code: function checkJs (mode, callback) {
    var exercise = this
  
    if (!exercise.passed)
      return callback(null, true) // shortcut if we've already had a failure
  
    gyp.rebuild(copyFauxTempDir, function (err) {
      if (err) {
        exercise.emit('fail', 'Compile mock C++ to test JavaScript: ' + err.message)
        return callback(null, false)
      }
  
      execWith(copyFauxTempDir, 111, 'FAUX 111\nDone!\n', function (err, pass) {
        if (err)
          return callback(err)
        if (!pass) {
          exercise.emit('fail', 'JavaScript code loads addon and invokes `delay(x, cb)` method')
          return callback(null, false)
        }
  
        execWith(copyFauxTempDir, 1111, 'FAUX 1111\nDone!\n', function (err, pass) {
          if (err)
            return callback(err)
  
          exercise.emit(pass ? 'pass' : 'fail'
              , 'JavaScript code loads addon and invokes `delay(x, cb)` method')
          callback(null, pass)
        })
      })
    })
  }
docs: run `node-gyp rebuild` on a mocked version of the addon that prints what we want so we can test that their JS is doing what it is supposed to be doing and there is no cheating! (e.g. console.log(...))