Mastering JavaScript Module Federation: The Power of Shared Scope and Dependency Sharing

In the rapidly evolving landscape of web development, building scalable and maintainable applications often involves adopting sophisticated architectural patterns. Among these, the concept of microfrontends has gained significant traction, allowing teams to develop and deploy parts of an application independently. At the heart of enabling seamless integration and efficient code sharing between these independent units lies Webpack's Module Federation plugin, and a critical component of its power is the shared scope.

This comprehensive guide delves deep into the shared scope mechanism within JavaScript Module Federation. We'll explore what it is, why it's essential for dependency sharing, how it works, and practical strategies for implementing it effectively. Our aim is to equip developers with the knowledge to leverage this powerful feature for enhanced performance, reduced bundle sizes, and improved developer experience across diverse global development teams.

What is JavaScript Module Federation?

Before diving into the shared scope, it's crucial to understand the foundational concept of Module Federation. Introduced with Webpack 5, Module Federation is a build-time and run-time solution that allows JavaScript applications to dynamically share code (like libraries, frameworks, or even entire components) between separately compiled applications. This means you can have multiple distinct applications (often referred to as 'remotes' or 'consumers') that can load code from a 'container' or 'host' application, and vice versa.

The primary benefits of Module Federation include:

• Code Sharing: Eliminate redundant code across multiple applications, reducing overall bundle sizes and improving load times.
• Independent Deployment: Teams can develop and deploy different parts of a large application independently, fostering agility and faster release cycles.
• Technology Agnosticism: While primarily used with Webpack, it facilitates sharing across different build tools or frameworks to some extent, promoting flexibility.
• Runtime Integration: Applications can be composed at runtime, allowing for dynamic updates and flexible application structures.

The Problem: Redundant Dependencies in Microfrontends

Consider a scenario where you have multiple microfrontends that all depend on the same version of a popular UI library like React, or a state management library like Redux. Without a mechanism for sharing, each microfrontend would bundle its own copy of these dependencies. This leads to:

• Bloated Bundle Sizes: Each application unnecessarily duplicates common libraries, leading to larger download sizes for users.
• Increased Memory Consumption: Multiple instances of the same library loaded in the browser can consume more memory.
• Inconsistent Behavior: Different versions of shared libraries across applications can lead to subtle bugs and compatibility issues.
• Wasted Network Resources: Users might download the same library multiple times if they navigate between different microfrontends.

This is where Module Federation's shared scope comes into play, offering an elegant solution to these challenges.

Understanding Module Federation's Shared Scope

The shared scope, often configured via the shared option within the Module Federation plugin, is the mechanism that enables multiple independently deployed applications to share dependencies. When configured, Module Federation ensures that a single instance of a specified dependency is loaded and made available to all applications that require it.

At its core, the shared scope works by creating a global registry or container for shared modules. When an application requests a shared dependency, Module Federation checks this registry. If the dependency is already present (i.e., loaded by another application or the host), it uses that existing instance. Otherwise, it loads the dependency and registers it in the shared scope for future use.

The configuration typically looks like this:

            // webpack.config.js

const { ModuleFederationPlugin } = require('webpack');

module.exports = {
  // ... other webpack configurations
  plugins: [
    new ModuleFederationPlugin({
      name: 'container',
      remotes: {
        'app1': 'app1@http://localhost:3001/remoteEntry.js',
        'app2': 'app2@http://localhost:3002/remoteEntry.js',
      },
      shared: {
        'react': {
          singleton: true,
          eager: true,
          requiredVersion: '^18.0.0',
        },
        'react-dom': {
          singleton: true,
          eager: true,
          requiredVersion: '^18.0.0',
        },
      },
    }),
  ],
};

            

              
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Key Configuration Options for Shared Dependencies:

• singleton: true: This is perhaps the most critical option. When set to true, it ensures that only a single instance of the shared dependency is loaded across all consuming applications. If multiple applications attempt to load the same singleton dependency, Module Federation will provide them with the same instance.
• eager: true: By default, shared dependencies are loaded lazily, meaning they are only fetched when they are explicitly imported or used. Setting eager: true forces the dependency to be loaded as soon as the application starts, even if it's not immediately used. This can be beneficial for critical libraries like frameworks to ensure they are available from the outset.
• requiredVersion: '...': This option specifies the required version of the shared dependency. Module Federation will attempt to match the requested version. If multiple applications require different versions, Module Federation has mechanisms to handle this (discussed later).
• version: '...': You can explicitly set the version of the dependency that will be published to the shared scope.
• import: false: This setting tells Module Federation not to automatically bundle the shared dependency. Instead, it expects it to be provided externally (which is the default behavior when sharing).
• packageDir: '...': Specifies the package directory to resolve the shared dependency from, useful in monorepos.

How Shared Scope Enables Dependency Sharing

Let's break down the process with a practical example. Imagine we have a main 'container' application and two 'remote' applications, `app1` and `app2`. All three applications depend on `react` and `react-dom` version 18.

Scenario 1: Container Application Shares Dependencies

In this common setup, the container application defines the shared dependencies. The `remoteEntry.js` file, generated by Module Federation, exposes these shared modules.

Container's Webpack Config (`container/webpack.config.js`):

            const { ModuleFederationPlugin } = require('webpack');

module.exports = {
  // ...
  plugins: [
    new ModuleFederationPlugin({
      name: 'container',
      filename: 'remoteEntry.js',
      exposes: {
        './App': './src/App',
      },
      shared: {
        'react': {
          singleton: true,
          eager: true,
          requiredVersion: '^18.0.0',
        },
        'react-dom': {
          singleton: true,
          eager: true,
          requiredVersion: '^18.0.0',
        },
      },
    }),
  ],
};

            

              
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Now, `app1` and `app2` will consume these shared dependencies.

`app1`'s Webpack Config (`app1/webpack.config.js`):

            const { ModuleFederationPlugin } = require('webpack');

module.exports = {
  // ...
  plugins: [
    new ModuleFederationPlugin({
      name: 'app1',
      filename: 'remoteEntry.js',
      exposes: {
        './Feature1': './src/Feature1',
      },
      remotes: {
        'container': 'container@http://localhost:3000/remoteEntry.js',
      },
      shared: {
        'react': {
          singleton: true,
          requiredVersion: '^18.0.0',
        },
        'react-dom': {
          singleton: true,
          requiredVersion: '^18.0.0',
        },
      },
    }),
  ],
};

            

              
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`app2`'s Webpack Config (`app2/webpack.config.js`):

The configuration for `app2` would be similar to `app1`, also declaring `react` and `react-dom` as shared with the same version requirements.

How it works at runtime:

1. The container application loads first, making its shared `react` and `react-dom` instances available in its Module Federation scope.
2. When `app1` loads, it requests `react` and `react-dom`. Module Federation in `app1` sees that these are marked as shared and `singleton: true`. It checks the global scope for existing instances. If the container has already loaded them, `app1` reuses those instances.
3. Similarly, when `app2` loads, it also reuses the same `react` and `react-dom` instances.

This results in only one copy of `react` and `react-dom` being loaded into the browser, significantly reducing the total download size.

Scenario 2: Sharing Dependencies Between Remote Applications

Module Federation also allows remote applications to share dependencies amongst themselves. If `app1` and `app2` both use a library that is *not* shared by the container, they can still share it if both declare it as shared in their respective configurations.

Example: Let's say `app1` and `app2` both use a utility library `lodash`.

`app1`'s Webpack Config (adding lodash):

            
// ... within ModuleFederationPlugin for app1
shared: {
  // ... react, react-dom
  'lodash': {
    singleton: true,
    requiredVersion: '^4.17.21',
  },
},

            

              
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`app2`'s Webpack Config (adding lodash):

            
// ... within ModuleFederationPlugin for app2
shared: {
  // ... react, react-dom
  'lodash': {
    singleton: true,
    requiredVersion: '^4.17.21',
  },
},

            

              
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In this case, even if the container doesn't explicitly share `lodash`, `app1` and `app2` will manage to share a single instance of `lodash` between themselves, provided they are loaded in the same browser context.

Handling Version Mismatches

One of the most common challenges in dependency sharing is version compatibility. What happens when `app1` requires `react` v18.1.0 and `app2` requires `react` v18.2.0? Module Federation provides robust strategies for managing these scenarios.

1. Strict Version Matching (Default behavior for `requiredVersion`)

When you specify a precise version (e.g., '18.1.0') or a strict range (e.g., '^18.1.0'), Module Federation will enforce this. If an application tries to load a shared dependency with a version that doesn't satisfy the requirement of another application already using it, it might lead to errors.

2. Version Ranges and Fallbacks

The requiredVersion option supports semantic versioning (SemVer) ranges. For example, '^18.0.0' means any version from 18.0.0 up to (but not including) 19.0.0. If multiple applications require versions within this range, Module Federation will typically use the highest compatible version that satisfies all requirements.

Consider this:

• Container: shared: { 'react': { requiredVersion: '^18.0.0' } }
• `app1`: shared: { 'react': { requiredVersion: '^18.1.0' } }
• `app2`: shared: { 'react': { requiredVersion: '^18.2.0' } }

If the container loads first, it establishes `react` v18.0.0 (or whatever version it actually bundles). When `app1` requests `react` with `^18.1.0`, it might fail if the container's version is less than 18.1.0. However, if `app1` loads first and provides `react` v18.1.0, and then `app2` requests `react` with `^18.2.0`, Module Federation will try to satisfy `app2`'s requirement. If the `react` v18.1.0 instance is already loaded, it might throw an error because v18.1.0 does not satisfy `^18.2.0`.

To mitigate this, it's best practice to define shared dependencies with the broadest acceptable version range, usually in the container application. For example, using '^18.0.0' allows for flexibility. If a specific remote application has a hard dependency on a newer patch version, it should be configured to explicitly provide that version.

3. Using `shareKey` and `shareScope`

Module Federation also allows you to control the key under which a module is shared and the scope it resides in. This can be useful for advanced scenarios, such as sharing different versions of the same library under different keys.

4. The `strictVersion` Option

When strictVersion is enabled (which is the default for requiredVersion), Module Federation throws an error if a dependency cannot be satisfied. Setting strictVersion: false can allow for more lenient version handling, where Module Federation might try to use an older version if a newer one isn't available, but this can lead to runtime errors.

Best Practices for Using Shared Scope

To effectively leverage Module Federation's shared scope and avoid common pitfalls, consider these best practices:

1. Centralize Shared Dependencies: Designate a primary application (often the container or a dedicated shared library application) to be the source of truth for common, stable dependencies like frameworks (React, Vue, Angular), UI component libraries, and state management libraries.
2. Define Broad Version Ranges: Use SemVer ranges (e.g., '^18.0.0') for shared dependencies in the primary sharing application. This allows other applications to use compatible versions without forcing strict updates across the entire ecosystem.
3. Document Shared Dependencies Clearly: Maintain clear documentation about which dependencies are shared, their versions, and which applications are responsible for sharing them. This helps teams understand the dependency graph.
4. Monitor Bundle Sizes: Regularly analyze the bundle sizes of your applications. Module Federation's shared scope should lead to a reduction in the size of dynamically loaded chunks as common dependencies are externalized.
5. Manage Non-Deterministic Dependencies: Be cautious with dependencies that are frequently updated or have unstable APIs. Sharing such dependencies might require more careful version management and testing.
6. Use `eager: true` Judiciously: While `eager: true` ensures a dependency is loaded early, overuse can lead to larger initial loads. Use it for critical libraries that are essential for the application's startup.
7. Testing is Crucial: Thoroughly test the integration of your microfrontends. Ensure that shared dependencies are correctly loaded and that version conflicts are handled gracefully. Automated testing, including integration and end-to-end tests, is vital.
8. Consider Monorepos for Simplicity: For teams starting with Module Federation, managing shared dependencies within a monorepo (using tools like Lerna or Yarn Workspaces) can simplify the setup and ensure consistency. The `packageDir` option is particularly useful here.
9. Handle Edge Cases with `shareKey` and `shareScope`: If you encounter complex versioning scenarios or need to expose different versions of the same library, explore the `shareKey` and `shareScope` options for more granular control.
10. Security Considerations: Ensure that shared dependencies are fetched from trusted sources. Implement security best practices for your build pipeline and deployment process.

Global Impact and Considerations

For global development teams, Module Federation and its shared scope offer significant advantages:

• Consistency Across Regions: Ensures that all users, regardless of their geographic location, experience the application with the same core dependencies, reducing regional inconsistencies.
• Faster Iteration Cycles: Teams in different time zones can work on independent features or microfrontends without constantly worrying about duplicating common libraries or stepping on each other's toes regarding dependency versions.
• Optimized for Diverse Networks: Reducing the overall download size through shared dependencies is particularly beneficial for users on slower or metered internet connections, which are prevalent in many parts of the world.
• Simplified Onboarding: New developers joining a large project can more easily understand the application's architecture and dependency management when common libraries are clearly defined and shared.

However, global teams must also be mindful of:

• CDN Strategies: If shared dependencies are hosted on a CDN, ensure the CDN has good global reach and low latency for all target regions.
• Offline Support: For applications requiring offline capabilities, managing shared dependencies and their caching becomes more complex.
• Regulatory Compliance: Ensure that the sharing of libraries complies with any relevant software licensing or data privacy regulations in different jurisdictions.

Common Pitfalls and How to Avoid Them

1. Incorrectly Configured `singleton`

Problem: Forgetting to set singleton: true for libraries that should only have one instance.

Solution: Always set singleton: true for frameworks, libraries, and utilities that you intend to share uniquely across your applications.

2. Inconsistent Version Requirements

Problem: Different applications specifying vastly different, incompatible version ranges for the same shared dependency.

Solution: Standardize version requirements, especially in the container app. Use broad SemVer ranges and document any exceptions.

3. Over-sharing Non-essential Libraries

Problem: Trying to share every single small utility library, leading to complex configuration and potential conflicts.

Solution: Focus on sharing large, common, and stable dependencies. Small, rarely used utilities might be better bundled locally to avoid complexity.

4. Not Handling the `remoteEntry.js` File Correctly

Problem: The `remoteEntry.js` file not being accessible or served correctly to consuming applications.

Solution: Ensure your hosting strategy for remote entries is robust and that the URLs specified in the `remotes` configuration are accurate and accessible.

5. Ignoring `eager: true` Implications

Problem: Setting eager: true on too many dependencies, leading to a slow initial load time.

Solution: Use `eager: true` only for dependencies that are absolutely critical for the initial rendering or core functionality of your applications.

Conclusion

JavaScript Module Federation's shared scope is a powerful tool for building modern, scalable web applications, particularly within a microfrontend architecture. By enabling efficient dependency sharing, it tackles issues of code duplication, bloat, and inconsistency, leading to improved performance and maintainability. Understanding and correctly configuring the shared option, especially the singleton and requiredVersion properties, is key to unlocking these benefits.

As global development teams increasingly adopt microfrontend strategies, mastering Module Federation's shared scope becomes paramount. By adhering to best practices, carefully managing versioning, and conducting thorough testing, you can harness this technology to build robust, high-performing, and maintainable applications that serve a diverse international user base effectively.

Embrace the power of shared scope, and pave the way for more efficient and collaborative web development across your organization.