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# PoC Type-Checking JavaScript Using JSDocs
## Intro
TypeScript can be used on plain JavaScript files documented with JSDoc comments
to check types without a build step. This is a proof of concept to show
viability and benefits of doing this for a "typical" component.
* [Handbook: Type Checking JavaScript Files][handbook]
* [JSDoc Reference][jsdoc]
## New files
* `tsconfig.json`: at the root of a TypeScript "project" configures how to find
and check files.
* `types/globals.ts`: defines available globals, types and various utilities.
* `types/XPCShellContentUtils.sys.mts`: an example of type definitions file for
an external module which was automatically generated by `tsc`.
## How to use and expectations
Use [npm or yarn to install][download] TypeScript.
Then run `tsc` in the extensions directory to check types:
```
mozilla-central/toolkit/components/extensions $ tsc
```
You can also use an editor which supports the [language server][langserv].
VSCode should pick it up automatically, but others like Vim might need
[some configuring][nvim].
Other than continuing to use JSDocs, trying to follow guidelines below, and
hopefully remembering to run `tsc` occasionally, for now there is explicitly
*no expectation* that all new code must be fully typed, or pass typecheck
on every commit.
If you are in a hurry, or run into a confusing type problem, feel free to
slap a @ts-ignore and/or ask for help from someone more familiar. Hopefully
as workflow gets better integrations, we all learn some typescript along
the way, and the whole process remains non-disruptive.
## Guidelines for type-friendly code
Using more modern and idiomatic code patterns can enable better type inference
by the TypeScript compiler.
These fall under 5 main categories:
1) Declare and/or initialize all class fields in the constructor.
* (general good practice)
2) Use real getters and redefineGetter instead of defineLazyGetter.
* (also keeps related code closer together)
3) When extending, don't override class fields with getters, or vice versa.
* https://github.com/microsoft/TypeScript/pull/33509
4) Declare and assign object literals at the same time, not separately.
* (don't use `let foo;` at the top of the file, use `var foo = {`)
5) Don't re-use local variables unnecessarily with different types.
* (general good practice, local variables are "free")
### @ts-ignore recommendations
*Don't* use `@ts-ignore` for class fields and function or method signatures.
*Feel free* to use it:
* locally inside methods,
* when the alternative is more noise than signal,
* when doing so isn't preventing passthrough of type-inference through other
parts of the codebase.
[handbook]: https://www.typescriptlang.org/docs/handbook/type-checking-javascript-files.html
[jsdoc]: https://www.typescriptlang.org/docs/handbook/jsdoc-supported-types.html
[download]: https://www.typescriptlang.org/download
[langserv]: https://github.com/typescript-language-server/typescript-language-server
[nvim]: https://www.reddit.com/r/neovim/comments/131l9cw/theres_another_typescript_lsp_that_wraps_the/
|
