JavaScript Runtimes & ESM

Node.js has evolved from experimental ESM support to making it fully production-ready. Key milestones:

// package.json - enable ESM for all .js files
{ "type": "module" }

// Node.js-specific import.meta properties
console.log(import.meta.url);       // file:///path/to/file.js
console.log(import.meta.dirname);   // /path/to (Node 21.2+)
console.log(import.meta.filename);  // /path/to/file.js (Node 21.2+)
console.log(import.meta.resolve('./other.js')); // Resolve specifier

// require(esm) - CJS can now load ESM (Node 23+)
// In a .cjs file:
const { add } = require('./math.mjs'); // Works!

Deno was built ESM-first from day one. Version 2.x adds full Node.js and npm backwards compatibility while keeping the ESM-native experience.

// Deno uses ESM natively - no configuration needed
import { serve } from "https://deno.land/std/http/server.ts";

// npm packages via npm: specifier
import express from "npm:express@5";

// JSR registry (ESM-native package registry)
import { parse } from "jsr:@std/csv";

// deno.json - project configuration with import maps
{
  "imports": {
    "lodash": "npm:[email protected]",
    "@std/": "jsr:@std/"
  },
  "tasks": {
    "dev": "deno run --watch main.ts",
    "test": "deno test"
  }
}

// Deno-specific features
console.log(import.meta.url);    // file:///path/to/file.ts
console.log(import.meta.main);   // true if entry point
console.log(import.meta.dirname); // Available in Deno 2.x

Bun handles ESM and CommonJS interchangeably in any file without requiring extensions or configuration. Its Zig-based runtime focuses on speed.

// Bun treats all files as modules by default
// ESM and CJS work interchangeably
import express from 'express';     // ESM import of CJS package
const fs = require('node:fs');     // CJS require in ESM file
// Both work in the same file!

// Bun's built-in bundler with ESM output
// bunfig.toml
[build]
entrypoints = ["./src/index.ts"]
outdir = "./dist"
format = "esm"

// Bun 1.3: ESM bytecode compilation
// bun build --bytecode --format=esm ./src/index.ts

// Built-in TypeScript and JSX support
// No tsconfig needed - just works
import { Component } from './MyComponent.tsx';

Edge runtimes run JavaScript at CDN edge locations for low-latency responses. They all use ES Module format as their standard.

// Cloudflare Workers (ESM format)
export default {
  async fetch(request, env) {
    const url = new URL(request.url);
    return new Response('Hello from the edge!', {
      headers: { 'content-type': 'text/plain' }
    });
  }
};

// Vercel Edge Functions
export const config = { runtime: 'edge' };
export default async function handler(request) {
  return new Response('Hello from Vercel Edge');
}

// Netlify Edge Functions
export default async (request, context) => {
  return new Response('Hello from Netlify Edge');
};