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Tags

Some libraries like encoding/json use tags for more information in a struct.

Tags can be fetched by using reflect.

Let's check an example:

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type Person struct {
    FirstName string `json:"first_name,omitempty" other:"hello"`
}

func main() {
    person := Person{
        FirstName: "Michael",
    }

    personType := reflect.TypeOf(person)
    if firstNameField, ok := personType.FieldByName("FirstName"); ok {
        fmt.Println(firstNameField.Tag.Get("json"))
        fmt.Println(firstNameField.Tag.Get("other"))
    }
}

output:

first_name,omitempty
hello

JSON Tags

If you use, for example encoding/json, you influence how your fields get Marshaled or Unmarshaled Source.

We can set the key name of the json object, by using tags. Let's check an example:

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type Person struct {
    FirstName string `json:"first_name,omitempty"`
}

func main() {
    person := Person{
        FirstName: "Michael",
    }

    data, err := json.Marshal(person)
    if err != nil {
        fmt.Println(err)
    }

    fmt.Println(string(data))
}

output:

{"first_name":"Michael"}

If we use omitempty on a field and the field value is set to a zero value, then the field will be truncated:

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person := Person{}

data, err := json.Marshal(person)
if err != nil {
    fmt.Println(err)
}

fmt.Println(string(data))

output:

{}

If we do not use omitempty the field will be included always:

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type Person struct {
    FirstName string `json:"first_name"`
}

func main() {
    person := Person{}

    data, err := json.Marshal(person)
    if err != nil {
        fmt.Println(err)
    }

    fmt.Println(string(data))
}

output:

{"first_name":""}