Set json encoding precision (#162)

types.go

@@ -49,9 +49,13 @@ func newNumericDateFromSeconds(f float64) *NumericDate {
 // MarshalJSON is an implementation of the json.RawMessage interface and serializes the UNIX epoch
 // represented in NumericDate to a byte array, using the precision specified in TimePrecision.
 func (date NumericDate) MarshalJSON() (b []byte, err error) {
+	var prec int
+	if TimePrecision < time.Second {
+		prec = int(math.Log10(float64(time.Second) / float64(TimePrecision)))
+	}
 	f := float64(date.Truncate(TimePrecision).UnixNano()) / float64(time.Second)
 
-	return []byte(strconv.FormatFloat(f, 'f', -1, 64)), nil
+	return []byte(strconv.FormatFloat(f, 'f', prec, 64)), nil
 }
 
 // UnmarshalJSON is an implementation of the json.RawMessage interface and deserializses a

types_test.go

@@ -18,12 +18,10 @@ func TestNumericDate(t *testing.T) {
 
 	jwt.TimePrecision = time.Microsecond
 
-	raw := `{"iat":1516239022,"exp":1516239022.12345}`
+	raw := `{"iat":1516239022.000000,"exp":1516239022.123450}`
 
-	err := json.Unmarshal([]byte(raw), &s)
+	if err := json.Unmarshal([]byte(raw), &s); err != nil {
-
+		t.Fatalf("Unexpected error: %s", err)
-	if err != nil {
-		t.Errorf("Unexpected error: %s", err)
 	}
 
 	b, _ := json.Marshal(s)
@@ -65,3 +63,49 @@ func TestSingleArrayMarshal(t *testing.T) {
 		t.Errorf("Serialized format of string array mismatch. Expecting: %s  Got: %s", string(expected), string(b))
 	}
 }
+
+func TestNumericDate_MarshalJSON(t *testing.T) {
+	// Do not run this test in parallel because it's changing
+	// global state.
+	oldPrecision := jwt.TimePrecision
+	t.Cleanup(func() {
+		jwt.TimePrecision = oldPrecision
+	})
+
+	tt := []struct {
+		in        time.Time
+		want      string
+		precision time.Duration
+	}{
+		{time.Unix(5243700879, 0), "5243700879", time.Second},
+		{time.Unix(5243700879, 0), "5243700879.000", time.Millisecond},
+		{time.Unix(5243700879, 0), "5243700879.000001", time.Microsecond},
+		{time.Unix(5243700879, 0), "5243700879.000000954", time.Nanosecond},
+		//
+		{time.Unix(4239425898, 0), "4239425898", time.Second},
+		{time.Unix(4239425898, 0), "4239425898.000", time.Millisecond},
+		{time.Unix(4239425898, 0), "4239425898.000000", time.Microsecond},
+		{time.Unix(4239425898, 0), "4239425898.000000000", time.Nanosecond},
+		//
+		{time.Unix(0, 1644285000210402000), "1644285000", time.Second},
+		{time.Unix(0, 1644285000210402000), "1644285000.210", time.Millisecond},
+		{time.Unix(0, 1644285000210402000), "1644285000.210402", time.Microsecond},
+		{time.Unix(0, 1644285000210402000), "1644285000.210402012", time.Nanosecond},
+		//
+		{time.Unix(0, 1644285315063096000), "1644285315", time.Second},
+		{time.Unix(0, 1644285315063096000), "1644285315.063", time.Millisecond},
+		{time.Unix(0, 1644285315063096000), "1644285315.063096", time.Microsecond},
+		{time.Unix(0, 1644285315063096000), "1644285315.063096046", time.Nanosecond},
+	}
+
+	for i, tc := range tt {
+		jwt.TimePrecision = tc.precision
+		by, err := jwt.NewNumericDate(tc.in).MarshalJSON()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got := string(by); got != tc.want {
+			t.Errorf("[%d]: failed encoding: got %q want %q", i, got, tc.want)
+		}
+	}
+}