How JavaScript ES Modules Resolve Circular Dependencies

This article explains how the JavaScript ECMAScript Module (ESM) specification resolves circular module dependencies. It covers the two-phase lifecycle of parsing and evaluation, the mechanics of live bindings that allow modules to reference uninitialized values, how the Temporal Dead Zone affects circular references at runtime, and best practices for avoiding common circular dependency pitfalls.

The Two-Phase Lifecycle

Unlike CommonJS, which evaluates code synchronously while building module exports, ECMAScript Modules operate in distinct phases:

1. Construction (Parsing): The engine reads the source code, finds all import and export statements, and builds a directed graph of module dependencies.
2. Instantiation (Linking): The engine creates module records and allocates memory space for all exported variables. It connects import statements directly to these memory locations (bindings) without executing any code.
3. Evaluation: The engine finally executes the top-level code in the modules, filling the previously allocated memory spaces with actual values.

Because the dependency graph and export bindings are constructed before any code executes, the JavaScript engine knows about the existence of all imports and exports across circular boundaries ahead of runtime.

Live Bindings

The key mechanism enabling circular dependency resolution in ESM is live bindings.

In CommonJS, require() returns a snapshot copy of module.exports. If a cycle occurs, a module might receive an incomplete copy of an object. In ESM, import creates a direct, live reference to the exported identifier in the exporting module’s scope.

When Module A imports a variable from Module B, Module A holds a reference to Module B’s internal slot. When Module B eventually assigns a value to that variable, Module A immediately accesses the updated value through its reference.

The Resolution Flow in Action

Consider a scenario where a.js imports b.js, and b.js imports a.js:

1. Graph Traversal: The engine traverses the dependency tree (using Depth-First Search) and instantiates memory locations for all exports in both a.js and b.js.
2. Evaluation Order: The engine evaluates the leaves of the dependency tree first. It begins executing b.js.
3. Reading Circular Imports: When b.js accesses an export from a.js, the binding exists, but a.js has not yet executed its top-level code.
4. Completion: Once b.js finishes evaluation, execution returns to a.js, which then evaluates its own code and initializes its exports.

Temporal Dead Zone (TDZ) and Runtime Errors

While the module engine links cyclic dependencies without crashing, runtime behavior depends on how identifiers are declared:

• Hoisted Functions: Function declarations are fully hoisted during the instantiation phase. If b.js calls a function exported by a.js, it works immediately, even if a.js has not completed top-level evaluation.
• let and const Variables: Variables declared with let, const, or class enter the Temporal Dead Zone until their initialization line executes. If b.js attempts to read a const or let value from a.js before a.js evaluates, JavaScript throws a ReferenceError: Cannot access '...' before initialization.
• var Variables: Variables declared with var are initialized to undefined during instantiation, which avoids TDZ errors but may introduce logic bugs if accessed prematurely.

Strategies to Avoid Circular Dependency Issues

• Defer Execution: Avoid accessing imported bindings in the top-level scope. Access them inside functions that are invoked only after all modules have finished evaluation.
• Extract Shared Logic: Move shared dependencies, constants, or types to a separate third module (e.g., common.js or types.js) to break the circular link.
• Use Dependency Injection: Pass required dependencies as function or class constructor arguments rather than importing them directly across cycles.