"omitempty" option omits boolean fields with false values

The omitempty option in the Go standard library’s encoding/json package omits boolean fields with false values.

The “omitempty” option specifies that the field should be omitted from the encoding if the field has an empty value, defined as false, 0, a nil pointer, a nil interface value, and any array, slice, map, or string of length zero.

This means that if you use the omitempty option on a boolean field, the field is omitted when it is false:

type AppConfig struct {
    FlagValue bool `json:"flag_value,omitempty"`
}

func main() {
    ac, _ := json.Marshal(AppConfig{true})
    fmt.Println(string(ac))
    ac, _ = json.Marshal(AppConfig{false})
    fmt.Println(string(ac)) 
}

output
{"flag_value":true}
{}

Which is unexpected, because you expect the second line to be {"flag_value":false}.

To fix this, you can use a pointer to a boolean instead:

type AppConfig struct {
    FlagValue *bool `json:"flag_value,omitempty"`
}

func main() {
    fv := true
    ac, _ := json.Marshal(AppConfig{&fv})
    fmt.Println(string(ac))
    fv = false
    ac, _ = json.Marshal(AppConfig{&fv})
    fmt.Println(string(ac))
}

output
{"flag_value":true}
{"flag_value":false}

To differentiate between unset and zero values for any data type, use pointers.