Converting an old JS project to TS

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Converting an old JS project to TS

It is not as daunting as it seems


3 min read

Having written my previous post on using TypeScript, I felt it was the opportune time to convert an old project to TS.

I decided on my export Figma SVGs via CLI as my first JS-to-TS conversion. I built this about 2 years ago, and hastily shipped it out back then as:

  1. I wanted to get it out as soon as possible (build fast amirite)
  2. I didn't know the benefits of TS yet


  1. Install required dependencies
  2. Create a tsconfig.json file
  3. Rename '.js' to '.ts'
  4. Update package.json entry point
  5. Fix errors, typing, and refactor
  6. Update tests and add jest.config (optional)


Converting an old project from JS to TS is not as daunting as it seems. While obviously, my project is a small one compared to way bigger open source projects, the steps are pretty much similar. A PR of the changes I have done to do this conversion can be seen here.

Install the required dependencies

For my project, these are ts-node, tsc, typescript, and ts-jest.

ts-node is the TS version of node, and allows you to execute TS files via ts-node <filename>.ts. tsc is the TS compiler (which comes included when you install typescript).

ts-jest is the jest transformer that allows jest to test our project after we have converted it to TS.

Create a tsconfig.json file

tsconfig is a configuration file in the root of the TS project. It is used to tell the TS compiler how you want to compile your project.

Rename '.js' to '.ts'

This is a straightforward step as it renames the file type of every file in the project from .js to .ts. If you have JSX files, you will need to rename them to .tsx.

Update package.json

I needed to change the entry point of my project from main.js to main.ts.

Fix errors, typing, and refactor

This was the most time-consuming part of the conversion process. As I was using ES6 in my tsconfig, I can now use modules. This meant doing this import axios from 'axios' instead using require const axios = require('axios')

I also needed to start typing the arguments used by my functions.

Aside from that, I took the opportunity to refactor several parts of my project. One such refactor was to consolidate the constants used in the project into a single file.

Update tests and add jest.config

A jest.config is also necessary as jest will use a preset to use the TS files. I also updated my tests.


To answer what a friend asked on twitter:

Any challenges u met?

โ€” Estee Tey (@estee_tey) September 3, 2022

I do not think there were major ones that blocked me from completing this TS conversion. Rather, it was my inexperience in converting a project to TS that I felt was a barrier. I had read several guides to get a bird's eye view of what I needed to do.

I highly recommend you to start using typescript in your project(s) if you have yet to ๐Ÿ˜