go / intermediate
Snippet
Implementing Custom JSON Serialization for Date Fields
By default, Go's json.Marshal formats types using default serialization rules. By implementing the json.Marshaler interface (defining a MarshalJSON() ([]byte, error) method), you can customize exactly how a type is rendered in JSON format, which is useful for custom date formatting, masking sensitive fields, or changing structures.
snippet.go
go
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package mainimport ("encoding/json""fmt""time")type CustomDate time.Timefunc (cd CustomDate) MarshalJSON() ([]byte, error) {formatted := fmt.Sprintf("\"%s\"", time.Time(cd).Format("2006-01-02"))return []byte(formatted), nil}type Event struct {Name string `json:"name"`Date CustomDate `json:"date"`}func main() {evt := Event{Name: "Gala Concert",Date: CustomDate(time.Date(2026, 7, 5, 0, 0, 0, 0, time.UTC)),}data, _ := json.Marshal(evt)fmt.Println(string(data))}
Breakdown
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type CustomDate time.Time
Creates a custom type alias based on time.Time to avoid changing the global behavior of time.Time.
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func (cd CustomDate) MarshalJSON() ([]byte, error) {
Implements the json.Marshaler interface to override default serialization for CustomDate.
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formatted := fmt.Sprintf("\"%s\"", time.Time(cd).Format("2006-01-02"))
Formats the time as a YYYY-MM-DD string enclosed in JSON double quotes.
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data, _ := json.Marshal(evt)
Serializes the Event struct, automatically calling MarshalJSON for the Date field.