Flow required module not found - reactjs

Question

Using Webpack 2, Flow 0.46.0
I have a pretty large app I am developing, so am using Webpack resolve modules to create alias import names e.g. '../../../../constants/ServiceURI' to 'constants/ServiceURI'
Everything works fine until I add flow. There must be a way to use mapper or resolve_dirname to fix this, but I cannot figure out how. No matter what I do it breaks flow.
I really want to use flow, but this is a blocker for me.
Project structure:
./flowconfig
./webpack.config.js
./src
/js
/constants
/actions
/...
/css
Webpack config looks like:
resolve: {
modules: [
path.resolve('./src/js'),
path.resolve('./src/js/constants'),
'node_modules'
],
extensions: ['.js', '.jsx']
Flow config looks like:
[ignore]
.*/node_modules/*
[include]
<PROJECT_ROOT>/src/js/
[libs]
[options]
esproposal.class_static_fields=enable
esproposal.class_instance_fields=enable
esproposal.export_star_as=enable
esproposal.decorators=ignore
# Tried this
module.name_mapper='^constants$' -> '<PROJECT_ROOT>/src/js/constants'
# Tried using this too
module.system.node.resolve_dirname=./src/js
module.system=haste
munge_underscores=true
[version]
0.46.0
Flow Error:
rc/js/actions/ActionActivity.js:6
6: import { ACTIVITY_API } from 'constants/ServiceURI'
^^^^^^^^^^^^^^^^^^^^^^
constants/ServiceURI. Required module not found

Answer 1

The regular expression you're using in the module.name_mapper does not match constants/ServiceURI. It only matches exactly constants. You additionally need to match anything that comes after constants to be able to resolve all modules inside the constants directory.
module.name_mapper='^constants/\(.*\)$' -> '<PROJECT_ROOT>/src/js/constants/\1'
Where \( and \) create a capturing group (the slashes are required), which you can refer to as \1. For more information see module.name_mapper.