feat: proxy request to esm.sh and convert to systemjs

deno.jsonc

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+{
+  "importMap": "./importmap.json", 
+  "tasks": {
+    "dev": "deno run --watch src/main.ts",
+    "test": "deno test --allow-env --allow-read"
+  }
+}

importmap.json

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+{
+  "imports": {
+    "@rollup/plugin-virtual": "https://esm.sh/@rollup/plugin-virtual",
+    "rollup": "https://esm.sh/rollup",
+    "testing/asserts": "https://deno.land/std/testing/asserts.ts",
+    "testing/mock": "https://deno.land/std/testing/mock.ts",
+    "http": "https://deno.land/std/http/mod.ts"
+  }
+}

src/esm-proxy-request-handler.test.ts

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+import { assertEquals } from 'testing/asserts';
+import { returnsNext, stub } from 'testing/mock';
+import { esmProxyRequestHandler } from './esm-proxy-request-handler.ts';
+
+Deno.test('should return an string of code in systemjs format', async () => {
+  const fetchStub = stub(globalThis, 'fetch', returnsNext([
+      Promise.resolve({ text: () => Promise.resolve('export * from "https://esm.sh/stable/vue@3.3.2/es2022/vue.mjs";') } as unknown as Response),
+  ]));
+  const req = new Request('https://esm.sh/vue');
+  const res = await esmProxyRequestHandler(req);
+  const systemjsCode = await res.text();
+  assertEquals(systemjsCode.startsWith('System.register('), true);
+  fetchStub.restore();
+});

src/esm-proxy-request-handler.ts

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+import { toSystemjs } from './to-systemjs.ts';
+
+export async function esmProxyRequestHandler(req: Request) {
+  const modifiedUrl = new URL(req.url, 'https://esm.sh');
+  const esmCode = await fetch(modifiedUrl.toString(), { headers: req.headers }).then((res) => res.text());
+  const systemjsCode = await toSystemjs(esmCode);
+  return new Response(systemjsCode);
+}

src/main.ts

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+import { serve } from 'http';
+import { esmProxyRequestHandler } from './esm-proxy-request-handler.ts';
+
+serve(esmProxyRequestHandler, { port: 8000 });

src/main_bench.ts

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+import { add } from "./main.ts";
+
+Deno.bench(function addSmall() {
+  add(1, 2);
+});
+
+Deno.bench(function addBig() {
+  add(2 ** 32, 2 ** 32);
+});

src/to-systemjs.ts

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+import { ModuleFormat, rollup } from 'rollup';
+import virtual from '@rollup/plugin-virtual';
+//import { minify } from 'npm:rollup-plugin-esbuild';
+
+export const toSystemjs = async (esmCode: string): Promise<string>  => {
+
+  const inputOptions = {
+    external: () => true,
+    input: 'esmCode',
+    plugins: [
+      virtual({ esmCode }),
+      //minify(),
+    ],
+    treeshake: false,
+  };
+
+  const outputOptions = {
+    file: 'out.mjs',
+    format: 'systemjs' as ModuleFormat,
+    sourcemap: false,
+  };
+
+  const bundle = await rollup(inputOptions);
+  const { output } = await bundle.generate(outputOptions);
+  await bundle.close();
+  return output[0].code.replace(/https:\/\/esm\.sh\//g, 'https://systemjs.sh/');
+}