Default values when unmarshalling json in go

unmarshal.defaults.go

package foo

type Foo struct {
  Field string `json:"field"`
}

// UnmarshalJSON is the implementation of the json.Unmarshaler interface.
func (t *Foo) UnmarshalJSON(data []byte) error {
    type innerFoo Foo
    inner := &innerFoo{
        Field: "default value",
    }

    if err := json.Unmarshal(data, inner); err != nil {
        return err
    }

    *t = Foo(*inner)
    return nil
}

example to set default values on a struct when unmarshing json