When Modules Look Back at Each Other: Understanding Circular Dependency in JavaScript

At some point, your code starts to connect in ways you didn’t fully plan.

One file depends on another.

That file, in turn, depends on the first.

Nothing seems unusual at first.

Each module simply imports what it needs.

But then something strange happens.

A value becomes undefined.

A function behaves unexpectedly.

And the problem isn’t where you’re looking—

it’s in how everything is connected.

The Loop You Don’t See

Imagine two modules:

One reaches out to the other for a value.

The other does the same.

It forms a loop:

A → B → A

Not a direct error.

But not a stable structure either.

Because now, each module is waiting for the other to be ready.

The Problem Isn’t the Loop—It’s the Timing

Circular dependency isn’t really about files pointing to each other.

It’s about when things become available.

When JavaScript loads modules, it doesn’t execute everything immediately.

It builds a map of dependencies first.

Then it starts running them.

But in a circular situation, something subtle happens.

One module begins executing.

It pauses to load another.

That second module starts—

and then reaches back to the first.

But the first one isn’t finished yet.

So what does it get?

Not an error.

Just… something incomplete.

When “Not Ready Yet” Becomes a Bug

This is where the behavior starts to feel confusing.

A variable that clearly exists suddenly becomes undefined.

Not because it doesn’t exist—

but because it hasn’t been initialized yet.

The system didn’t fail.

It simply gave you what was available at that moment.

And that moment was too early.

A System That Tries to Keep Going

ES Modules don’t break when this happens.

They allow the cycle.

They create connections first, then fill in the values as execution progresses.

So in theory, everything eventually becomes available.

But only if you access it at the right time.

And that’s where things become fragile.

The Hidden Risk of Immediate Access

The problem appears when you try to use something too soon.

At the top level of a module:

console.log(valueFromOtherModule)

There’s no delay.

No waiting.

So if that value isn’t ready yet, you get something incomplete.

Not because the system is broken—

but because you asked too early.

Why Moving Code Changes Everything

A small change can make a big difference.

Instead of accessing values immediately…

you wrap them in a function:

export function getValue() {
  return valueFromOtherModule
}

Now the access is delayed.

It only happens when the function is called.

By that time, both modules have likely finished initializing.

The loop still exists.

But the timing no longer breaks things.

A Different Way to See the Problem

It’s tempting to think circular dependencies are inherently bad.

But they’re not.

They’re just relationships.

What matters is how those relationships behave over time.

A circular dependency becomes a problem when:

• values are needed immediately
• initialization order matters
• logic runs too early

Without those conditions, the loop can exist quietly.

Breaking the Loop Without Removing It

Sometimes the structure itself needs to change.

Instead of two modules pointing at each other:

A ↔ B

You introduce a third:

A → shared ← B

Now the dependency becomes directional.

And the cycle disappears.

Not by force—

but by redesign.

A Final Thought

Circular dependencies don’t fail loudly.

They don’t crash your application.

They reveal themselves in subtle ways—

through timing, through undefined, through behavior that feels slightly off.

And once you see it clearly…

the problem shifts.

It’s no longer about avoiding connections.

It’s about understanding when those connections are safe.

Because in the end, the issue isn’t that modules depend on each other.

It’s that sometimes…

they depend on each other too soon.