Vaadin 19: Why ts-loader is used only with useClientSideIndexFileForBootstrapping?

[xdenser]

I am migrating Vaadin 18.0.1 project to Vaadin 19.0.2 and and we do not use useClientSideIndexFileForBootstrapping,
Typescript compilation worked in 18.0.1 and now it stopped in 19.0.2.
OK I can add it back in my wbpack.config.js, but condition loks strange to me.
See lines 210, 241 in
https://github.com/vaadin/flow/blob/master/flow-server/src/main/resources/webpack.generated.js
61df7f7

[haijian-vaadin]

Thank you @xdenser for the feedback, this is a bug, we will fix it.
The bug can be reproduced by having a project with a (Java + HTML) view:

1. Download a v20(19) hybrid project from start.vaadin.com, set UI as Flow (JAVA + HTML)
2. Run the application with mvn -Dspring-boot.jvmArguments="-Dvaadin.useDeprecatedV14Bootstrapping" -Dvaadin.useDeprecatedV14Bootstrapping spring-boot:run
   See the error

ERROR in ./views/helloworld/hello-world-view.ts 5:0
Module parse failed: Unexpected character '@' (5:0)
You may need an appropriate loader to handle this file type, currently no loaders are configured to process this file. See https://webpack.js.org/concepts#loaders
| import { customElement, html, LitElement } from 'lit-element';
| 
> @customElement('hello-world-view')
| export class HelloWorldView extends LitElement {
|   createRenderRoot() {
 @ ../target/frontend/generated-flow-imports.js 103:0-55

[platosha]

As far as I remember, I had to disable it this way because ts-loader errors (TS18002) when no TypeScript files are used in the project, which is common among Java-only apps. TypeStrong/ts-loader#405

[haijian-vaadin]

Use fork-ts-checker-webpack-plugin https://github.com/TypeStrong/fork-ts-checker-webpack-plugin in addition to ts-loader might solve this issue.
If this doesn't work, a fallback option would be to use a wrapper for ts-loader and catch and handle the error there.

[vaadin-bot]

This ticket/PR has been released with platform 19.0.6. For prerelease versions, it will be included in its final version.