Learn TypeScript w/ Mike North

Steps 1, 2 and 3 - Adding our first types

October 27, 2023

Table of Contents

In this chapter, we’ll follow steps 1, 2 and 3 as described in the approach for converting large projects from JavaScript to TypeScript.

  1. Get TypeScript into your build toolchain, type-checking your existing .js files, in the most permissive mode possible
  2. Rename some files from .js to .ts, fixing only the things necessary to get the compile working
  3. Forbid implicit anys, replacing them throughout the codebase with explicit anys, {}s or more descriptive types

    • Improve error handling by enabling useUnknownInCatchVariables in tsconfig
    • Install community-supported types from DefinitelyTyped where necessary

Getting started with the big project

Let’s look back at our project git repo. We’ll be working a little bit with the packages/chat-stdlib library we created earlier, and the main chat app in packages/chat.

From the root of the git repo, enter the folder

cd packages/chat
yarn dev

If this is your first time running the project, it may take a minute or two to build. Start the app up to make sure everything works properly. Look for the console output

Serving on http://localhost:3000

This is an indication that the UI should be viewable on http://localhost:3000

You should see something like this

chat app home screen

Type-checking JavaScript

To enable type-checking of JavaScript files, add the following line to the compilerOptions object in packages/chat/tsconfig.json

"checkJs": true,

This project uses parcel to build and run the UI, which knows which parts of TypeScript syntax it can delete in order to arrive at JavaScript, but it performs no type-checking. To have TypeScript compile our project, you can run

yarn typecheck

You should see a couple of errors, all within the src/ folder

Found 8 errors in 4 files. Errors Files 1 src/index.js:8 1 src/ui/App.jsx:22 4 src/ui/components/Channel.jsx:19 2 src/utils/networking.js:40

Go ahead and fix these. Some hints:

This comment

* @type {React.FC<React.PropsWithChildren<any>>} param0

when added above any React function component will allow for an arbitrary props object, and a correctly-typed children property.

React.useState() defaults to a state type of undefined.

const [list, setList] = React.useState()
const list: undefined
if (list) list.length
Property 'length' does not exist on type 'never'.2339Property 'length' does not exist on type 'never'.
const list: never

You can provide a type, by providing a default argument, even if it has a possibility of being null or undefined

* @type {any[]}
const initialList = null;
const [list, setList] = React.useState(initialList)
const list: any[]
if (list) list.length
const list: any[]

Renaming from .js to .ts

This can be a bit tedious to do without tools, so I’ve written a script for you to invoke from within the packages/chat folder.

node scripts/rename-to-ts.mjs

I recommend making a git commit immediately after the rename (but not necessarily a PR) so that git history is preserved. Sometimes git is not perfect at understanding a rename in combination with other changes in a single commit, and you end up with ultimately a deletion of the old file and an addition of a new one.

and don’t forget to make this small change to /index.html

--- a/index.html
+++ b/index.html
@@ -8,6 +8,6 @@
<body class="font-sans antialiased h-screen">
<div id="appContainer" class="w-full h-full"></div>
- <script src="src/index.js" type="text/javascript"></script>
+ <script src="src/index.ts" type="text/javascript"></script>

Keep running

yarn typecheck && yarn test

and fixing small things until both commands pass

Forbidding implicit anys

Lets go to packages/chat/tsconfig.json and add the following option

"noImplicitAny": true


yarn typecheck

You might see

Found 51 errors in 18 files.

That’s a lot of errors. This is a great opportunity to demonstrate how we can refine a sub-part of our project at a time. Create a new file packages/chat/src/utils/tsconfig.json.

"extends": "../../tsconfig.json",
"compilerOptions": {
"noImplicitAny": true
"include": ["."]

and typecheck again, but just for the src/utils folder

yarn tsc -P src/utils

This should be much more approachable! Please fix all the type errors. Keep in mind that in a .ts file, the type information provided by JSDoc comments is ignored

Repeat the same procedure for the src/data and src/ui folders (in that order). You can move the inner tsconfig.json from folder to folder, safely.

Finally, delete this tsconfig.json and set the "noImplicitAny": true in ../chat/tsconfig.json. Run

yarn typecheck

Fix any remaining errors in the ./tests folder, and consider yourself done with step 3

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