I am running across a bug where Webpack generates an invalid Javascript file when running in watch mode. This bug occurs 100% of the time, the first time I modify Cards.js in watch mode. Once the file is in a bad state, I keep on getting the following error:

 undefined is not a function
__webpack_require__@http://localhost:8080/com/realestate/frontend/page/Common.js:51:30
Object.webpackJsonp.0@http://localhost:8080/com/realestate/frontend/page/Cards.js:21:2
__webpack_require__@http://localhost:8080/com/realestate/frontend/page/Common.js:51:30
webpackJsonpCallback@http://localhost:8080/com/realestate/frontend/page/Common.js:22:20
(?)()@http://localhost:8080/com/realestate/frontend/page/Cards.js:1:1Common.js:1256 printUncaughtErrorCommon.js:1285 window.onerror
Common.js:51 Uncaught TypeError: undefined is not a function

The only way to fix the problem is to clean-build the entire project (further incremental changes to the file do not fix the problem).

Here is a copy of Common.js: https://gist.github.com/cowwoc/b752cf667c049af05cec
Here is a copy of Cards.js: https://gist.github.com/cowwoc/5cfcc58edc3b1a8aee40

I am modifying Cards.js line 54 over and over in order to trigger this problem.

I walked through the code and discovered that the browser hits line 51 with module[moduleId] equal to 29. This is problematic for two reasons:

1. Usually module[moduleId] points to a function, not a number.
2. Module 29 is generated by a call to require("./ResourceTable.scss"). If you look for it in Common.js you will notice it is not defined.