Typescript error runtime error: Cannot read property ‘prototype’ of undefined when extending

Possible reasons this is happening:

1. Quadruple-check that BundleConfig is concatenating the files in the correct order. This is by far the most common cause of that error.
2. Verify you don’t have any top-level export directives in Test.ts. This would cause the file to become an external module and Test1 would no longer be visible.

Failing that, you should post the emitted JavaScript to the question so we can diagnose what’s causing the issue.