How to specify default values when parsing JSON in Go

Question

I want to parse a JSON object in Go, but want to specify default values for fields that are not given. For example, I have the struct type:

type Test struct          


        

Answer 1

In case u have a list or map of Test structs the accepted answer is not possible anymore but it can easily be extended with a UnmarshalJSON method:

func (t *Test) UnmarshalJSON(data []byte) error {
  type testAlias Test
  test := &testAlias{
    B: "default B",
  }

  _ = json.Unmarshal(data, test)

  *t = Test(*test)
  return nil
}

The testAlias is needed to prevent recursive calls to UnmarshalJSON. This works because a new type has no methods defined.

https://play.golang.org/p/qiGyjRbNHg

Answer 2

This is possible using encoding/json: when calling json.Unmarshal, you do not need to give it an empty struct, you can give it one with default values.

For your example:

var example []byte = []byte(`{"A": "1", "C": "3"}`)

out := Test{
    A: "default a",
    B: "default b",
    // default for C will be "", the empty value for a string
}
err := json.Unmarshal(example, &out) // <--
if err != nil {
    panic(err)
}
fmt.Printf("%+v", out)

Running this example in the Go playground returns {A:1 B:default b C:3}.

As you can see, json.Unmarshal(example, &out) unmarshals the JSON into out, overwriting the values specified in the JSON, but leaving the other fields unchanged.