pegjs-loader

0.5.8 verified Sat Apr 25 auth: no javascript

A webpack loader that compiles PEG.js grammar files into JavaScript parser modules. Current version 0.5.8 is stable with weekly npm downloads around 1,000. Requires peer dependencies PEG.js ^0.10.0 and webpack >=1. Supports options like cache, optimize, trace, allowedStartRules, and dependencies. Differentiated from alternatives like jison-loader by its tight integration with PEG.js and support for multiple parser options via query parameters.

Common errors

error Error: Cannot find module 'pegjs' ↓

cause pegjs not installed in node_modules

fix

npm install --save-dev pegjs

error Module build failed: Error: Couldn't find preset 'es2015' relative to directory ↓

cause Webpack config requires babel-loader for .js files, but pegjs-loader doesn't use babel

fix

Ensure pegjs-loader is not processed by babel; exclude node_modules or pegjs-loader from babel-loader rule

error TypeError: Cannot read property 'cacheable' of undefined ↓

cause Bug in pegjs-loader v0.2.0

fix

Upgrade pegjs-loader to v0.2.2 or later

error Error: path must be a string ↓

cause Passing non-string options as query parameters

fix

Ensure all loader options are strings (e.g., 'true' instead of true)

Warnings

breaking PEG.js updated to 0.10.0, requiring changes in grammar syntax ↓

fix Update grammars to be compatible with PEG.js 0.10.0. Refer to PEG.js changelog.

deprecated webpack 1 loaders syntax is deprecated; webpack 2+ uses 'rules' instead of 'loaders' ↓

fix Use module.rules: [{ test: /\.pegjs$/, use: 'pegjs-loader' }] in webpack 2+.

gotcha Options must be passed as query parameters; they cannot be set in a separate options object ↓

fix Append options as query string: 'pegjs-loader?cache=true'.

breaking PEG.js 0.9.0 breaking changes (e.g., no more global PEG object) ↓

fix Ensure grammar files are compatible with PEG.js 0.9.0+

Install

npm install pegjs-loader

yarn add pegjs-loader

pnpm add pegjs-loader

Imports

pegjs-loader
wrong
```
import pegjs from 'pegjs-loader'
```
correct
```
module.exports = { module: { loaders: [{ test: /\.pegjs$/, loader: 'pegjs-loader' }] } }
```
This is a webpack loader, not a JavaScript module. It is configured in webpack.config.js, not imported in application code.
parser
wrong
```
const parser = require('pegjs-loader!./parser.pegjs');
```
correct
```
const parser = require('./parser.pegjs');
```
If configured in webpack config, simply require the .pegjs file. Inline loader syntax is also possible but less common.

parser (with options)

wrong

const parser = require('./parser.pegjs?cache=true');

correct

const parser = require('pegjs-loader?cache=true!./parser.pegjs');

Options are passed as query parameters to the loader, not to the grammar file.

Quickstart

Configure webpack to use pegjs-loader on .pegjs files, then require and parse with the compiled parser.

// webpack.config.js
module.exports = {
  entry: './src/index.js',
  output: { filename: 'bundle.js' },
  module: {
    loaders: [
      {
        test: /\.pegjs$/,
        loader: 'pegjs-loader?cache=true&optimize=size'
      }
    ]
  }
};

// src/index.js
const parser = require('./grammar.pegjs');
const result = parser.parse('input string');
console.log(result);