rektide / esm

Tomorrow's ECMAScript modules today!

esm

A fast, production ready, zero-dependency ES module loader for Node 6+!

See the release post 📖 and video 🎥 for all the details.

Install

• New projects

  Run npm init esm or yarn create esm.

  💡 Use the -y flag to answer “yes” to all prompts.

• Existing projects

  Run npm i esm or yarn add esm.

Getting started

There are two ways to enable esm.

1. Enable esm for packages:

   Use esm to load the main ES module and export it as CommonJS.

   index.js

   // Set options as a parameter, environment variable, or rc file.
   require = require("esm")(module/*, options*/)
   module.exports = require("./main.js")

   main.js

   // ESM syntax is supported.
   export {}

   💡 These files are automagically created with create-esm.

2. Enable esm for local runs:

   node -r esm main.js

   💡 Omit the filename to enable esm in the REPL.

Features

The esm loader bridges the ESM of today to the ESM of tomorrow.

👏 By default, 💯 percent CJS interoperability is enabled so you can get stuff done fast.
🔒 .mjs files are limited to basic functionality without support for esm options.

Out of the box esm just works, no configuration necessary, and supports:

• import / export
• import.meta
• Dynamic import
• Improved errors
• Live bindings
• Loading .mjs files as ESM
• The file URI scheme
• Node --eval and --print flags
• Node --check flag (Node 10+)

Options

Specify options with one of the following:

• The "esm" field in package.json
• JSON6 in an .esmrc or .esmrc.json file
• JSON6 or file path in the ESM_OPTIONS environment variable
• CJS/ESM in an .esmrc.js or .esmrc.mjs file

{
"await":	A boolean for top-level await in modules without ESM exports.
"cjs":	A boolean or object for toggling CJS features in ESM. Features { "cache": A boolean for storing ES modules in require.cache. "extensions": A boolean for respecting require.extensions in ESM. "interop": A boolean for __esModule interoperability. "mutableNamespace": A boolean for mutable namespace objects. "namedExports": A boolean for importing named exports of CJS modules. "paths": A boolean for following CJS path rules in ESM. "vars": A boolean for __dirname, __filename, and require in ESM. }
"mainFields":	An array of fields, e.g. ["main"], to check when importing a package.
"mode":	A string mode: "auto" detect files with import, import.meta, export, "use module", or .mjs as ESM "all" script files are treated as ESM "strict" to treat only .mjs files as ESM
}

DevOpts

{
"cache":	A boolean for toggling cache creation or string path of the cache directory.
"debug":	A boolean for unmasking methods and stack traces.
"sourceMap":	A boolean for including inline source maps.
"warnings":	A boolean for logging development parse and runtime warnings.
}

Tips

• Load esm with the “require” option of
  ava, mocha, nodemon, nyc, qunit, tape, ts-node, or webpack
• Load esm before @babel/register, newrelic, or sqreen
• Load esm with the --node-arg=-r --node-arg=esm option of node-tap
• Load esm with the --node-args="-r esm" option of pm2
• Load esm with wallaby.js
• Use esm to load jasmine