Dynamically fill in "homepage" in package.json with environment varaiables

Hi everyone,
I have this use case where I need to deploy my react app to both root url and a subdirectory.
I use subdirectories to deploy demo instances of the app.

For example:

http://dev.box/                          <-- this is for master branch
http://dev.box/branch-demo/branch-1      <-- CI systems will build and deploy branches to subdirectories
http://dev.box/branch-demo/branch-2

From previous discussions [1], I can use "homepage" field to adjust css and js paths in generated index.html. But for my use case, I'll need to change the "homepage" base on which location I'm deploying to.

I imagine solutions like:

HOMEPAGE='branch-demo/branch-1' npm run build

or like this in package.json (if possible):

{
  "homepage": "${HOMEPAGE}"
}

Any suggestions on how I can achieve dynamic values for homepage base on env variables?
Thanks!

[EnoahNetzach]

Once #937 (or similar) will land it would be possible via the PUBLIC_URL environment variable.

[gaearon]

Closing in favor of #937, let's track that instead.