Skip to main content

10. Runtime target matrix

This note enumerates the JS runtimes the bridge targets and the feature deltas. It is informative.

1. Node 22 LTS (October 2024 LTS release; maintenance support through April 2027)​

Capabilities:

  • Built-in fetch (Node 18+).
  • node:* built-ins (fs, net, http, child_process, worker_threads, crypto, stream, util).
  • ESM + CJS interop.
  • --experimental-vm-modules for advanced module patterns (rarely used by bridge).
  • Native WebSocket (Node 22+).
  • crypto.subtle for WebCrypto.

Limitations:

  • No Deno.* global.
  • No browser APIs (no window, no document, no localStorage).
  • No Bun.* global.

Bridge behaviour: Node 22 LTS is the default runtime = "node22" target. The [ts.capabilities] table maps directly to the node:* import surface.

2. Deno 2 (October 2024 release)​

Capabilities:

  • Built-in fetch, WebSocket, crypto.subtle.
  • Deno.* global (Deno-specific APIs like Deno.readFile, Deno.serve, Deno.test).
  • ESM-only by default; CJS via npm: specifier and node: prefix.
  • Native TypeScript without compilation (Deno's V8 has its own TS layer).
  • Import maps via deno.json's imports field.
  • JSR support via jsr: specifier (built-in).

Limitations:

  • No module.exports syntax (ESM-only at the file level).
  • node:fs works but via Deno's polyfill (subtle differences).
  • The Deno.* global is not on Node / Bun.

Bridge behaviour: runtime = "deno2" produces emit that uses Deno-native APIs where they differ from Node (Deno.serve instead of http.createServer). JSR consumes are native.

3. Bun 1.1 (1.0 GA September 2023; 1.1 release April 2024)​

Capabilities:

  • Built-in fetch, WebSocket, crypto.subtle.
  • Bun.* global (Bun-specific APIs like Bun.file, Bun.serve, Bun.write).
  • ESM + CJS interop (fastest of the three native runtimes).
  • Built-in TypeScript without compilation.
  • Built-in JSX without compilation.
  • Built-in bun build bundler (used by MEP-52 phase 17 for browser bundle).
  • Compatible with most node:* built-ins via a Node.js compat layer.

Limitations:

  • Newer than Node / Deno; some edge-case Node APIs are not yet implemented.
  • The Bun.* global is not on Node / Deno.

Bridge behaviour: runtime = "bun1.1" produces emit that uses Bun-native APIs where they differ. The node:* import path works via Bun's compat layer.

4. Browser ES2024​

Capabilities:

  • ESM via <script type="module"> or via <link rel="modulepreload">.
  • All standard Web APIs (window, document, fetch, WebSocket, Worker, IndexedDB, localStorage, crypto.subtle).
  • Import maps via <script type="importmap"> (Chrome 89+, Firefox 108+, Safari 16.4+).
  • TC39 stage 4 features (top-level await, private class fields, dynamic import).

Limitations:

  • No CJS support (CJS-only packages require a bundler).
  • No node:* built-ins (no fs, net, child_process, worker_threads).
  • DOM-API surface area (different from server-side runtimes).

Bridge behaviour: runtime = "browser" produces emit that uses Web APIs only. The TargetBrowserBundle path bundles all deps into a single ESM artefact via bun build (primary) or esbuild (fallback). The bridge refuses to lock a consumed package whose engines.node or engines.workerd declares incompatibility with the browser.

5. Edge (Cloudflare Workers, Vercel Edge, Deno Deploy)​

Capabilities:

  • Subset of Web APIs (fetch, Request, Response, Headers, crypto.subtle, WebSocket).
  • Some platform-specific globals (Cloudflare's caches, Vercel's geolocation, Deno Deploy's Deno.serve).

Limitations:

  • No node:fs (no filesystem).
  • No node:net (no TCP socket).
  • No node:child_process (no process spawn).
  • No node:worker_threads (limited worker support via the platform's own Worker primitive).
  • Bundle size limits (Cloudflare Workers: 1 MB compressed; Vercel Edge: 4 MB; Deno Deploy: 10 MB).

Bridge behaviour: runtime = "edge" is the strictest target. The [ts.capabilities] table's fs, proc, worker flags must be false for an edge build. The bridge's edge-runtime gate (phase 17) cross-checks the consumed packages' import surface against the allowed edge API list.

6. Cross-runtime consistency​

MEP-72's gate requires:

  • The same Mochi source compiled for runtime = "node22", "deno2", and "bun1.1" produces identical stdout when run with the same fixture inputs (matches MEP-52 phase 1+'s "run on all three" gate).
  • The runtime = "browser" and "edge" builds produce bundles that pass their respective gate suites; runtime parity with Node is not required (browser and edge have intentionally restricted APIs).

7. Deferred runtime targets​

  • Node 20 LTS (maintenance until April 2026): not gated because Node 22 LTS is current.
  • Older browsers (pre-ES2024): not gated; the ES2024 target is the floor.
  • React Native (Hermes engine): not gated; React Native's JS engine has its own constraints.
  • Tauri (V8 / WebView2): not gated; would inherit the browser target.

8. Cross-references​