How JavaScript Handles Circular Dependencies in ES Modules

Circular module dependencies occur when Module A imports Module B, and Module B directly or indirectly imports Module A. JavaScript handles these circular dependencies in ECMAScript Modules (ESM) through a multi-phase lifecycle—parsing, instantiation, and evaluation—combined with a mechanism known as “live bindings.” Instead of copying values, imports act as direct references (pointers) to the exported memory locations. This allows modules to reference exports from cyclic dependencies before their evaluation is fully complete, though careful structuring is required to avoid accessing uninitialized variables.

The Module Loading Lifecycle

To manage cyclic references without causing infinite loops or execution deadlocks, JavaScript engines process ES modules in three distinct phases:

  1. Construction (Parsing): The engine fetches and parses all files into Module Records. It identifies all import and export statements to build a complete Dependency Graph before running any code.
  2. Instantiation: The engine allocates memory locations for all exported variables and links the import statements directly to those memory locations. No values are assigned yet, and no executable code runs.
  3. Evaluation: The engine executes the module code top-to-bottom. Memory locations are populated with their actual runtime values.

Because instantiation happens before evaluation, cyclic references can be wired together in memory before any code execution starts.

Live Bindings vs. CommonJS Value Copying

Unlike CommonJS (require), which caches the module.exports object as a static copy at the time of execution, ES modules use live bindings.

When a module imports a value in ESM, it receives a read-only live reference to the exporter’s variable. If the exporting module updates that variable later during its evaluation phase, the importing module immediately observes the updated value through the reference.

Execution Order and the Temporal Dead Zone

When a circular dependency exists, one module must begin evaluating before the other has finished. This introduces risks related to the Temporal Dead Zone (TDZ) and execution order:

Code Example: Safe vs. Unsafe Circular Dependencies

Consider two mutually dependent files, moduleA.js and moduleB.js:

// moduleA.js
import { bFunction, bValue } from './moduleB.js';

export const aValue = 'Value from A';

export function aFunction() {
  return `A calling: ${bFunction()}`;
}

// Unsafe top-level access:
// console.log(bValue); // ReferenceError: Cannot access 'bValue' before initialization
// moduleB.js
import { aValue, aFunction } from './moduleA.js';

export const bValue = 'Value from B';

export function bFunction() {
  return 'Hello from B';
}

// Safe inside a function executed later:
export function useA() {
  return aValue; 
}

If moduleA.js is the entry point: 1. moduleA.js imports moduleB.js, pausing moduleA execution. 2. moduleB.js evaluates first, declaring bValue and bFunction. 3. moduleA.js resumes execution, declaring aValue and aFunction. 4. Calling useA() after evaluation succeeds because aValue is accessed via a live binding after initialization.

Best Practices for Managing Circular Dependencies