Vanilla JS specs won't pass type checking in Cypress

[eric-burel]

In Cypress, if you run vulcan-next-starter/basic.spec.ts, everything is fine.
But if you run vulcan-next-starter/basic.spec.js, (a JS file) then basic.spec.ts build will be triggered, and will fail with errors regarding mocha globals (describe etc.).

To reproduce:

• git checkout bugfix/11-webpack-cypress-js-import && yarn && yarn run dev
• open Cypress yarn run cypress:open
• run spec basic.spec.ts > ok
• run spec basic.spec.js > not ok! It acts as if TypeScript types were unknown.

• If you disable typechecking in cypress/webpack.config.js, it will work fine. But it hides the underlying issue.

Currently we are forced to use transpileOnly option of ts-loader in Cypress to bypass this.

Steps to solve

• Decide if we should keep it possible to use vanilla JS or not in Vulcan.
• Define on which criteria we should take this decision (speed of build, complexity of Vulcan, beginner-friendliness for instance)
• If we keep .js, have a better support for JS files within Typescript