The new option fixes #2644, which is a reactivation of #287, and has been in discussion in #2034.

Background

The current behavior of the compiler when using --outDir is to duplicate the input structure in the output under the value of --outDir. the root of the input is computed as the longest common path of all source (.ts and not .d.ts) files. so an input of FolderA\FolderB\1.ts and FolderA\FolderB\2.ts would result in computing the commonSourceRoot to FolderA\FolderB\. now if a new file FolderA\3.ts is added to the input, the commonSourceRoot will pop out to FolderA\.

This is generally undesirable and can be confusing. it also does not allow for folders that do not contain .ts files to be replicated in the output.

--rootDir flag

The new flag is used to customize the commonSourceRoot instead of automatically calculating it.

The same rules apply, i.e. all source files (.ts) should be under the rootDir, otherwise the output generation may not be correct.

The name rootDir is meant to resemble outDir as both flags go together.