babel-cli: add --skip-initial-build option to only compile on changes when watching

[lxe]

Currently, the -w option would build the files, and then keep the process running, recompiling the files on changes.

Consider this 'watch' workflow:

#/usr/bin/env bash

watch () {
  # compile sources and watch for changes
  babel src -d dest -w &

  # restart server when anything in dest/ changes
  nodemon --watch dest dest/server &

  wait
}

This causes 2 problems:

• Since nodemon dest/server can start before babel finishes compilation of
• Nodemon would pick up every change the in dist/ and restart the server on initialization multiple times.

Let's compile the files before starting the watch commands:

#/usr/bin/env bash

watch () {
  # compile sources
  babel src -d dest

  # compile sources and watch for changes
  babel src -d dest -w &

  # restart server when anything in dest/ changes
  nodemon dest/server &

  wait
}

This solves the first problem, however now we are recompiling things twice, which causes nodemon to flap.

We can solve all that with sleep, but that's far from ideal.

Instead, lets introduce --just-watch:

#/usr/bin/env bash

watch () {
  # compile sources
  babel src -d dest

  # just watch for changes and recompile when needed
  babel src -d dest -w --just-watch &

  # restart server when anything in dest/ changes
  nodemon dest/server &

  wait
}

This way nodemon always starts cleanly.

[loganfsmyth]

Do you happen to know if there's precedent for a flag like this with any other tools? Babel can't be the only tool where this is an issue. I don't have a problem with this flag, but I feel like it'd be clearer with a name like "--skip-initial-build`

[lxe]

@loganfsmyth this is similar (in spirit) to the --ignoreInitial chokidar flag, which won't emit events on "initial" change. The need for this is better illustrated by programatical usage of build of tools, such as babel (via gulp, etc), where we can listen on events and dispatch actions with ease. It's harder when using vanilla CLI tools and bash.

As far as the name is concerned, --skip-initial-build is definitely better.

[lukehorvat]

If we're going to think about nodemon, we might want to handle removal of files too. Babel's -w doesn't clean up removed/renamed files from the output directory.

Currently I use three package.json scripts to handle it:

{
  "build": "rimraf dest && babel src -d dest",
  "start": "npm run build && node dest",
  "watch": "nodemon -w src -x npm start"
}

But it recompiles the entire app every time something changes, which is not ideal.

[codecov-io]

Current coverage is 87.83%

Merging #3489 into master will not change coverage

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  Hits           8895       8895          
  Misses         1233       1233          
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[lxe]

I'm not sure why this had such major coverage impact

[loganfsmyth]

Babel is generally pretty strict about arguments. It would be good to show an error if commander.skipInitialBuild is true but commander.watch is false.

[loganfsmyth]

We should also squash these commits before landing this.

[loganfsmyth]

Thanks!