Provide a watching build mode that writes to the disk in development

See https://github.com/facebookincubator/create-react-app/issues/1018#issuecomment-261792488 for a discussion which leads to the creation of this issue.

I propose adding a new command (working name build-dev) which watches for changes and rebuild the bundle (effectively, given the current CRA implementation, it should execute webpack --watch), then writes it onto filesystem.

The main motivation is an easier integration path with non-Node.js dev servers: such servers could just serve continuously updating bundle from the filesystem.

The only desired configuration options is an --output, which could probably be specified via a command line argument:

% npm run build-dev -- --output ../static/www

Alternative command name suggestions:

• watch
• build-watch

[gaearon]

My concern: the concept of a dev server is already confusing to many people and I’ve seen countless issues from people trying to find dev server output in the filesystem, adding scripts with hrefs to local files, when they didn’t need it.

So I’m worried introducing this feature is useful in some advanced cases but might make everything more confusing in simple cases for people who mistakingly start with build-dev instead because they haven’t seen a dev server before when dev server is exactly what they need.

[leftdevel]

(As the OP of #1018) : For me it all boils down to:

• Write the dev bundle that results from running npm start, to the file system.
• ‘watch’ for changes (in dev mode).

@gaearon makes a great point saying that CRA biggest win is saving the developer from configuring the dev server which is the most difficult feature. I’d be happy if at least we could access the dev bundle.js file from the filesystem, I wouldn’t care if the dev server is started, I would just ignore it.

My argument is that:

CRA is quite appealing to non Node.js developers. There are many experienced developers in python, php, .net out there, but not all of us/them are that experienced in frontend set ups.

CRA makes it a no-brainer to use react. And to be honest, these days it’s way more difficult to set up a React project than an ember or angularjs 1.x project. React API is pretty simple and powerful, but when you introduce React to your organization projects, it’s not just React, but all its friends which are hard to master (webpack, babel, presets, linters, etc).

Not sure if you guys have read this blog post yet https://goo.gl/HviDRn, but it’s about a company that has a backend in Flask and were evaluating a frontend framework. They talk about how difficult is to keep up with the shifting ecosystem around React, and I’d totally agree on their pain points. At the end they chose Emberjs because of their CLI tools that makes it super easy to integrate.

I think CRA was created with the mindset of helping people that are just starting with React, and playing around with a few simple tutorials, but it has the potential of becoming a profesional tool for people that don’t have that much time to spend learning webpack, babel, if to use fetch or superagent, what Promise polyfill to use, etc.

We need something opinionated that just works. Note that I’m totally fine with the already existing features in CRA.

Sorry for the long post. ** flies away **