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Multi-line Comment | JSDoc Comment

The /** */ syntax is used for creating a multi-line comment in JavaScript and other languages like TypeScript. When used in the specific format /** ... */ with an @type annotation inside it, it serves a special purpose known as a JSDoc comment. JSDoc comments are used to provide metadata about the code, including type information, descriptions, and other annotations.

What Is /** */?​

  • Multi-line Comment: This syntax is primarily for creating multi-line comments in JavaScript and TypeScript. Anything between /** and */ is ignored by the JavaScript engine and serves as documentation or notes for developers.
  • JSDoc Comment: When used with specific tags (e.g., @type, @param, @returns), it becomes a JSDoc comment, which provides additional information and type annotations that can be used by IDEs, documentation generators, and static analysis tools.

Purpose of /** @type {...} */​

The specific format /** @type {import('module').Type} */ is used for type annotations. Here’s a breakdown:

  1. Type Annotation: Specifies the type of a variable, function, or object. This is especially useful in JavaScript, which is dynamically typed, as it allows developers to use type checking tools like TypeScript without converting the entire codebase to TypeScript.
  2. IDE Support: Enhances code editors with features like autocompletion, type checking, and inline documentation. For example, in Visual Studio Code, using these annotations will provide you with IntelliSense support.
  3. Documentation: Helps generate documentation using tools that support JSDoc comments.

Example: Using JSDoc for Type Annotation​

Here’s an example of how you might use /** @type {...} */ in a JavaScript file to annotate the type of a configuration object:

/** @type {import('@docusaurus/types').Config} */
const config = {
  title: 'My Docusaurus Site',
  tagline: 'Dinosaurs are cool',
  url: 'https://your-docusaurus-test-site.com',
  baseUrl: '/',
  onBrokenLinks: 'throw',
  onBrokenMarkdownLinks: 'warn',
  favicon: 'img/favicon.ico',
  organizationName: 'facebook', // Usually your GitHub org/user name.
  projectName: 'docusaurus', // Usually your repo name.

  presets: [
    [
      '@docusaurus/preset-classic',
      /** @type {import('@docusaurus/preset-classic').Options} */
      ({
        docs: {
          sidebarPath: require.resolve('./sidebars.js'),
        },
        blog: {
          showReadingTime: true,
        },
        theme: {
          customCss: require.resolve('./src/css/custom.css'),
        },
      }),
    ],
  ],
};

module.exports = config;

Why Do We Need It?​

  1. Type Safety: Ensures that the values assigned to variables or passed to functions match the expected types, catching potential errors at development time rather than at runtime.
  2. Autocompletion and IntelliSense: Improves the development experience by providing contextual suggestions and type information in your code editor.
  3. Documentation: Provides clear, standardized documentation for your code, making it easier for others (or yourself in the future) to understand and use.

Conclusion​

The /** @type {import('module').Type} */ comment is a powerful tool for adding type information to JavaScript code. It enhances the development experience by providing type safety, better editor support, and improved documentation. While optional, it is highly recommended, especially in larger codebases where type-related errors can be more common.