How to Migrate from JavaScript to TypeScript

Migrating code from JavaScript to TypeScript can be complex and challenging but also rewarding. TypeScript offers several advantages, such as simpler refactoring and more effective error detection when writing code in the text editor. However, migration also involves integrating TypeScript-specific tools into the development process and adopting new approaches to coding and project management.

Let's dig deeper and find out more.

The Advantages of TypeScript  

The transition to TypeScript goes beyond a simple language change. It's a productivity strategy that brings value to the code and the development process. Here are some of the main benefits that TypeScript will give you: 

  • Static Typing: Detecting potential errors and inconsistencies captured by the TypeScript Language Server helps prevent mistakes and improves code quality by providing static analysis of data types. 
  • Improved Development Experience: TypeScript features, such as auto-completion and type inference, significantly enhance the development experience by providing contextual suggestions and reducing the need to write redundant code. 
  • Easy Access to Documentation: Development tools such as the Type System provide precise information about the interfaces and types used in the code, increasing confidence in its correctness. 
  • Early Error Detection: TypeScript's type checking helps identify errors in the early stages of the development process, saving time and reducing the cognitive load on developers. Errors can be captured before execution, helping cut costs in terms of time and resources. 

Initial Steps: Setting Up TypeScript 

The conversion process to TypeScript begins with TypeScript configuration through the 'TS config.json' file. For a hybrid JavaScript and TypeScript codebase, we recommend setting the 'allowJS' rule to true.
Another crucial decision involves the "strict" rule, which, when enabled, offers superior error detection. 

A successful transition to TypeScript also depends on the right tools, such as a compiler that converts TypeScript to JavaScript. Some popular options include Vite, Parcel, and TSX.  

Tools for Converting to TypeScript 

Numerous tools can facilitate the conversion process. For instance, MongoDB to TypeScript generators convert models into types, GraphQL Cogen generates fully typed queries, and Eslint helps detect conversion or refactoring opportunities from JavaScript to TypeScript.

Refactoring often involves creating new TypeScript files and migrating existing code from JavaScript files. This strategy can generate compilation errors, so renaming all JavaScript files to TypeScript is essential. 

Strategies for Typing the Code 

One can approach code typing in two ways: "as you go" or "from scratch." The "as you go" strategy allows for incremental progress, which is useful in situations with time constraints or limited resources. On the other hand, the "from scratch" strategy involves dedicating time and effort to complete the conversion from zero to one hundred percent. 

Typing should begin from the root of data definitions and expand towards more specific components. This process avoids problems where components may expect certain types that are not fulfilled. Tools like Zod for data validation and schema generation can significantly simplify data validations. 

Handling Dependencies 

When migrating from JavaScript to TypeScript, managing dependencies is a crucial step. Fortunately, many dependencies already have their corresponding types defined. If the type definitions are absent, you can seek out types in DefinitelyTyped and install the missing third-party packages, extract them from JSDoc comments, if any, or create declaration files manually.  

Advantages of Converting to TypeScript 

Once a project is fully converted to TypeScript, the following benefits can be observed: 

  1. Type Safety: TypeScript's static type checking leads to more reliable and robust code.
  2. Superior Tools and Auto-completion: Greater developer productivity with intelligent auto-completion and code navigation.
  3. Improved Collaboration: Better understanding of the codebase and early problem detection.
  4. Improved Maintainability: TypeScript's static typing reduces the likelihood of introducing bugs during code changes.
  5. Compatibility with the Ecosystem and Community: TypeScript has a large ecosystem and community that can enhance your development experience.
  6. Integration with Modern JavaScript Features: Write modern JavaScript code with TypeScript's support for the latest ECMAScript features.
  7. Future Compatibility: TypeScript is actively maintained and developed by Microsoft and provides a path to adopt future JavaScript features.

The path to TypeScript, while challenging, leads to more solid, maintainable, and efficient code. The initial hurdles of conversion are overshadowed by the long-term benefits that TypeScript brings to the development experience. A "proper approach" to transitioning to TypeScript is subjective and can depend on various factors such as project scale, complexity, team experience, and timelines. 

Key Takeaways 

  •  Incremental Transition: Consider an incremental approach instead of converting the entire codebase at once. This could involve setting up TypeScript and writing new features while leaving existing JavaScript code untouched until you're ready to refactor it. 
  • Type Definitions: Begin by adding types to the most critical parts of your application, such as the data models and interfaces. This will provide a solid base to expand upon.
  • Leverage Tooling: Use TypeScript conversion tools and linters to automate parts of the process and highlight conversion or refactoring opportunities.
  • Handling Dependencies: For each of your dependencies, check if TypeScript definitions are already available. If not, you may need to create custom-type definitions.
  • Testing and Validation: Regularly test your application throughout the conversion process to catch and resolve any breaking changes introduced during the conversion.
  • Training and Learning: Ensure your development team understands TypeScript's benefits and fundamental principles. This could involve tutorials, workshops, or pair programming sessions.
  • Peer Review: Incorporate thorough code reviews to ensure newly converted TypeScript code maintains high quality and conforms to best practices.

Remember, the key to a successful migration is planning, understanding the benefits and challenges, and taking a systematic transition approach while ensuring your application's functionality is maintained. 


This piece was written by Pablo Ventura from Encora.  

About Encora

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