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Feat/fullstack esm #6

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c69a289
feat: add Node builtin shims and improve external dependency resoluti…
RiyaSharma03 Mar 19, 2026
4342820
feat: preserve React context identity across HMR re-executions
RiyaSharma03 Mar 26, 2026
33f25c2
fix: improve CORS headers and unwrap ESM default exports in almostesm
RiyaSharma03 Mar 27, 2026
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79 changes: 59 additions & 20 deletions almostesm/src/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,10 @@ fs.mkdirSync(CACHE_DIR, { recursive: true });
// CORS for browser access
app.use((req: Request, res: Response, next: NextFunction) => {
res.header("Access-Control-Allow-Origin", "*");
res.header("Access-Control-Expose-Headers", "X-Externals");
res.header("Access-Control-Allow-Methods", "GET, POST, OPTIONS");
res.header("Access-Control-Allow-Headers", "Content-Type");
res.header("Access-Control-Expose-Headers", "X-Externals, X-Resolved-Version");
if (req.method === "OPTIONS") { res.sendStatus(204); return; }
next();
});

Expand Down Expand Up @@ -124,6 +127,21 @@ async function handlePkgRequest(res: Response, pkgName: string, version: string,
`module.exports = require("${requireSpecifier}");\n`
);

// Write a shim file for Node built-ins and build an alias map.
// RN packages sometimes import Node APIs (buffer, stream, etc.) for
// native code paths that are dead code in the browser bundle.
const shimFile = path.join(tmpDir, "__node-shim.js");
fs.writeFileSync(shimFile, "module.exports = {};");
const nodeBuiltinAliases: Record<string, string> = {};
for (const name of [
"buffer", "stream", "fs", "path", "os", "crypto", "util", "events",
"http", "https", "net", "tls", "zlib", "child_process", "worker_threads",
"url", "querystring", "string_decoder", "assert", "tty", "domain",
]) {
nodeBuiltinAliases[name] = shimFile;
nodeBuiltinAliases[`node:${name}`] = shimFile;
}

// Externalize bare package imports (e.g. "react") so shared deps are
// loaded once. For subpath imports (e.g. "css-in-js-utils/lib/foo"),
// generally inline them since they're internal implementation details.
Expand Down Expand Up @@ -173,30 +191,43 @@ async function handlePkgRequest(res: Response, pkgName: string, version: string,
} else {
pkg = args.path.split("/")[0];
}
if (!externalSet.has(pkg)) return null;

// Track installed version for the base package
if (!externalizedMap[pkg]) {
const version = getInstalledVersion(pkg);
if (version) externalizedMap[pkg] = version;
}

// Bare import: always externalize
if (args.path === pkg) {
return { path: pkg, external: true };
}

// Subpath import: for version-sensitive packages (react, react-dom,
// react-native), always externalize to avoid inlining mismatched
// versions. For other packages, try to resolve locally and inline.
if (alwaysExternalSubpaths.has(pkg)) {
return { path: args.path, external: true };
if (externalSet.has(pkg)) {
// Track installed version for the base package
if (!externalizedMap[pkg]) {
const version = getInstalledVersion(pkg);
if (version) externalizedMap[pkg] = version;
}

// Bare import: always externalize
if (args.path === pkg) {
return { path: pkg, external: true };
}

// Subpath import: for version-sensitive packages (react, react-dom,
// react-native), always externalize to avoid inlining mismatched
// versions. For other packages, try to resolve locally and inline.
if (alwaysExternalSubpaths.has(pkg)) {
return { path: args.path, external: true };
}

try {
require.resolve(args.path, { paths: [args.resolveDir] });
return null;
} catch {
return { path: args.path, external: true };
}
}

// Not in the explicit externals set — try to resolve locally.
// If resolution fails (implicit peer dep like expo-modules-core
// imported by expo-router without being listed as a dependency),
// externalize it so esbuild doesn't crash the entire build.
try {
require.resolve(args.path, { paths: [args.resolveDir] });
return null;
return null; // resolvable locally → inline it
} catch {
console.log(`[auto-external] ${args.path} (not in deps, unresolvable)`);
return { path: args.path, external: true };
}
});
Expand All @@ -212,6 +243,7 @@ async function handlePkgRequest(res: Response, pkgName: string, version: string,
outfile: outFile,
platform: "browser",
target: "es2020",
alias: nodeBuiltinAliases,
// For RN/Expo packages: prioritize .web.* extensions, handle JSX in .js,
// and inline font/image assets as data URLs.
...(isReactNative && {
Expand All @@ -237,7 +269,14 @@ async function handlePkgRequest(res: Response, pkgName: string, version: string,
});

const bundled = fs.readFileSync(outFile, "utf-8");
const wrapped = `// Bundled: ${requireSpecifier}@${version}\n// Externals: ${externals.join(", ") || "none"}\n${bundled}\nif (typeof __module !== "undefined") { module.exports = __module; }\n`;
// When esbuild bundles an ESM package as IIFE, __toCommonJS wraps the
// exports with __esModule: true and a .default getter. If we pass this
// wrapper through as-is, consuming esbuild bundles apply __toESM which
// tries to chain getters across two separate IIFE closures — this breaks
// for packages like color-convert where the .default getter never
// resolves correctly. Fix: unwrap the .default so consumers get the
// actual value directly (CJS-style), which __toESM always handles.
const wrapped = `// Bundled: ${requireSpecifier}@${version}\n// Externals: ${externals.join(", ") || "none"}\n${bundled}\nif (typeof __module !== "undefined") { module.exports = (__module && __module.__esModule && __module.default !== undefined) ? __module.default : __module; }\n`;

fs.writeFileSync(cacheFile, wrapped);
const externalsJson = JSON.stringify(externalizedMap);
Expand Down
4 changes: 3 additions & 1 deletion almostmetro/example/package-lock.json
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9 changes: 5 additions & 4 deletions almostmetro/package-lock.json
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32 changes: 31 additions & 1 deletion almostmetro/src/transforms/react-refresh.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -56,8 +56,12 @@ export function createReactRefreshTransformer(base: Transformer): Transformer {
return result;
}

// Check if module uses createContext (needs HMR identity preservation)
const usesCreateContext =
params.src.includes('createContext') || result.code.includes('createContext');

// Preamble: set up refresh hooks scoped to this module
const preamble =
let preamble =
'var _prevRefreshReg = window.$RefreshReg$;\n' +
'var _prevRefreshSig = window.$RefreshSig$;\n' +
'var _refreshModuleId = ' + JSON.stringify(params.filename) + ';\n' +
Expand All @@ -73,6 +77,26 @@ export function createReactRefreshTransformer(base: Transformer): Transformer {
' return function(type) { return type; };\n' +
'};\n';

// Context identity preservation: patch React.createContext so re-executions
// return the same context object, preventing useContext identity mismatches
if (usesCreateContext) {
preamble +=
'var _hmrCtxIdx = 0;\n' +
'var _hmrOrigCC;\n' +
'try {\n' +
' var _hmrReact = require("react");\n' +
' _hmrOrigCC = _hmrReact.createContext;\n' +
' if (!window.__HMR_CONTEXTS__) window.__HMR_CONTEXTS__ = {};\n' +
' _hmrReact.createContext = function(defaultValue) {\n' +
' var key = _refreshModuleId + ":ctx:" + (_hmrCtxIdx++);\n' +
' if (window.__HMR_CONTEXTS__[key]) return window.__HMR_CONTEXTS__[key];\n' +
' var ctx = _hmrOrigCC(defaultValue);\n' +
' window.__HMR_CONTEXTS__[key] = ctx;\n' +
' return ctx;\n' +
' };\n' +
'} catch(_e) {}\n';
}

// Postamble: register each component and accept HMR
let postamble = '\n';
for (const name of components) {
Expand All @@ -81,6 +105,12 @@ export function createReactRefreshTransformer(base: Transformer): Transformer {
' $RefreshReg$(' + name + ', ' + JSON.stringify(name) + ');\n' +
'}\n';
}
if (usesCreateContext) {
postamble +=
'if (_hmrOrigCC) {\n' +
' try { require("react").createContext = _hmrOrigCC; } catch(_e) {}\n' +
'}\n';
}
postamble +=
'window.$RefreshReg$ = _prevRefreshReg;\n' +
'window.$RefreshSig$ = _prevRefreshSig;\n' +
Expand Down