@rcompat/runtime

Runtime detection for JavaScript environments.

What is @rcompat/runtime?

A cross-runtime module that tells you which JavaScript runtime your code is executing in. Uses runtime keys for accurate detection without any runtime overhead.

Installation

npm install @rcompat/runtime

pnpm add @rcompat/runtime

yarn add @rcompat/runtime

bun add @rcompat/runtime

Usage

Basic detection

import runtime from "@rcompat/runtime";

console.log(runtime);
// "node" | "bun" | "deno" | "workerd" | "vercel" | "netlify" | "fastly" | "browser"

Conditional logic

import runtime from "@rcompat/runtime";

if (runtime === "bun") {
  // Bun-specific code
} else if (runtime === "deno") {
  // Deno-specific code
} else if (runtime === "node") {
  // Node.js-specific code
}

Feature detection

import runtime from "@rcompat/runtime";

function getFileReader() {
  switch (runtime) {
    case "bun":
      return Bun.file;
    case "deno":
      return Deno.readFile;
    default:
      return require("fs").promises.readFile;
  }
}

Supported Runtimes

| Runtime | Value | Environment | |----------------|-------------|----------------------------| | Node.js | "node" | Standard Node.js | | Bun | "bun" | Bun runtime | | Deno | "deno" | Deno runtime | | Cloudflare | "workerd" | Cloudflare Workers | | Vercel | "vercel" | Vercel Edge Functions | | Netlify | "netlify" | Netlify Edge Functions | | Fastly | "fastly" | Fastly Compute@Edge | | Browser | "browser" | Web browsers |

API Reference

Default Export

declare const runtime:
  | "node"
  | "bun"
  | "deno"
  | "workerd"
  | "vercel"
  | "netlify"
  | "fastly"
  | "browser";

A string identifying the current runtime environment.

How It Works

This package uses runtime keys, a WinterCG proposal for runtime detection. The exports field in package.json routes to different files based on the runtime condition, meaning detection happens at import time with zero runtime overhead.

{
  "exports": {
    ".": {
      "bun": "./lib/public/bun.js",
      "deno": "./lib/public/deno.js",
      "workerd": "./lib/public/cloudflare.js",
      "node": "./lib/public/node.js"
    }
  }
}

Examples

Runtime-specific logging

import runtime from "@rcompat/runtime";

const logger = {
  info(message) {
    const prefix = `[${runtime}]`;
    console.log(prefix, message);
  }
};

logger.info("Server started");
// [node] Server started
// [bun] Server started
// etc.

Environment checks

import runtime from "@rcompat/runtime";

const isServerRuntime = ["node", "bun", "deno"].includes(runtime);
const isEdgeRuntime = ["workerd", "vercel", "netlify", "fastly"].includes(runtime);
const isClientRuntime = runtime === "browser";

if (isEdgeRuntime) {
  // Limited APIs available
}

Polyfill loading

import runtime from "@rcompat/runtime";

async function setup() {
  if (runtime === "node") {
    // Node.js doesn't have fetch in older versions
    globalThis.fetch ??= (await import("node-fetch")).default;
  }
}

Debug information

import runtime from "@rcompat/runtime";

function getDebugInfo() {
  return {
    runtime,
    timestamp: Date.now(),
    userAgent: runtime === "browser" ? navigator.userAgent : undefined,
  };
}

Cross-Runtime Compatibility

| Runtime | Supported | |-----------------|-----------| | Node.js | ✓ | | Bun | ✓ | | Deno | ✓ | | Cloudflare Workers | ✓ | | Vercel Edge | ✓ | | Netlify Edge | ✓ | | Fastly Compute | ✓ | | Browser | ✓ |

No configuration required — just import and use.

License

MIT

Contributing

See CONTRIBUTING.md in the repository root.