Awesome

Persistent TypeScript compiler

This is a TypeScript compiler that can be used as a Bazel "persistent worker." If it is launched with --persistent_worker, then it will run in a loop, reading compilation arguments (in protobuf format) from stdin; doing a compile; and then writing the results (also in protobuf format) to stdout. (The format is defined by Bazel.)

For Bazel projects that do a lot of TypeScript compilation, this has two performance benefits:

It avoids compiler startup time.
It avoids unnecessarily re-parsing source files. Specifically, the runtime file, lib.d.ts, will only be read once; any any other .ts or .d.ts files will also only be read once.

In our internal usage at Asana, using bazeltsc has led to roughly a 2x to 4x speedup in TypeScript compilation (the numbers are affected by a variety of factors).

Installation

You will need to get bazeltsc into a place where Bazel can find it. One way to do this is by using Bazel's rules_nodejs; but you can do it any way you like.

Instructions if you are using rules_nodejs:

Follow the rules_nodejs setup instructions.
Add bazeltsc to your package.json:
```
npm install --save-dev bazeltsc
```
Run the Bazel equivalent of npm install:
```
bazel run @nodejs//:npm install
```
Copy tsc.bzl from node_modules/bazeltsc/tsc.bzl into your repo somewhere. (We intend to clean this up as soon as we figure out how to get Bazel to be able to find tsc.bzl directly from inside node_modules.)
Use the example from the example directory -- especially the WORKSPACE and BUILD file -- to finish setting things up.
It is essential that when you run Bazel, you invoke it with --strategy=TsCompile=worker. This is what tells Bazel to use bazeltsc as a persistent worker instead of as a regular tool that is invoked once per compilation.

You will probably want to add that to a .bazelrc file in the root directory of your project, so that you don't have to specify it on the command line:
```
# This will be used every time someone does `bazel build ...`:
build --strategy=TsCompile=worker
```

Experimenting with bazeltsc

Normally, you just let Bazel launch bazeltsc. But it is helpful to understand how bazeltsc is launched by Bazel, and how you can experiment with it yourself.

bazeltsc can run in any of three modes:

As a bazel persistent worker. Bazel will launch it with this command line:
```
bazeltsc --persistent_worker
```
At that point, bazeltsc is reading protobuf-formatted compilation requests from stdin; compiling; and returning protobuf-formatted results on stdout.
As a thin wrapper around tsc. Examples:
```
bazeltsc --outDir target foo.ts bar.ts
bazeltsc @argfile
```
This provides no functionality beyond what tsc itself provides; it is supported in case, for whatever reason, there are times when you want to tell Bazel not to use persistent workers.
In "debug" mode. If you want to experiment interactively with bazeltsc, run it like this:
```
bazeltsc --debug
```
Then, at the > prompt, enter a full tsc command line. This will let you see the speed difference between an initial compilation and a subsequent one.

For example:
```
bazeltsc --debug
> x.ts
Compilation took 890ms. Exit code: 0.
> x.ts
Compilation took 351ms. Exit code: 0.
```