Support .mjs file extensions

Created by: lencioni

Is this a bug report?

Yes?

Can you also reproduce the problem with npm 4.x?

Yes

Which terms did you search for in User Guide?

mjs, extension, static, media

Environment

1. node -v: v6.2.2
2. npm -v:3.10.10
3. yarn --version (if you use Yarn):
4. npm ls react-scripts (if you haven’t ejected):

waypoint-test@0.1.0 /Users/joe_lencioni/waypoint-test
└── react-scripts@1.0.14

Then, specify:

1. Operating system: macOS Sierra
2. Browser and version (if relevant): Chrome

Steps to Reproduce

(Write your steps here:)

1. Clone https://github.com/colloquet/waypoint-test.git
2. npm install
3. npm start

Expected Behavior

Page loads without errors.

Actual Behavior

Page shows error: "InvalidCharacterError: Failed to execute 'createElement' on 'Document': The tag name provided ('/static/media/index.73a4b08a.mjs') is not a valid name."

Reproducible Demo

See repro instructions above

I believe this is happening because webpack has been configured to treat files with filenames ending in .mjs as static files, so when you import one instead of returning a reference to the export, it will output the contents in static/media and return the path to that file as a string.

Originally reported: https://github.com/brigade/react-waypoint/issues/221