@lezer/javascript

What is @lezer/javascript?

@lezer/javascript is a parser for JavaScript written using the Lezer parser system. It provides a way to parse JavaScript code into a syntax tree, which can be used for various purposes such as syntax highlighting, code analysis, and transformation.

What are @lezer/javascript's main functionalities?

Parsing JavaScript Code

This feature allows you to parse JavaScript code into a syntax tree. The code sample demonstrates how to parse a simple JavaScript statement and print the resulting syntax tree.

const {parser} = require('@lezer/javascript');
const input = 'const x = 10;';
const tree = parser.parse(input);
console.log(tree.toString());

Syntax Tree Traversal

This feature allows you to traverse the syntax tree. The code sample shows how to use a TreeCursor to iterate over the nodes in the syntax tree and print their types.

const {parser} = require('@lezer/javascript');
const {TreeCursor} = require('@lezer/common');
const input = 'const x = 10;';
const tree = parser.parse(input);
let cursor = tree.cursor();
do {
  console.log(cursor.node.type.name);
} while (cursor.next());

Custom Syntax Highlighting

This feature allows you to apply custom syntax highlighting to JavaScript code. The code sample demonstrates how to define a highlighting style and apply it to a parsed syntax tree.

const {parser} = require('@lezer/javascript');
const {highlightTree} = require('@lezer/highlight');
const {styleTags, tags} = require('@lezer/highlight');
const input = 'const x = 10;';
const tree = parser.parse(input);
const highlightStyle = styleTags({
  'VariableName': tags.variableName,
  'Keyword': tags.keyword,
  'Number': tags.number
});
let highlighted = '';
highlightTree(tree, highlightStyle, (from, to, classes) => {
  highlighted += input.slice(from, to) + ' [' + classes + '] ';
});
console.log(highlighted);

Other packages similar to @lezer/javascript

acorn

Acorn is a small, fast, JavaScript-based JavaScript parser. It is designed to be modular and can be extended with plugins. Compared to @lezer/javascript, Acorn is more widely used and has a larger community, but it may not integrate as seamlessly with the Lezer ecosystem.

esprima

Esprima is a high-performance, standard-compliant ECMAScript parser written in JavaScript. It is known for its speed and accuracy. While Esprima is similar to @lezer/javascript in terms of functionality, it is more mature and has been used in many production environments.

README

@lezer/javascript

This is a JavaScript grammar for the lezer parser system.

It parses modern JavaScript, and supports a "ts" dialect to parse TypeScript, and a "jsx" dialect to parse JSX.

The top option can be set to "SingleExpression" or "SingleClassItem" to parse an expression or class item instead of a full program.

The code is licensed under an MIT license.

Changelog

1.4.21 (2024-12-03)

Bug fixes

Add support for const modifiers on TypeScript type parameters.

Allow TypeScript syntax, where the condition is just a variable.

Fix a bug where some TypeScript < tokens didn't appear in the syntax tree.