When Modules Look Back at Each Other: Understanding Circular Dependency in JavaScript
A circular dependency occurs when modules depend on each other in a loop, which can lead to partially initialized values being accessed before they are ready, causing unexpected behavior.
At some point, your code starts to connect in ways you didnât 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 isnât where youâre lookingâ
itâs in how everything is connected.
The Loop You Donât See
Imagine two modules:
One reaches out to the other for a value.
The other does the same.
It forms a loop:
A â B â ANot a direct error.
But not a stable structure either.
Because now, each module is waiting for the other to be ready.
The Problem Isnât the LoopâItâs the Timing
Circular dependency isnât really about files pointing to each other.
Itâs about when things become available.
When JavaScript loads modules, it doesnât 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 startsâ
and then reaches back to the first.
But the first one isnât finished yet.
So what does it get?
Not an error.
Just⌠something incomplete.
When âNot Ready Yetâ Becomes a Bug
This is where the behavior starts to feel confusing.
A variable that clearly exists suddenly becomes undefined.
Not because it doesnât existâ
but because it hasnât been initialized yet.
The system didnât 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 donât 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 thatâs 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)Thereâs no delay.
No waiting.
So if that value isnât ready yet, you get something incomplete.
Not because the system is brokenâ
but because you asked too early.
Why Moving Code Changes Everything
A small change can make a big difference.
Instead of accessing values immediatelyâŚ
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
Itâs tempting to think circular dependencies are inherently bad.
But theyâre not.
Theyâre 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 â BYou introduce a third:
A â shared â BNow the dependency becomes directional.
And the cycle disappears.
Not by forceâ
but by redesign.
A Final Thought
Circular dependencies donât fail loudly.
They donât crash your application.
They reveal themselves in subtle waysâ
through timing, through undefined, through behavior that feels slightly off.
And once you see it clearlyâŚ
the problem shifts.
Itâs no longer about avoiding connections.
Itâs about understanding when those connections are safe.
Because in the end, the issue isnât that modules depend on each other.
Itâs that sometimesâŚ
they depend on each other too soon.