Declare a variable

Declare a variable to hold the current user ID so the rest of the module can reference it. The variable should be named currentUserId, be block scoped, initially null, and include a one-line comment explaining why it is mutable.

Before I push this change, make the variable declaration reviewer-friendly: explain when it becomes non-null, show the few lines of code that will assign it in the login flow, and flag any race conditions a reviewer should check for. Keep the comment concise and concrete.

I declared currentUserId as let currentUserId = null and some tests now fail only under concurrency. I suspect race conditions between token refresh and route guard. Who owns the sequence I need to coordinate with, what precise questions do I ask them, and what small code change will make the race visible in tests so I can fix it?

On this codebase we keep introducing module-level mutable variables like currentUserId and then spend time hunting bugs when state diverges. Where is the real productivity loss — readability, testing, or initialization order — and what habit change and architectural rule would stop us from doing this again?