How to Handle omitempty with Structs, Pointers, and Zero Values

The omitempty tag only omits a field from JSON output if the field's value is the zero value for its type, which is nil for pointers and empty strings for strings. For a pointer field, omitempty works as expected because the zero value is nil; for a struct field, it only omits if the entire struct is the zero value (all fields are zero), not if just one field is zero. To omit a struct field based on a specific internal field, you must use a pointer to the struct or implement custom marshaling.

type Config struct {
    // Omitted if ptr is nil
    Ptr *string `json:"ptr,omitempty"`
    
    // Omitted only if the entire struct is zero-valued (Name="", ID=0)
    Info Info `json:"info,omitempty"`
    
    // Omitted if ptrInfo is nil
    PtrInfo *Info `json:"ptrInfo,omitempty"`
}

type Info struct {
    Name string
    ID   int
}

The omitempty tag tells Go to skip a field in JSON output only if it holds its default "empty" value. For pointers, this means nil; for strings, an empty string; for numbers, zero. If you use a regular struct, it is only considered empty if every single field inside it is zero. If you want to omit a struct based on just one of its fields, you must make the struct field a pointer so it can be nil.