Add flags and dead code elimination

[gaearon]

Hacks like #52 are frustrating.
We need a system that lets us write stuff like

if (__REPO__) {
  var path = ...
} else if (__DEPENDENCY__) {
  var path = ...
} else if (__EJECTED__) {
  var path = ...
}

and eliminate all other branches on eject.

[Jiansen]

@gaearon , if I understood your concerns correctly, the frustrating part is that code like the following will become more difficult to maintain with the increased number of commands in later versions.

var relativePath = isInNodeModules ? '../../..' : '..';
if (process.argv[2] === '--debug-template') {
  relativePath = '../template';
}

If so, with match-js or a similar approach, I should be able to refactor the code similar to the following, though not exactly in your proposed form

relativePath = match( env )(
  CASE(REPO, ...),
  CASE(DEPENDENCY, ...),
  CASE(EJECTED, ...),
  CASE(otherwise, ...)
);

where env could be process.argv or other value that represents the execution environment.

Currently, I see that --debug-template and --smoke-test are both set at process.argv[2], and there is no check for EJECTED because scripts/eject.js is the only involved script.

To help me understand your problem correctly, could you, from a user's perspective, list some test commands that set flag(s)?

In my forked repo, I will test my ideas. I hope that we can move towards a solution that you wanted.

Cheers

[gaearon]

These flags are all internal and we don’t want to expose this code for the user. They are only useful for testing and local development of create-react-app itself. So if statements are not a problem and we don’t need to refactor them. What we need is a way to cut some code after ejecting.

[Jiansen]

@gaearon ,

The above change deletes dead code on ejecting; However, this does not prevent users from using npm start --debug-template before react-scripts is ejected. At lease, we don't expose the flag to general users.

I will think if we could hide the flag and cut off the code on the time of building react-scripts.

If you think the current progress is OK, I will do some tests and create a PR.

[gaearon]

This was in #257.