Add first class Javascript/Typescript support to the Mill build tool

[monyedavid]

Related Issues

#3927

Checklist

• example/jslib/basic
  • 1-simple/: A minimal Typescript module demonstrating typechecking/running/testing/bundling of a Node CLI tool
  • 2-react/: A minimal Typescript module demonstrating typechecking/running/testing/bundling of a React-based web application, served using a static HTML page
  • 3-custom-build-logic/: A Typescript module with custom build logic

[lihaoyi]

Each top-level bullet of the original bounty should be its own PR. For this PR, just focus on example/jslib/basic/, and we can review/merge/payout the first part of the bounty first. Extending it for further top-level bullets can come later

[lihaoyi]

It's a start and good that tests pass, but the code in the various build.mill example projects is far too messy to serve their purpose as documentation examples.

For reference, look at the equivalent build.mill files in example/pythonlib/basic/ or example/kotlinlib/basic/ for what I would expect the build.mill files to look like. Everything else should be pushed upstream into javascriptlib/src/ library traits and helpers, and those will have to be organized appropriately (see pythonlib/src/ as an example)

[lihaoyi]

@jodersky could you give this a review when you have some time? Since your work on the Python side is very similar you may have ideas that are transferable to JS/TS

[jodersky]

Will do!

[jodersky]

I think this would be easier to review and improve if each example were its own pull request, starting with only the basic example.

A couple of things stand out to me:

• I think that testing can be factored out into its own module, without having to subclass modules and have custom tasks. Eg. instead of defining the "main" module to be a JestModule with testSources etc, the test sources could become the normal sources of a nested JestModule which would extend something like `TestModule. This is how Mill's Java and Scala support does it, and is what I used for the Python support as well.

• It will also be easier to approach this by documentation to examples first, and also commenting what every task is supposed to do.

In general, I'd suggest to take a look at what we're doing with pythonlib and use it as inspiration on how to approach this. The reason is that it's still fairly minimal and to-the-point, but it does strive to follow general tendencies and structure of the established javalib and scalalib.

[monyedavid]

I think this would be easier to review and improve if each example were its own pull request, starting with only the basic example.

A couple of things stand out to me:

• I think that testing can be factored out into its own module, without having to subclass modules and have custom tasks. Eg. instead of defining the "main" module to be a JestModule with testSources etc, the test sources could become the normal sources of a nested JestModule which would extend something like `TestModule. This is how Mill's Java and Scala support does it, and is what I used for the Python support as well.
• It will also be easier to approach this by documentation to examples first, and also commenting what every task is supposed to do.

In general, I'd suggest to take a look at what we're doing with pythonlib and use it as inspiration on how to approach this. The reason is that it's still fairly minimal and to-the-point, but it does strive to follow general tendencies and structure of the established javalib and scalalib.

the abstraction (TestModule) is done in another pr, as that extends beyond the scope of this task set (and many other things here really)

I can open that next after this is done / move them here whichever you prefer @jodersky

Separating into smaller pr's would be somewhat difficult, its length is due to the modifications made to TypescriptModule and creating the other two modules. I promise the other pr's would be much smaller.

Also as there are different Test frame works I will be working with through the project,
we have a TestModule extends TypeScriptModule, which contains core functionality for its extenders.
JestModule, MochaModule and others extend TestModule, making it easier to use and reducing duplicate code

[lihaoyi]

I think this looks in decent enough state to merge. It's not perfect, but it would be useful to get some of the follow on use cases in place first before polishing further.

@monyedavid there's a test failure that needs to be fixed, then I'll merge this and close out the bounty

[lihaoyi]

Merged this, thanks @monyedavid for bearing with all our review and feedback! I think the PR is in a great state now to build upon for later, and I expect follow-up PRs to be easier since you are now more familiar with the Mill tool, codebase, and coding conventions.

Please send me your international bank transfer details over email and I will wire over the bounty