Configuration via constructor, not global state

Global init() or configure() functions that mutate module-level state break every scenario with multiple concurrent consumers: test suites that need different API keys per test, monorepo apps using the same package with different configurations, and multi-tenant servers handling requests with per-user credentials. ISO 25010 testability and modifiability both require that configuration be isolatable per instance. Module-level state also prevents the garbage collector from cleaning up instances and leaks across test runs.

Replace global configuration state with a constructor or factory function that captures config in closure.

// Before — global state:
let globalConfig: Config
export function init(config: Config) { globalConfig = config }
export function doSomething() { /* uses globalConfig */ }

// After — per-instance via factory:
export function createClient(config: Config) {
  return {
    doSomething() { /* uses config from closure */ }
  }
}

This pattern allows test suites to create isolated instances (const clientA = createClient({ apiKey: 'test-key-a' })) and monorepo apps to configure multiple instances independently. All configuration options should have TypeScript types and documented defaults.