rails-webpacker | Multiple examples using react | Application Framework library

module.exports just tells what should be exported from this file to be reused in others.

If I just read at what does the 3 lines of code you shared,

const envConfig = module.exports = environment

far left-hand side "environment" is assigned to "module.exports" left-hand side "module.exports" to right-hand side "envConfig"

const aliasConfig = module.exports = {...}

"module.exports" is now assigned another object and lost reference to the first object. "aliasConfig" now references the '{...}' object

module.exports = merge(envConfig.toWebpackConfig(), aliasConfig)

"module.exports" is assigned the result of the merge function (if it is using webpack-merge package, this object is a deep merge of the two). It lost reference to the previous objects.

Declaring module.exports the first times was not necessary.

Webpack config works like any nodejs module. Here is the link to the documentation on module.exports https://nodejs.org/api/modules.html#modules_module_exports

Also note that using the double assignment syntax can be considered dangerous because of how hoisting works in javascript. If I recall it's easy to create global variables that we don't want/expect that way.