Summary of the problem of handling circular dependency of ES6 modules during Webpack packaging

The circular dependency problem of ES6 modules is a common challenge during Webpack packaging. A circular dependency refers to two or more modules referring to each other, forming a closed loop. When dealing with circular dependencies, Webpack adopts some strategies to ensure that the modules load correctly.

1. The concept of circular dependency

Suppose there are two modules A and B, which depend on each other:

// 
import { bFunc } from './';
export function aFunc() {
  bFunc();
}
// 
import { aFunc } from './';
export function bFunc() {
  aFunc();
}

In this example,andA circular dependency relationship is formed.

2. How Webpack handles circular dependencies

2.1. Temporary Quotation

Webpack uses the ES6 module's "temporary reference" feature to handle circular dependencies. When a module is loading, if a module is being parsed (i.e. not completed), Webpack will set its export to a placeholder. This means that other modules can still reference it when the module is not fully loaded.

For example, useandCircular dependencies:

• When Webpack loadsWhen it finds that it needs to be referenced。
• Webpack starts loadingand find it in the process that requires reference。
• Webpack willThe export is set to a placeholder (e.g.undefined) and then continue to load. onceComplete loading,The placeholder of the ,is replaced with the actual export.

2.2. Solutions and Considerations

Design modules with caution：

• If possible, try to avoid circular dependencies. Refactor the code, separate the public dependency module, or communicate through indirect means such as event mechanisms and callback functions.

Use default export：

• In some cases, using default exports can simplify the problem of circular dependencies, as default exports allow you to use placeholders during module loading.

Detect cyclic dependencies：

• Webpack can detect circular dependencies and issue warnings at compile time. Be aware of these warnings and consider refactoring the code to eliminate circular dependencies.

Usage Tools：

• Analytical tools can be used (e.g.madge) to identify and visualize the dependencies between modules and help discover circular dependencies.

2.3. Example

Here is an example showing how Webpack handles circular dependencies:

// 
import { bFunc } from './';
export function aFunc() {
  ('aFunc called');
  bFunc();
}
// 
import { aFunc } from './';
export function bFunc() {
  ('bFunc called');
  aFunc();
}
// 
import { aFunc } from './';
aFunc();

In this example, even if there is a circular dependency, Webpack is still able to successfully load and callaFuncandbFunc, but be careful about the infinite recursive calls that may result.

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