esmod

1.0.0 verified Fri May 01 auth: no javascript

esmod is a fast in-place transpiler for converting ES6 import/export statements to CommonJS require/exports. Version 1.0.0 is currently stable, with a focus on speed by leveraging AST-based transformation rather than regex-only approaches. It distinguishes itself from tools like esbuild or Babel by being designed for lightweight, single-file transformations without module resolution or bundling — ideal for build tool authors and custom pipelines.

Common errors

error TypeError: Cannot read properties of undefined (reading 'code') ↓

cause Calling transform() without waiting for a Promise, but it's actually synchronous and returns an object directly.

fix

Access result.code immediately after calling transform() without await.

error SyntaxError: Unexpected token 'export' ↓

cause Input contains export statements but options.from is not set to 'esm', so transform treats input as CommonJS.

fix

Set { from: 'esm' } in options.

error Error: No transform applied because source and target are the same. ↓

cause Both from and to options are set to 'esm', resulting in no transformation.

fix

Set 'to' to 'cjs' to convert to CommonJS.

Warnings

gotcha transform() throws synchronous errors; do not use try/catch with async error patterns. ↓

fix Wrap in try/catch (not .catch()) since it's not a Promise.

gotcha Input code must contain only valid ES module syntax; mixing require() and import will cause parse errors. ↓

fix Ensure code is pure ES module syntax before passing to transform.

deprecated options.from and options.to are optional; default is ESM-to-ESM (no-op), which may be unexpected. ↓

fix Always specify both targets to avoid accidental no-op.

Install

npm install esmod

yarn add esmod

pnpm add esmod

Imports

transform
wrong
```
const transform = require('esmod').transform
```
correct
```
import { transform } from 'esmod'
```
ESM-only package; CommonJS require() does not work natively unless in a Node ESM context.
transformSync
wrong
```
import transformSync from 'esmod'
```
correct
```
import { transformSync } from 'esmod'
```
transformSync is a named export, not a default export.
default transform
wrong
```
const { transform } = require('esmod')
```
correct
```
import transform from 'esmod'
```
The default export is a shorthand for the transform function.

Quickstart

Converts an ES module file (.mjs) to CommonJS (.cjs) using esmod's transform function with explicit source and target formats.

import { transform } from 'esmod';
import { readFileSync, writeFileSync } from 'fs';

const code = readFileSync('input.mjs', 'utf8');
const result = transform(code, { from: 'esm', to: 'cjs' });
writeFileSync('output.cjs', result.code);
console.log('Transformed successfully!');