mock.js

const Module = require('module')

const applyMocks = function (id, mocks) {
  const filePath = Module._resolveFilename(id, module.parent)
  if (!Module._cache[filePath]) {
    Module._load(id, module.parent, false)
  }

  const mod = Module._cache[filePath]
  const originalExports = mod.exports
  const originalRequire = mod.require
  mod.require = function (id) {
    if (id in mocks) {
      return mocks[id]
    }

    return originalRequire.call(this, id)
  }

  mod.loaded = false
  mod.load(mod.filename)
  const mocked = mod.exports
  mod.require = originalRequire
  mod.exports = originalExports
  return mocked
}

module.exports = applyMocks

p.js

const q = require('./q')
module.exports = function () {
  return q()
}

q.js

module.exports = function () { return 'original' }

test.js

const applyMocks = require('./mock')

const p = require('./p')
console.log(p()) // logs 'original'

const mocked = applyMocks('./p', {
  './q': function () { return 'overwritten' }
})
console.log(mocked()) // logs 'overwritten'

const p2 = require('./p')
console.log(p2()) // logs 'original'