Angular CLI 1.1.1中tsconfig.spec.json为何采用CommonJS模块？

Great question! This difference isn't arbitrary—it's all about matching the module system to the specific needs of your application code vs. your test code. Let's break it down:

For tsconfig.app.json (Application Code): ES2015 Modules #

• Tree Shaking & Bundle Optimization: ES2015 (ES6) modules use static import/export syntax, which tools like Webpack (used under the hood in Angular CLI 1.1.1) can analyze at build time. This enables tree shaking, a process that eliminates unused code from your final bundle, drastically reducing its size and improving app performance.
• Modern Frontend Standards: Angular is built around ES2015+ features, so using ES modules aligns with the framework's design and takes advantage of native JS module capabilities. Even if you target older browsers (like ES5), the TypeScript compiler can transpile ES modules to compatible formats while retaining the benefits of static analysis during development.
• Browser Compatibility Roadmap: ES modules are natively supported in modern browsers, so using them as your source module system keeps your code future-proof and aligned with web platform evolution.

For tsconfig.spec.json (Test Code): CommonJS Modules #

• Test Toolchain Compatibility: Back in Angular CLI 1.1.1, the default testing stack (Karma + Jasmine) had far more mature support for CommonJS modules. CommonJS is the standard module system for Node.js, and since Karma runs tests in a Node-backed environment, using CommonJS ensures seamless loading of test dependencies, mocks, and test files.
• Dynamic Loading Flexibility: Test code often relies on dynamic module loading—think requiring mock data on the fly, or loading test utilities that don't fit neatly into static import statements. CommonJS's require() function handles these dynamic use cases more reliably than ES modules did at the time.
• Stability & Reliability: The ES module ecosystem for testing was still evolving when Angular CLI 1.1.1 was released. Using CommonJS avoided potential bugs or compatibility issues that could arise from trying to force ES modules into a test environment that wasn't fully optimized for them.

In short, Angular CLI picks the module system that best serves each context: ES2015 for app performance and modern standards, CommonJS for test stability and toolchain compatibility.

内容的提问来源于stack exchange，提问作者Manoj