyright (c) 2016-present, Charlike Mike Reagent <> & contributors.
Released under the MPL-2.0 License.

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