Web Tree-sitter

WebAssembly bindings to the Tree-sitter parsing library.

Setup

You can download the web-tree-sitter.js and web-tree-sitter.wasm files from the latest GitHub release and load them using a standalone script:

<script src="/the/path/to/web-tree-sitter.js"></script>

<script>
  const { Parser } = window.TreeSitter;
  Parser.init().then(() => { /* the library is ready */ });
</script>

You can also install the web-tree-sitter module from NPM and load it using a system like Webpack:

const { Parser } = require('web-tree-sitter');
Parser.init().then(() => { /* the library is ready */ });

or Vite:

import { Parser }  from 'web-tree-sitter';
Parser.init().then(() => { /* the library is ready */ });

With Vite, you also need to make sure your server provides the tree-sitter.wasm file to your public directory. You can do this automatically with a postinstall script in your package.json:

"postinstall": "cp node_modules/web-tree-sitter/tree-sitter.wasm public"

You can also use this module with deno:

import { Parser } from "npm:web-tree-sitter";
await Parser.init();
// the library is ready

To use the debug version of the library, replace your import of web-tree-sitter with web-tree-sitter/debug:

import { Parser } from 'web-tree-sitter/debug'; // or require('web-tree-sitter/debug')

Parser.init().then(() => { /* the library is ready */ });

This will load the debug version of the .js and .wasm file, which includes debug symbols and assertions.

Note

The web-tree-sitter.js file on GH releases is an ES6 module. If you are interested in using a pure CommonJS library, such as for Electron, you should use the web-tree-sitter.cjs file instead.

Basic Usage

First, create a parser:

const parser = new Parser();

Then assign a language to the parser. Tree-sitter languages are packaged as individual .wasm files (more on this below):

const { Language } = require('web-tree-sitter');
const JavaScript = await Language.load('/path/to/tree-sitter-javascript.wasm');
parser.setLanguage(JavaScript);

Now you can parse source code:

const sourceCode = 'let x = 1; console.log(x);';
const tree = parser.parse(sourceCode);

and inspect the syntax tree.

console.log(tree.rootNode.toString());

// (program
//   (lexical_declaration
//     (variable_declarator (identifier) (number)))
//   (expression_statement
//     (call_expression
//       (member_expression (identifier) (property_identifier))
//       (arguments (identifier)))))

const callExpression = tree.rootNode.child(1).firstChild;
console.log(callExpression);

// { type: 'call_expression',
//   startPosition: {row: 0, column: 16},
//   endPosition: {row: 0, column: 30},
//   startIndex: 0,
//   endIndex: 30 }

Editing

If your source code changes, you can update the syntax tree. This will take less time than the first parse.

// Replace 'let' with 'const'
const newSourceCode = 'const x = 1; console.log(x);';

tree.edit({
  startIndex: 0,
  oldEndIndex: 3,
  newEndIndex: 5,
  startPosition: {row: 0, column: 0},
  oldEndPosition: {row: 0, column: 3},
  newEndPosition: {row: 0, column: 5},
});

const newTree = parser.parse(newSourceCode, tree);

Parsing Text From a Custom Data Structure

If your text is stored in a data structure other than a single string, you can parse it by supplying a callback to parse instead of a string:

const sourceLines = [
  'let x = 1;',
  'console.log(x);'
];

const tree = parser.parse((index, position) => {
  let line = sourceLines[position.row];
  if (line) return line.slice(position.column);
});

Getting the `.wasm` language files

There are several options on how to get the .wasm files for the languages you want to parse.

From npmjs.com

The recommended way is to just install the package from npm. For example, to parse JavaScript, you can install the tree-sitter-javascript package:

npm install tree-sitter-javascript

Then you can find the .wasm file in the node_modules/tree-sitter-javascript directory.

From GitHub

You can also download the .wasm files from GitHub releases, so long as the repository uses our reusable workflow to publish them. For example, you can download the JavaScript .wasm file from the tree-sitter-javascript releases page.

Generating `.wasm` files

You can also generate the .wasm file for your desired grammar. Shown below is an example of how to generate the .wasm file for the JavaScript grammar.

IMPORTANT: Emscripten, Docker, or Podman need to be installed.

First install tree-sitter-cli, and the tree-sitter language for which to generate .wasm (tree-sitter-javascript in this example):

npm install --save-dev tree-sitter-cli tree-sitter-javascript

Then just use tree-sitter cli tool to generate the .wasm.

npx tree-sitter build --wasm node_modules/tree-sitter-javascript

If everything is fine, file tree-sitter-javascript.wasm should be generated in current directory.

Running .wasm in Node.js

Notice that executing .wasm files in Node.js is considerably slower than running Node.js bindings. However, this could be useful for testing purposes:

const Parser = require('web-tree-sitter');

(async () => {
  await Parser.init();
  const parser = new Parser();
  const Lang = await Parser.Language.load('tree-sitter-javascript.wasm');
  parser.setLanguage(Lang);
  const tree = parser.parse('let x = 1;');
  console.log(tree.rootNode.toString());
})();

Running .wasm in browser

web-tree-sitter can run in the browser, but there are some common pitfalls.

Loading the .wasm file

web-tree-sitter needs to load the tree-sitter.wasm file. By default, it assumes that this file is available in the same path as the JavaScript code. Therefore, if the code is being served from http://localhost:3000/bundle.js, then the Wasm file should be at http://localhost:3000/tree-sitter.wasm.

For server side frameworks like NextJS, this can be tricky as pages are often served from a path such as http://localhost:3000/_next/static/chunks/pages/index.js. The loader will therefore look for the Wasm file at http://localhost:3000/_next/static/chunks/pages/tree-sitter.wasm. The solution is to pass a locateFile function in the moduleOptions argument to Parser.init():

await Parser.init({
  locateFile(scriptName: string, scriptDirectory: string) {
    return scriptName;
  },
});

locateFile takes in two parameters, scriptName, i.e. the Wasm file name, and scriptDirectory, i.e. the directory where the loader expects the script to be. It returns the path where the loader will look for the Wasm file. In the NextJS case, we want to return just the scriptName so that the loader will look at http://localhost:3000/tree-sitter.wasm and not http://localhost:3000/_next/static/chunks/pages/tree-sitter.wasm.

For more information on the module options you can pass in, see the emscripten documentation.

"Can't resolve 'fs' in 'node_modules/web-tree-sitter"

Most bundlers will notice that the web-tree-sitter.js file is attempting to import fs, i.e. node's file system library. Since this doesn't exist in the browser, the bundlers will get confused. For Webpack, you can fix this by adding the following to your webpack config:

{
  resolve: {
    fallback: {
      fs: false
    }
  }
}

Uh oh!