Flow: Non-descriptive error message if webpack.config.js is invalid

[Artur-]

Take the Spring Boot starter project and replace webpack.config.js with

foo

If you run node_modules/.bin/webpack manually, you will get

ReferenceError: foo is not defined
    at Object.<anonymous> (.../webpack.config.js:1:63)

If you run mvn, you get

2020-02-03 12:15:47.107 ERROR 60528 --- [  restartedMain] o.a.c.c.C.[Tomcat].[localhost].[/]       : Exception sending context initialized event to listener instance of class [com.vaadin.flow.spring.VaadinServletContextInitializer$DevModeServletContextListener]

java.lang.RuntimeException: Unable to initialize Vaadin DevModeHandler
    at com.vaadin.flow.spring.VaadinServletContextInitializer$DevModeServletContextListener.contextInitialized(VaadinServletContextInitializer.java:349) ~[vaadin-spring-12.2.0.alpha1.jar:na]
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4680) [tomcat-embed-core-9.0.22.jar:9.0.22]
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5143) [tomcat-embed-core-9.0.22.jar:9.0.22]
...
Caused by: java.lang.IllegalStateException: Webpack exited prematurely
    at com.vaadin.flow.server.DevModeHandler.<init>(DevModeHandler.java:201) ~[flow-server-2.2.0.alpha10.jar:2.2.0.alpha10]
    at com.vaadin.flow.server.DevModeHandler.createInstance(DevModeHandler.java:298) ~[flow-server-2.2.0.alpha10.jar:2.2.0.alpha10]
    at com.vaadin.flow.server.DevModeHandler.start(DevModeHandler.java:248) ~[flow-server-2.2.0.alpha10.jar:2.2.0.alpha10]

This does not even contain a reference to webpack.config.js nor to the problem that actually occured in webpack

[caalador]

It does say Webpack exited prematurely, is there any more log before the illegal state?

Also as you replaced the config content with foo up in the logs do you have:

"Flow generated webpack configuration was not mentioned "
"in the configuration file: {}."
"Please verify that './webpack.generated.js' is used "
"in the merge or remove the file to generate a new one."

[mvysny]

Same thing here with Vaadin 14.5.1: I only see the exception but there is no indication in the entire stdout as to why the webpack would fail:

Caused by: java.lang.IllegalStateException: Webpack exited prematurely
        at com.vaadin.flow.server.DevModeHandler.doStartWebpack(DevModeHandler.java:657)
        at com.vaadin.flow.server.DevModeHandler.doStartDevModeServer(DevModeHandler.java:583)
        at com.vaadin.flow.server.DevModeHandler.runOnFutureComplete(DevModeHandler.java:530)
        at com.vaadin.flow.server.DevModeHandler.lambda$new$0(DevModeHandler.java:146)
        at java.util.concurrent.CompletableFuture.uniWhenComplete(CompletableFuture.java:774)

This started to happen after I applied the checker from #8475 . Without the checker the Vaadin would fail in runtime with Uncaught DOMException: CustomElementRegistry.define: 'vaadin-lumo-styles' has already been defined as a custom element: the checker apparently catches the issue early on (which is a good thing), but then Vaadin logs nothing of the issue and fails with this useless exception.

This is a horrible dev UX experience - my project doesn't start anymore, and I have no idea why. I would rather revert the checker and go back to failing at runtime, at least there I'll receive an "informative" error message.

[caalador]

Is this only in v14 dev mode server start?
For v19+ there is the server output when running the server (both plain and spring starter):

[INFO]  com.vaadin.flow.server.DevModeHandler    : Running webpack to compile frontend resources. This may take a moment, please stand by...
...\skeleton-starter-flow\webpack.config.js:1
foo
^

ReferenceError: foo is not defined
    at Object.<anonymous> (...\skeleton-starter-flow\webpack.config.js:1:1)
    at Module._compile (node:internal/modules/cjs/loader:1092:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1121:10)
    at Module.load (node:internal/modules/cjs/loader:972:32)
    at Function.Module._load (node:internal/modules/cjs/loader:813:14)
    at Module.require (node:internal/modules/cjs/loader:996:19)
    at require (node:internal/modules/cjs/helpers:92:18)
    at WEBPACK_OPTIONS (...\skeleton-starter-flow\node_modules\.pnpm\[email protected][email protected]\node_modules\webpack-cli\bin\utils\convert-argv.js:114:13)
    at requireConfig (...\skeleton-starter-flow\node_modules\.pnpm\[email protected][email protected]\node_modules\webpack-cli\bin\utils\convert-argv.js:116:6)
    at ...\skeleton-starter-flow\node_modules\.pnpm\[email protected][email protected]\node_modules\webpack-cli\bin\utils\convert-argv.js:123:17

[mvysny]

@caalador could be - I've only tested Vaadin 14.5.1.