acorn-loose

What is acorn-loose?

The acorn-loose npm package is a variant of the Acorn JavaScript parser that is designed to handle syntax errors more gracefully. It allows you to parse JavaScript code that may not be entirely correct, making it useful for applications like code editors, linters, and other tools that need to work with incomplete or incorrect code.

What are acorn-loose's main functionalities?

Parsing JavaScript with Syntax Errors

This feature allows you to parse JavaScript code that contains syntax errors. The acorn-loose parser will attempt to create an Abstract Syntax Tree (AST) even if the code is not entirely correct.

const acornLoose = require('acorn-loose');
const code = 'var a = 1 +';
const ast = acornLoose.parse(code);
console.log(ast);

Handling Incomplete Code

This feature is useful for parsing incomplete code snippets, such as those that might be found in a code editor while the user is still typing.

const acornLoose = require('acorn-loose');
const incompleteCode = 'function test() {';
const ast = acornLoose.parse(incompleteCode);
console.log(ast);

Other packages similar to acorn-loose

esprima

Esprima is a high-performance, standard-compliant ECMAScript parser written in JavaScript. Unlike acorn-loose, Esprima is more strict and does not handle syntax errors as gracefully. It is best suited for applications where the input code is expected to be correct.

espree

Espree is the parser that powers ESLint. It is built on top of Acorn and is designed to be used in linting tools. While it can handle some syntax errors, it is generally more strict compared to acorn-loose.

README

Acorn loose parser

An error-tolerant JavaScript parser written in JavaScript.

This parser will parse any text into an ESTree syntax tree that is a reasonable approximation of what it might mean as a JavaScript program.

It will, to recover from missing brackets, treat whitespace as significant, which has the downside that it might mis-parse a valid but weirdly indented file. It is recommended to always try a parse with the regular acorn parser first, and only fall back to this parser when that one finds syntax errors.

Community

Acorn is open source software released under an MIT license.

You are welcome to report bugs or create pull requests on github.

Installation

The easiest way to install acorn-loose is from npm:

npm install acorn-loose

Alternately, you can download the source and build acorn yourself:

git clone https://github.com/acornjs/acorn.git
cd acorn
npm install

Interface

parse(input, options) takes an input string and a set of options (the same options as acorn takes), and returns a syntax tree, even if the code isn't syntactically valid. It'll insert identifier nodes with name "✖" as placeholders in places where it can't make sense of the input. Depends on the acorn package, because it uses the same tokenizer.

var acornLoose = require("acorn-loose");
console.log(acornLoose.parse("1 / * 4 )[2]", {ecmaVersion: 2020}));

Like the regular parser, the loose parser supports plugins. You can take the LooseParser class exported by the module, and call its static extend method with one or more plugins to get a customized parser class. The class has a static parse method that acts like the top-level parse method.

isDummy(node) takes a Node and returns true if it is a dummy node inserted by the parser. The function performs a simple equality check on the node's name.