code: function getStuffAndDeleteSecret(opts, someArgument) {
    // We depend on "stuffs" repository.
    const stuffs = opts.stuffs
  
    // We may now carry on.
    return stuffs.getStuff(someArgument).then(stuff => {
      // Modify return value. Just to prove this is testable.
      delete stuff.secret
      return stuff
    })
  }
docs: By exporting this function as-is, we can inject mocks as the first argument!!!!!