How Can I Handle Diverse JSON Formats When Unmarshalling in Go?

Adapting Unmarshal to Handle Diverse JSON Formats

When accessing external APIs, developers often encounter varying JSON response formats, presenting challenges when unmarshalling data into a consistent structure. In Go, it's possible to gracefully handle these differences with a few techniques.

Take the example of an API endpoint that returns JSON messages in two formats, either as a string "message" or an array of error codes ["ERROR_CODE"]. To manage this variation, a custom struct can be defined to hold the response:

<code class="go">type Response struct {
    Message interface{} `json:"message"`
}</code>

By setting the Message field as an interface type, Go's dynamic typing system allows for the storage of diverse data types. This flexibility offers a starting point for handling the different response formats.

To unmarshal the JSON into the Response struct, a call to json.Unmarshal would be utilized. However, since the Message field can hold various types, a special case arises when the JSON represents an array (the error response). By default, JSON arrays are unmarshaled into values of type []interface{}.

The key to handling this distinction lies in examining the type of the unmarshalled Message field. This can be achieved with a type assertion or a type switch, as exemplified below:

<code class="go">    switch x := r.Message.(type) {
    case string:
        fmt.Println("Success, message:", x)
    case []interface{}:
        fmt.Println("Error, code:", x)
    default:
        fmt.Println("Something else:", x)
    }</code>

In this snippet, the type of r.Message is examined. If it's a string, it indicates a success message. If it's a slice of interfaces, it represents an error response with the error code. Otherwise, it's considered an unexpected response.

By utilizing this technique, developers can effectively unmarshal JSON responses with varying formats into a unified struct, enabling subsequent processing and decision-making based on the message type.

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