esm_prateek

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 npm init esm or yarn 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
• CJS/ESM in an .esmrc.js or .esmrc.mjs file
• JSON6 in an .esmrc or .esmrc.json file
• JSON6 or file path in the ESM_OPTIONS environment variable
• The ESM_DISABLE_CACHE environment variable

{
"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. }
"force":	A boolean to apply these options to all module loads.
"mainFields":	An array of fields, e.g. ["main"], checked 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 cache directory path.
"sourceMap":	A boolean for including inline source maps.
}

Tips

Bundling

• Add a “module” field to package.json with the path to the main ES module.

  This is automagically done with npm init esm or yarn create esm.

• Use esmify with browserify.

Extensions

• Enable ESM syntax for wallaby.js following their integration example.

Loading

• The jasmine test runner does not have a mechanism to load esm. However, esm can load a bootstrap file that programmaticly runs tests following their library usage example.

• Load esm before loaders/monitors like @babel/register, newrelic, sqreen, and ts-node.

• Load esm with the “node-args” options of
  

  • node-tap: --node-arg=-r --node-arg=esm
  • pm2: --node-args="-r esm"

• Load esm with “require” options of ava, mocha, nodemon, nyc, qunit, tape, and webpack.

  By Node’s rules, builtin require cannot sideload .mjs files. However, with esm, ES modules can be sideloaded as .js files or .mjs files may be loaded with dynamic import.