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vue3-reading: Vue 3 Source Code Analysis

website·Indexed 29 days ago

https://s-elo.github.io/vue3-reading/

A guide for reading and understanding the Vue 3 source code. It covers the core reactivity system, including the implementation of reactive and effect APIs, as well as the repository's build process, development scripts, testing strategies (unit, e2e, and type tests), and prerequisite knowledge such as ES6 Proxy, TypeScript, and data structures.

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What's inside vue3-reading

  1. Overview of software testing levels

    Software testing is categorized into four main levels to ensure continuous integration and delivery:

    • Unit Testing: Tests individual modules, components, or functions. Test cases are typically kept with the source code.
    • Integration Testing: Ensures different units work together as expected. It focuses on specific functional blocks and is often performed alongside end-to-end (E2E) testing.
    • System Testing: Validates the entire system's compatibility, performance, regression, scalability, and security (including network, IO, CPU, and memory usage).
    • Acceptance Testing: Performed from the end-user's perspective before the software is delivered to the production environment.

  2. Purpose and usage of the __esModule flag in CommonJS bundles

    The __esModule flag is a convention used by bundlers (like Rollup) to identify CommonJS modules that are intended to be treated as ES modules.

    Because CommonJS does not have a native 'default export' concept, importing a CJS module via ESM usually results in the entire module.exports object being treated as the default export. By adding Object.defineProperty(exports, '__esModule', { value: true }), the bundler is told that if a default property exists on module.exports, it should be treated as the actual default export of the module.

    // In a CJS file:
Object.defineProperty(exports, '__esModule', { value: true });

const name = 'leo';
module.exports.default = name; // This will now be treated as the default export by ESM importers
module.exports.age = 16;

  3. Implement basic object interception using ES6 Proxy

    The Proxy object allows you to define custom behavior for fundamental operations (e.g., property lookup, assignment) on a target object. This is a form of metaprogramming where you wrap a target object with a handler to intercept and redefine operations.

    Syntax: const proxy = new Proxy(target, handler)

    • target: The object to wrap (can be any object, array, function, or another proxy).
    • handler: An object containing traps (functions) that define the interception behavior.
    const origin = {}
const obj = new Proxy(origin, {
  get: function (target, propKey, receiver) {
    return '10'
  }
});

console.log(obj.a); // '10'
console.log(obj.b); // '10'
console.log(origin.a); // undefined (original object remains untouched)

  4. Understand TypeScript's role and value

    TypeScript is an open-source, typed superset of JavaScript that compiles to plain JavaScript, allowing it to run in any browser, host, or OS. Its primary value is provided during development (compile-time) rather than runtime, offering:

    • Enhanced editor intelligence and autocomplete.
    • Improved code readability for team collaboration.
    • Compile-time type checking to reduce low-level type errors.

  5. Handle TypeScript const enums in Vue 3 build process

    Vue 3 uses a custom build mechanism to handle export const enum declarations. Because these are TypeScript-specific and need to be inlined, the build process scans the repository for these enums, caches their values, and removes the declarations from the source code before the final bundling stage (esbuild). This prevents compilation errors and ensures the values are available for injection.

  6. Understand the reactive internal workflow

    The Vue 3 reactivity system follows a specific lifecycle for reactive and effect:

    1. Proxy Wrapping: reactive wraps a target object in a Proxy.
    2. Dependency Tracking (Track): When an effect is executed, it triggers the Proxy's get trap. The system records the relationship between the current activeEffect and the accessed property.
    3. Triggering Side Effects (Trigger): When a property is modified, the Proxy's set trap is triggered, which finds all effects associated with that property and executes them.

  7. Perform source transformations using Babel AST

    The Vue 3 build system (specifically in const-enum.js and the patchTypes plugin) performs source-to-source transformations by parsing code into an Abstract Syntax Tree (AST) using @babel/parser and applying changes via magic-string. This allows the build process to programmatically identify and modify specific language constructs, such as converting constant enums or stripping internal type markers.

  9. Enable TypeScript type checking in JavaScript files

    You can enable TypeScript type checking for a specific JavaScript file by adding the // @ts-check comment at the top of the file. This allows the IDE/compiler to flag type mismatches without requiring a full .ts conversion.
    // @ts-check
let x = 3;
x = ''; // Type 'string' is not assignable to type 'number'.ts(2322)

  10. Develop the Vue SFC Playground

    The dev-sfc command is used to develop the online playground. It executes two stages sequentially (run-s):

    1. Preparation (dev-sfc-prepare): Ensures dependent packages are bundled. If missing, it runs npm run build-compiler-cjs.
    2. Execution (dev-sfc-run): Runs the following in parallel:
      • dev compiler-sfc -f esm-browser: Bundles the SFC compiler for the browser.
      • dev vue -if esm-bundler-runtime: Bundles Vue with inline dependencies.
      • dev server-renderer -if esm-bundler: Bundles the server renderer.
      • dev-sfc-serve: Starts the playground development server.

  11. How scripts/build.js triggers Rollup

    The build method in scripts/build.js executes Rollup by passing configuration via the --environment flag. This allows rollup.config.js to dynamically adjust the build based on the target package and desired format.
    await execa(
  'rollup',
  [
    '-c',
    '--environment',
    [
      `COMMIT:${commit}`,
      `NODE_ENV:${env}`,
      `TARGET:${target}`, // Package name
      formats ? `FORMATS:${formats}` : ` `,
      prodOnly ? `PROD_ONLY:true` : ` `,
      sourceMap ? `SOURCE_MAP:true` : ` `
    ]
      .filter(Boolean)
      .join(',')
  ],
  { stdio: 'inherit' }
)

  12. Generate and bundle TypeScript declaration files

    Vue 3 generates bundled .d.ts files for its packages, which is required for type testing (e.g., dts-test). The process involves two main steps:

    1. Compilation: tsc -p tsconfig.build.json is executed. This uses a configuration that extends tsconfig.json with declaration: true and emitDeclarationOnly: true, outputting individual declaration files to a temp directory.
    2. Bundling: rollup -c rollup.dts.config.js is executed. This uses rollup-plugin-dts to bundle the individual files from temp into a single <package-name>.d.ts file in the dist folder using the esm format.