When trying to reRequire a file named index that is in the local path, reRequire will instead return what mock-require exports, specifically the startMocking function.

Example code:

index.js:

const fs = require('fs')
// ...
module.exports = { fs }

test.js:

const mock = require('mock-require')

mock('fs', {})

const index = mock.reRequire('./index')

console.log(index)

Expected output:

{ fs: {} }

Actual output:

function startMocking(path, mockExport) {
  const calledFrom = getCallerFile();

  if (typeof mockExport === 'string') {
    mockExport = getFullPathNormalized(mockExport, calledFrom);
  }

  pendingMockExports[getFullPathNormalized(path, calledFrom)] = mockExport;
}

Based on looking at the code (and specifically at getFullPath and isInNodePath, it looks like any file that can be resolved from the mock-require package will cause it to return that file rather than the actual local file.