I prefer typescript to javascript. There are multiple of reasons for TS, but sometimes I join to projects made in JavaScript.

Available paths

Then there are multiple options and path to choice. We have to consider some options. All of them has some benefits and costs:

1. Leave Javascript as is and develop the application as is.

• + It is good to have one language instead of two
• + You can start coding faster because you don’t have to migrate
• + Javascript is more flexible
• + All libraries you can use in JS without any tricks
• + Building is faster
• - You have no typescript

2. Migrate all the code to Typescript

• + It is goo to have one language than two
• + You get all typescript’s benefit from the beginning
• - It can take you a lot of time to migrate everything

3. Migrate to Typescript step by step, file by file

• + You can start coding new features in TS immediately
• + You will not spend huge time on migration at the beginning
• - You have 2 languages in your project
• - You have to think about the migration every day
• - You current tasks will take you a bit more time, because of migrations
• +/- You have to learn how to improve code with small steps

As you know I am a great fan of ongoing refactoring, so 3rd approach is natural for me. Now there is only one question:

How to migrate from JavaScript to TypeScript in small steps?

1. Add tsconfig.json file to your project.
   • Make sure that allowJs is set to true
   • You can get a simple sample template from https://www.typescriptlang.org/docs/handbook/migrating-from-javascript.html
2. Build the application and fix all errors if there are any
3. Don’t change extensions from .js to .ts you do don’t have to
4. Create a new file with .ts extension.
   • and use it in current application to be sure that everything is working correctly
5. If you have to touch a .js file, to modify or to import from .ts file. Then
   • Change file extension to .ts
   • Try to build the application. It probably fails.
   • Everywhere where are no types use any
     • or I recommend to use fakeAny. You can read more about that in post How to handle noImplicitAny during migration fom js to ts
6. If you need to use some types from migrated file or when you need to pass some strongly types parameters. Then add or import concrete types and use them instead of any or fakeAny.

Continue migrating in next months

As you can see with that approach you will have all benefits from TypeScript without a huge amount of wasted time. But you mustn’t forget to continue the migration in next month. To have more and more positive aspects of TypeScript.

What do you think about that approach?

Leave the comment to let me know. I will be grateful. Thank you.

• javascript
• typescript
• legacy
• clean code
• ongoing refactor