website/content/blog/type-checking-javascript-files-using-typescript.md

title	date	draft	tags	math	medium_enabled
Type Checking Javascript Files Using Typescript	2023-09-19T15:30:02-04:00	false		false	false

There's a nice and easy way to type check Javascript files without having to set up an entire project or adjust the code. It only takes understanding some of the flags within the Typescript compiler.

To install:

npm install -g typescript

Typecheck a file:

tsc --checkJs --allowJs example.js --outfile /dev/null

Flag	Description
--checkJs	Allow JavaScript files to be a part of your program.
--allowJs	Enable error reporting in type-checked JavaScript files

Since tsc is primarily a compiler, it will produce code output. We're only concerned with seeing if there's any type errors, so we throw away the compiled code by writing to /dev/null.

Now it's likely that we're using features of the standard library or the DOM in our Javascript code. We need to include a couple flags for Typescript to understand them.

Flag	Description
--target	Set the JavaScript language version for emitted JavaScript and include compatible library declarations.
--lib	Specify a set of bundled library declaration files that describe the target runtime environment

You can see tsc --helpfor the long list of options. Here's an example for targetting ES2022 with the DOM library.

tsc --checkJs --allowJs --target es2022 --lib es2022,dom example.js --outfile /dev/null