Switching fails due to unknown schema entry in `webpack.config.js`

[miketheman]

In #178 a new webpack configuration chunkFormat: "module" was added, and released in version 1.2.2.
This configuration flag does not exist in webpack 4, and was added in webpack 5.

jsbunlding-rails switching guide claims to support webpack 4 in its first line, but this new config option breaks the experience immediately with the auto-generated file, with this output:

webpack --config webpack.config.js

Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.

• configuration.output has an unknown property 'chunkFormat'. These properties are valid:
  object { auxiliaryComment?, chunkCallbackName?, chunkFilename?, chunkLoadTimeout?, crossOriginLoading?, devtoolFallbackModuleFilenameTemplate?, devtoolLineToLine?, devtoolModuleFilenameTemplate?, devtoolNamespace?, filename?, futureEmitAssets?, globalObject?, hashDigest?, hashDigestLength?, hashFunction?, hashSalt?, hotUpdateChunkFilename?, hotUpdateFunction?, hotUpdateMainFilename?, jsonpFunction?, jsonpScriptType?, library?, libraryExport?, libraryTarget?, path?, pathinfo?, publicPath?, sourceMapFilename?, sourcePrefix?, strictModuleExceptionHandling?, umdNamedDefine?, webassemblyModuleFilename? }
  -> Options affecting the output of the compilation. output options tell webpack how to write the compiled files to disk.

Commenting out the new config allows the build to complete:

diff --git a/webpack.config.js b/webpack.config.js
index 557a746..555104c 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -10,7 +10,7 @@ module.exports = {
   output: {
     filename: "[name].js",
     sourceMapFilename: "[file].map",
-    chunkFormat: "module",
+    // chunkFormat: "module",
     path: path.resolve(__dirname, "app/assets/builds"),
   },
   plugins: [

I looked for any mention of this in any issues/guides, didn't find any, so put this here to help any future migrators ease the transition between versions.

I could have tried to use jsbundling-rails 1.2.1 or earlier, but didn't try that, since I wanted to use the latest released version. I don't know if this will have further implications to my migration, but wanted to surface this while it was fresh.