ES modules use `import`/`export`, are statically analysable (enabling tree shaking) and always run in strict mode. Node's older CommonJS uses `require`/`module.exports` and resolves at runtime.
export const RPC = "https://sepolia.base.org";
export default function client() { /* ... */ }
import client, { RPC } from "./client.js";Prefer named exports: they refactor and autocomplete better than a default.