Example of API client instance module that can be replaced and refreshed with fake environment variables on the fly during testing

testable-env-instance.test.ts

import { expect, test } from 'vitest'
import myModule from './testable-env-instance'

test('defaults to env', () => {
  expect(myModule.token).toBe('INITIAL_TEST_VALUE')
})

test('env can be changed ondemand', () => {
  expect(myModule.token).toBe('INITIAL_TEST_VALUE')

  myModule._env.POSTMARK_API_TOKEN = 'hello'
  expect(myModule.token).toBe('hello')
})

testable-env-instance.ts

const {
  POSTMARK_API_TOKEN = 'INITIAL_TEST_VALUE',
} = process.env

class DummyClass {
  token = ''

  constructor(token: string) {
    this.token = token
  }
}

// initial client uses environment variable
let _instance = new DummyClass(POSTMARK_API_TOKEN)

// intercept `_instance._env.POSTMARK_API_TOKEN` prop, for testability
const proxy = new Proxy({}, {
  get(target, getProp, receiver) {
    if (getProp === '_env') {
      return new Proxy({}, {
        set(obj, setProp, value) {
          if (setProp === 'POSTMARK_API_TOKEN') {
            // refresh the client with our new "environment variable"
            _instance = new DummyClass(value)
            return true
          }
          return true
        }
      })
    }
    // for anything other than ._env, behave like the real client
    return Reflect.get(_instance, getProp)
  },
}) as DummyClass & { _env: Record<string, string> }

export default proxy