refactoring-from-javascript-to-typescript

Refactoring from JavaScript to TypeScript:

1. Understand the TypeScript Compiler:

• Get to know the TypeScript compiler (tsc) as it's the tool that will convert your TypeScript code into JavaScript.
• Familiarize yourself with compiler options in the tsconfig.json file, which controls how your TypeScript is compiled.
• Understand that tsc can also perform type checking and emit no code if type errors are found, ensuring quality in your codebase.
• Explore the watch mode (tsc --watch), which automatically compiles your files as you make changes, speeding up the development process.

2. Gradual Adoption:

• You don't have to convert your entire codebase at once. TypeScript supports gradual adoption, allowing you to migrate file by file.
• Use the allowJs option to include JavaScript files in the compilation process alongside TypeScript.
• Implement TypeScript in new features or modules first, while leaving older JavaScript code as-is until it can be refactored.
• Consider setting up a build process that compiles TypeScript and JavaScript side by side, so you can run both seamlessly during the transition.

3. Type Definitions for Libraries:

• Utilize DefinitelyTyped, a repository of type definitions for JavaScript libraries.
• This allows you to gain the benefits of TypeScript's type checking for existing JavaScript libraries.
• If DefinitelyTyped doesn’t have the type definitions for a library you're using, you can write your own declaration files (*.d.ts) to include custom type annotations.
• Regularly update your type definitions as you update your libraries to ensure compatibility and take advantage of new type features.

4. The 'any' Type for Quick Migration:

• Start with the any type for complex parts of the code that you can't type immediately.
• Gradually replace any with more specific types to increase type safety.
• Use tools like ESLint with TypeScript support to identify and warn about the usage of any, which can help in reducing its use over time.
• Document the intent to replace any with proper types using TODO comments to ensure that it's addressed eventually.

5. Leveraging Type Inference:

• Take advantage of TypeScript's type inference where you don't have to explicitly define types everywhere.
• As you refactor, TypeScript can infer types from your JavaScript code, making the transition smoother.
• Learn where type inference is strong (like initializing variables or return types of functions) and where you need to explicitly define types (like function parameter types).
• Write your code in a type-friendly way that maximizes TypeScript's ability to infer types, such as avoiding the use of complex and nested object literals that can be difficult for the compiler to infer.

6. Enforce Stricter Checks Over Time:

• Begin with loose type checking and progressively enable stricter compiler options, like strictNullChecks, noImplicitAny, and strictBindCallApply.

7. Refactor with Interfaces and Types:

• Create interfaces and types to describe the shapes of objects, which can help document the intent and structure of your code.

8. Modularize Your Code:

• TypeScript works best with modules. If your JavaScript isn't already modularized, consider refactoring it into modules during the TypeScript migration.

9. Use TypeScript's Advanced Types:

• Learn and apply advanced TypeScript types like enums, generics, and union types to improve code quality and maintainability.

10. Incorporate Type Guards:

• Implement type guards to safely narrow down types in your code, replacing runtime type detection logic.

11. Handling this Context Safely:

• Be aware of how TypeScript handles this and use arrow functions or explicitly type this in function parameters when needed.

12. Take Advantage of Tooling:

• Use TypeScript integrated development environment (IDE) support for refactoring, such as variable renaming, interface extraction, and type checking in real-time.

13. Testing During Refactoring:

• Ensure you have good test coverage and run your tests frequently during the refactoring process to catch any issues early.

14. Document with JSDoc:

• Use JSDoc annotations in your JavaScript code, which TypeScript can interpret to provide better type checking before you fully annotate with TypeScript types.

15. Community and Support:

• Engage with the TypeScript community for support. Many common issues have been faced and documented by others during their migrations.

JSDoc - Introduction and Importance:

1. What is JSDoc:

   JSDoc is a markup language used to annotate JavaScript code. It helps you document the structure, purpose, and usage of your code.

2. Self-documenting Code:

   Using JSDoc makes the code self-documenting. It means that the codebase is easier to understand and maintain, as the documentation lives close to the code it describes.

3. Improves Code Readability:

   JSDoc comments enhance the readability of the code by explaining what a function does, its parameters, and what it returns.

4. Facilitates Intellisense:

   JSDoc supports code completion and intellisense in many code editors, like Visual Studio Code, making it easier for developers to write and understand code.

5. Type Safety in JavaScript:

   Although JavaScript is dynamically typed, JSDoc comments can specify types, which can be checked by some transpilers or linters for type safety.

6. API Documentation:

   JSDoc comments can be used to generate API documentation automatically, which is useful for both internal and external APIs.

7. Onboarding New Developers:

   A well-documented codebase with JSDoc can significantly reduce the time required for new developers to understand the code.

8. Describing Data Structures:

   JSDoc is great for describing complex data structures, which helps in visualizing and understanding how data is organized.

9. Annotation of Asynchronous Code:

   JSDoc can annotate asynchronous functions, clarifying the promise structure and what the asynchronous code is expected to do.

10. Support for Overloaded Functions:

    JSDoc can document multiple signatures for a function, which is helpful in a dynamic language like JavaScript where functions can have different behaviors based on argument types.

11. Enhanced Debugging:

    Code with JSDoc annotations can be easier to debug, as the documentation provides insights into what the code is supposed to do.

12. Versioning Information:

    JSDoc can include versioning information in the documentation, which helps track changes and the evolution of code over time.

13. Deprecation Warnings:

    JSDoc annotations can indicate deprecated features, guiding developers to avoid using obsolete code.

14. Custom Tags:

    JSDoc allows the creation of custom tags, which can be tailored to fit the documentation needs of a particular project.

15. Integration with Type Checking Tools:

    Tools like TypeScript can consume JSDoc annotations to provide type checking in regular JavaScript files, bridging the gap between JavaScript and TypeScript's static typing.

Why Use JSDoc:

JSDoc is used for maintaining a high level of clarity and understanding within the code. It allows developers to quickly get an idea of what the code does without diving deep into the implementation details. This is especially important in large or complex projects where keeping track of all the moving parts can be challenging. By documenting functions, parameters, returns, and more, JSDoc helps to define clear interfaces within the code, making it easier to use and maintain. It also facilitates better tooling support for JavaScript development, like type checking and auto-completion, which leads to improved developer productivity and code quality.

8. Modularize Your Code:

• JavaScript started without a module system, which led to different patterns to keep code organized, like the Revealing Module Pattern or the use of global variables to share code across different files. This approach can lead to maintenance issues and name collisions as the codebase grows.

• Modules are a way to divide your code into separate, reusable pieces, with their own private scope. Each module can export certain parts of its code, like functions, objects, or types, to be used in other modules, and it can import just what it needs from others.

• TypeScript is designed to work seamlessly with this modular approach, supporting the ES Modules standard, which is the official standard for packaging JavaScript code for reuse. ES Modules allow you to use import and export statements to share code between different files.

• When you refactor JavaScript to TypeScript, it's recommended to structure your code as modules rather than as a single or several large files. This can have several benefits:

  • Scope Management: By keeping different parts of the code in separate modules, you avoid polluting the global scope, which reduces the chances of name conflicts.

  • Reusability: Modules can be imported by the parts of your application that need them, promoting DRY (Don't Repeat Yourself) principles.

  • Maintainability: Smaller, focused modules are usually easier to understand, test, and maintain than larger files.

  • Tree Shaking: When using modules, modern JavaScript bundlers like Webpack or Rollup can perform tree shaking, which is the elimination of unused code from the final bundle sent to the browser. This can lead to smaller and faster-loading web applications.

  • Clear Dependencies: With explicit import and export statements, it's easier to see the dependencies between different parts of your code, making it clearer how your application is structured.

• During the migration, you can start by wrapping your existing JavaScript code in module syntax. Then, as you convert your code to TypeScript, you can refine the exports and imports to make use of TypeScript's type system for even stronger guarantees about the interfaces between modules.

• Finally, TypeScript's type system and tooling are at their most powerful within a modular codebase. You get better autocompletion, easier refactoring, and more precise type checking when the type information can be clearly communicated between different parts of your application via module exports and imports.