Problem Description

I have a question about the jest.config.ts that I am setting up.
There I use a moduleNameMapper, so that imports a la import ... from "@base/..." also work in the tests. For this I use a function of ts-jest, which returns the corresponding regular expressions from the tsconfig.json.compilerOptions.paths. tsconfig.json is a json file which I am importing with typescript’s resolveJsonModule option. See here for the documentation on moduleNameMapper.

The function from ts-jest returns one of the following two objects to be used as the moduleNameWrapper, depending on other parameters passed. How that works is not relevant for the question, as I am also able to just enter the keys and values directly, not using the ts-jest function.

First Object: { '^@base/(.*)$': './src/$1' }
Second Object: { '^@base/(.*)$': '<rootDir>/src/$1' }
The first does not work (the modules are then not resolved), the second works correctly.

Question

Hence my question: What does . mean as a path for jest? What is the difference with <rootDir>? Why do they both not point to the same directory? My jest config is in the root directory of the project, so I would assume that both . and <rootDir would point to the root directory (the folder where the jest config is located). Also see the documentation for rootDir.

What I’ve tried so far

I tried putting both objects in as moduleNameWrapper in the jest.config, but only the one with rootDir worked. I would have expected ./src/$1 to have worked as well, but it did not.

The rootDir solution is unsatisfying, as producing the object including <rootDir> requires an additional options parameter, which sets a prefix. This prefix is hard coded, and could e.g. result in problems further in this project or when using this project as a foundation for the next.

This is the call to create the moduleNameMapper object via ts-jest:

pathsToModuleNameMapper(compilerOptions.paths, {prefix: "<rootDir>/src/"}),

If the second parameter is omitted, then the dot version is produced, which does not work.

Thanks.