Reproduction steps

• run npm i - install dependencies
• run npm run start - will build the project and output It Works! to the console
• run npm run case-one - will run jest test successfully. Navigating to src/case-one.spec.ts will highlight "custom/module" with red with error Cannot find module 'custom/module' or its corresponding type declarations.
• run npm run case-two - will fail with error scripts/case-two.ts:1:34 - error TS2307: Cannot find module 'custom/module' or its corresponding type declarations.
  • Navigating to scripts/case-two.ts will highlight import with the same error as for the test file.
• run npm run case-three - will fail with error src/helpers/test-helpers.ts:1:34 - error TS2307: Cannot find module 'custom/module' or its corresponding type declarations.
  • Navigating to scripts/case-three.ts will not show any errors, because type is used only in an imported function, and in that file (src/helpers/test-helpers.ts) custom module is resolved successfully because tsconfig actually picks it up.
• case-two-works and case-three-works scripts are just there to show that scripts can run in a transpile-only mode, but that is a workaround that would be preferable to avoid, because it can hide valid errors.