How to Handle `omitempty` with `time.Time` Fields in Go JSON Marshaling?

JSON omitempty with time.Time Field: A Custom Solution

The omitempty tag option fails to work with time.Time because it is a struct. This means that the "zero" value for a time.Time is a valid value with all its field values set to zero, which is not treated as "empty" by the json encoder.

To overcome this challenge, let's explore a custom approach that employs a pointer to time.Time (*time.Time) and implements custom encoding and decoding.

Pointer-Based Solution

By switching to a pointer, nil values are interpreted as "empty" by the JSON encoder:

type MyStruct struct {
    Timestamp *time.Time `json:",omitempty"`
    Date      *time.Time `json:",omitempty"`
    Field     string    `json:",omitempty"`
}

Custom Marshaler and Unmarshaler

If you prefer not to use pointers, you can implement custom Marshaler and Unmarshaler interfaces:

type MyStruct struct {
    Timestamp time.Time `json:"Timestamp"`
    Date      time.Time `json:"Date"`
    Field     string    `json:"Field"`
}

func (ms MyStruct) MarshalJSON() ([]byte, error) {
    type Alias MyStruct
    if ms.Timestamp.IsZero() {
        ms.Timestamp = time.Time{}
    }
    if ms.Date.IsZero() {
        ms.Date = time.Time{}
    }
    return json.Marshal(Alias(ms))
}

func (ms *MyStruct) UnmarshalJSON(data []byte) error {
    type Alias MyStruct
    aux := &Alias{}
    if err := json.Unmarshal(data, aux); err != nil {
        return err
    }
    *ms = MyStruct(aux)
    return nil
}

Here, the MarshalJSON method sets zero-valued time.Time instances to the equivalent Go zero value, enabling empty field handling.

Conclusion

The pointer-based solution is straightforward and effective. However, if using pointers is not feasible, the custom Marshaler/Unmarshaler approach provides a flexible way to handle empty time.Time values.

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