DI to Global Variables

Avoid Mutable Globals

Avoid mutating global variables, instead opting for dependency injection. This applies to function pointers as well as other kinds of values.

// BAD
// sign.go

var _timeNow = time.Now

func sign(msg string) string {
  now := _timeNow()
  return signWithTime(msg, now)
}

// GOOD
// sign.go

type signer struct {
  now func() time.Time
}

func newSigner() *signer {
  return &signer{
    now: time.Now,
  }
}

func (s *signer) Sign(msg string) string {
  now := s.now()
  return signWithTime(msg, now)
}

// BAD
// sign_test.go

func TestSign(t *testing.T) {
  oldTimeNow := _timeNow
  _timeNow = func() time.Time {
    return someFixedTime
  }
  defer func() { _timeNow = oldTimeNow }()

  assert.Equal(t, want, sign(give))
}

// GOOD
// sign_test.go

func TestSigner(t *testing.T) {
  s := newSigner()
  s.now = func() time.Time {
    return someFixedTime
  }

  assert.Equal(t, want, s.Sign(give))
}