Hidden Global State

1// config.ts
2export let config = loadFromEnv()          // mutable, reachable, global
3 
4// pricing.ts — signature says it needs an Order. It needs more.
5export function price(order: Order): Money {
6  return order.total.times(1 + config.vatRate)
7}
8// test A sets config.vatRate = 0.2 and forgets to restore it
9// test B, running later, silently asserts against 0.2
10 
11// declared: the dependency is in the type
12export function price(order: Order, rates: TaxRates): Money {
13  return order.total.times(1 + rates.vat)
14}
15 
16// the seam that makes ninety call sites survivable:
17export function price(order: Order, rates: TaxRates = config.taxRates): Money { ... }
18// step 1: add the parameter with a default (nothing breaks)
19// step 2: pass it explicitly from the module you are working in
20// step 3: when the last caller passes it, delete the default
21// step 4: when the last default is gone, delete the global

Step one is the important one: the default keeps every existing caller working, so the migration can proceed one module at a time and stop at any point without leaving the codebase in a worse state than it started (Expand and Contract).

1. 1
   Randomise test order in CI

   Turn the intermittent failure into a reproducible one before changing any production code.

   fails by Fixing the global first, and having no way to tell whether the suite got better.

2. 2
   Separate read from write

   Find every writer. If there are none outside tests, freeze it and most of the problem is gone for one line.

   fails by Assuming it is read-only because it was when it was written.

3. 3
   Add the parameter with a default

   Every function that reads the global takes it as an optional argument defaulting to the global. Nothing changes behaviour.

   fails by Making it required immediately, which produces a nine-hundred-line diff nobody can review.

4. 4
   Pass explicitly, module by module

   In the module you are already working in, thread the value from the entry point. Ship each module separately.

   fails by Doing it across the whole codebase at once, which is how the effort stalls and half-migrates.

5. 5
   Prove it in a test

   A test that runs the module with two different configurations in one process. It cannot pass until the module is genuinely free of the global.

   fails by Trusting the diff instead, and leaving one reader behind.

6. 6
   Remove the default

   When no caller relies on it, make the parameter required. The compiler confirms the migration is complete.

   fails by Leaving the default forever, which is the state most of these migrations actually end in.

7. 7
   Delete the global

   And add the CI grep that stops it being reintroduced next quarter.

   fails by Deleting it without the grep, and finding a new one in six months.

Steps one and two are worth doing even if the rest never happens: a reproducible suite and a frozen configuration eliminate the two costs that hurt daily, for a fraction of the work. If a migration has to be abandoned, abandon it after step two rather than in the middle of step four.