Unmarshalling a JSON that may or may not return an array?

Question

I\'m retrieving JSON from a third party website (home electricity usage), and depending on what I\'ve requested from the site, the JSON returned may or may not be an array.

Answer 1

Usually, whenever you have a JSON value of unknown type, you will use json.RawMessage to get it, peek into it, and unmarshal it correctly into the corresponding type. A simplified example:

// The A here can be either an object, or a JSON array of objects.
type Response struct {
    RawAWrapper struct {
        RawA json.RawMessage `json:"a"`
    }
    A  A   `json:"-"`
    As []A `json:"-"`
}

type A struct {
    B string
}

func (r *Response) UnmarshalJSON(b []byte) error {
    if err := json.Unmarshal(b, &r.RawAWrapper); err != nil {
        return err
    }
    if r.RawAWrapper.RawA[0] == '[' {
        return json.Unmarshal(r.RawAWrapper.RawA, &r.As)
    }
    return json.Unmarshal(r.RawAWrapper.RawA, &r.A)
}

Playground: http://play.golang.org/p/2d_OrGltDu.

Guessing the content based on the first byte doesn't seem too robust to me though. Usually you'll have some sort of a clue in your JSON (like a length or type field on the same level as the dynamic one) that tells you whether you have an object or an array.

See also:

• How can you decode multiple message types with golang websockets?
• Partly JSON unmarshal into a map in Go