Breaking the Circle of Confusion: A Redux Circular Dependency Journey

Recently, I stumbled across a bug in my Redux codebase that left me scratching my head. If you've ever felt that sudden wave of confusion when the test suite throws an error that makes no sense, you’ll know the feeling. Here's what happened and how I eventually found (and fixed) the issue.

What on Earth is a Circular Dependency?

A circular dependency occurs when two or more modules depend on each other—directly or indirectly—creating an infinite loop in the dependency chain. In other words, it's like two friends saying, "You go first," but no one ever moves. In JavaScript, this can result in undefined modules or incomplete data, which leads to bugs that can be incredibly hard to trace.

The Culprit: An Example

Imagine two JavaScript files, moduleA.js and moduleB.js:

// moduleA.js
import { functionB } from './moduleB.js';
export function functionA() {
  console.log('functionA called');
  functionB();
}

// moduleB.js
import { functionA } from './moduleA.js';
export function functionB() {
  console.log('functionB called');
  functionA();
}

Here, both modules depend on each other. When JavaScript tries to load them, it gets confused because neither can be fully loaded without the other being ready first. This leads to problems like undefined functions or incomplete modules—basically, a mess.

So, How Did I Find My Circular Dependency?

Ah, the dreaded error that kicked off this adventure:

Error: `reducer` is a required argument, and must be a function or an object of functions that can be passed to combineReducers.
 ❯ Module.configureStore node_modules/@reduxjs/toolkit/src/configureStore.ts:98:11

My first reaction? "Wait, what?!" — not my finest moment. I was sure my reducers were in place, so this error seemed out of nowhere. After some digging, I realized this wasn’t a missing reducer issue but a circular dependency sneaking into my Redux setup. Of course, figuring that out wasn't easy!

The Real Hero: dpdm

That's when I found my savior: the npm package dpdm. This gem analyzes your dependency tree and shows you where those sneaky circular dependencies live. Running the following command gave me a clear view:

dpdm --no-warning --no-tree ./src/index.tsx

And voilà! Here’s a taste of what it found in my project:

• Circular Dependencies
  01) src/stores/store.ts -> src/stores/rootReducer.ts -> src/stores/slice/authSlice.ts -> src/utilities/api.ts
  02) src/stores/rootReducer.ts -> src/stores/slice/bookingSlice.ts -> src/hooks/redux.tsx -> src/stores/types.ts -> src/stores/setUpStore.ts
  03) src/stores/types.ts -> src/stores/setUpStore.ts
  ...

The Fix: Refactoring Time!

The report was clear: there were a bunch of circular dependencies in my Redux files, primarily in store.ts, rootReducer.ts, and some slices. After splitting up some of the logic, breaking down unnecessary dependencies, and refactoring the code, I finally got things back in order.

Lessons Learned

• Circular dependencies are sneaky: They often don’t show up until runtime or during unit tests, making them hard to track down.
• Tools like dpdm are lifesavers: Don’t waste time manually searching through imports. Let tools do the heavy lifting.
• Refactoring is your friend: Sometimes circular dependencies are a sign of bad architecture. A good refactor not only fixes the immediate issue but also makes your codebase cleaner and more maintainable.

So, the next time you run into one of these pesky bugs, grab some coffee, have a laugh, and break that circle!

Happy debugging! ?