FR: Bazel support

[sgammon]

[REQUIRED] Describe your environment

• Operating System version: macOS Mojave (10.14.4)
• Browser version: Chrome 70-something
• Firebase SDK version: Latest
• Firebase Product: All of them

[REQUIRED] Describe the problem

We have a frontend project using Bazel, specifically rules_closure. It would be awesome to gain Bazel support in the Firebase JS SDK, so we could potentially build against the SDK's sources.

The gains delivered by this support might include a deeper ability to tree-shake unused code from the Firebase SDK, for a given application.

Steps to reproduce:

1. Try to use Firebase SDK in JS from Bazel with rules_closure
2. Realize you will probably have to write an overlay file and use new_git_repository, or find a fork, or build against externs
3. Externs are not ideal because they are out-of-scope of the Closure compiler

Thank you in advance Firebase team

[hsubox76]

Thanks for the request. You'll have to bear with me as I'm not very familiar with Bazel, so can you help point me toward some specific things to look into that would be needed to make this work? Is it that we need to provide a BUILD file somewhere in the package that provides a target you can use?

[sgammon]

@hsubox76, i think so, yes. i'm also new to Bazel. i'll try to get this right! here is the longer story.

essentially, I have a Bazel project based on rules_closure and rules_sass. In this project, I address a number of projects (internal and external) via a WORKSPACE file.

in each case, the symbols provided by these internal and external projects amount to the rule types:

• sass_library: for SASS/SCSS sources
• closure_js_library: JS sources, in traditional Closure or Closure module format (i.e. goog.provide or goog.module)
• proto_library: for .proto files, which I presume are in use somewhere within Firebase

I wanted to build against sources, with the above rules, for firebaseui-web. so I made a fork, mostly to make changes to support GSS, and fix things that have broken after recent updates in the Closure compiler.

with this fork, I wrote an overlay file to surface it in Bazel. the overlay file is private at the moment, but I'd be up to share it, it makes our fork of firebaseui-web usable via Bazel.

we had to do the same with MDL, which is the UI library firebaseui-web is based on.

so this got us to thinking, why not build against the actual firebase-js-sdk, with Bazel? we tried to write another overlay file, but realized pretty quickly that the firebase SDK is quite different.

for instance, it's written in typescript. I have read in different places that Closure Compiler is involved in the build, but I haven't been able to find how to get those intermediate (Closure-annotated) sources out of the build pipeline. we found the typescript > Closure-annotated-JS conversion to be non-trivial, so we filed the FR :)

I'd be happy to contribute the above-mentioned code to the open source effort, if it helps, and I'd be happy to help on a PR on the main SDK, with development or testing, as you and your team see fit.

hope this helps and thank you for your help on this

[sgammon]

@hsubox76 i forgot to say, I'm currently building against the Firebase SDK via the externs. it works well enough, but I can't rewrite symbols across the boundary between my code and Firebase, and I can't tree-shake code out of the Firebase JS SDK that I don't use.