prettier-plugin-pegjs

2.0.2 verified Sat Apr 25 auth: no javascript

A Prettier plugin for formatting PEG.js and Peggy grammar files. v2.0.2 is the latest stable, requires Node >= 16 and ships TypeScript definitions. It supports standard Prettier options plus an 'actionParser' to format action code (JavaScript/TypeScript by default, configurable). Unlike generic formatters, it deeply understands PEG syntax, preserving code within actions. Available for browser via standalone bundle.

Common errors

error Error: Cannot find module 'prettier-plugin-pegjs' ↓

cause Plugin not installed or not in node_modules

fix

Run 'npm install --save-dev prettier prettier-plugin-pegjs' or 'yarn add --dev prettier prettier-plugin-pegjs'.

error SyntaxError: Unexpected token (1:1) while parsing PEG grammar ↓

cause The grammar file is not valid PEG.js syntax (e.g., missing header or invalid rule)

fix

Check the grammar against PEG.js specification; ensure initial rule is uppercase.

error TypeError: prettier.format is not a function ↓

cause Using CommonJS require with ESM-only version or importing incorrectly

fix

Use dynamic import: const prettier = await import('prettier'); or migrate to ESM.

Warnings

gotcha The plugin parses PEG.js (original) grammar, not Peggy (fork) — ensure your grammar file uses .pegjs extension or specify parser explicitly ↓

fix Use .pegjs extension or set parser: 'pegjs' in Prettier config

breaking v2.0.0 dropped support for Node < 16, CommonJS via require() fails ↓

fix Upgrade to Node 16+ and use ESM imports (import) or dynamic import()

deprecated Using default export with require('prettier-plugin-pegjs') will return undefined in older versions — always use default import ↓

fix Migrate to import syntax or upgrade to v2 (ESM only)

gotcha actionParser option defaults to 'babel-ts' — may cause parse errors if actions contain other languages ↓

fix Set actionParser to appropriate parser (e.g., 'babel' for JavaScript without TypeScript)

Install

npm install prettier-plugin-pegjs

yarn add prettier-plugin-pegjs

pnpm add prettier-plugin-pegjs

Imports

default

wrong

const prettierPluginPegjs = require('prettier-plugin-pegjs')

correct

import prettierPluginPegjs from 'prettier-plugin-pegjs'

Package is ESM-only. Use dynamic import() for CommonJS projects.

printPrettier
wrong
```
import { printPrettier } from 'prettier-plugin-pegjs'
```
correct
```
import { printPrettier } from 'prettier-plugin-pegjs/standalone'
```
printPrettier is exported from the standalone subpath. Direct import from main entry only exports the Prettier plugin object.

PegjsParser

wrong

const PegjsParser = require('prettier-plugin-pegjs').PegjsParser

correct

import { PegjsParser } from 'prettier-plugin-pegjs'

TypeScript type export; only available for type imports in CJS.

Quickstart

Formats a PEG.js grammar string using Prettier with the plugin, demonstrating usage in a Node.js TypeScript environment.

import prettier from 'prettier';
import prettierPluginPegjs from 'prettier-plugin-pegjs';

const code = `Expression = head:Term tail:(_("+"/"-")_ Term)* { return tail.reduce(function(result, element) { if (element[1] === "+") { return result + element[3]; } if (element[1] === "-") { return result - element[3]; } }, head) }`;

const formatted = await prettier.format(code, {
  parser: 'pegjs',
  plugins: [prettierPluginPegjs],
  tabWidth: 2,
});
console.log(formatted);