How to Unmarshal JSON Data with Variable Types: String or Array of Strings?

Decoding JSON with Variant Structures

The endpoint you're consuming returns JSON in varying formats, sometimes as a string and other times as an array. To handle this inconsistency, the following question explores the best approach to designing a Go struct that can accommodate both types.

The Question:

How can I create a Go struct that can unmarshal JSON data with variable types, either a string or an array of strings? Is there an elegant solution beyond attempting to decode into two separate structs?

The Answer:

A more sophisticated approach involves unmarshaling the JSON into an interface{} type. This type-agnostic approach allows for dynamic handling of different value types.

To implement this technique, create a struct with a field of type interface{}, as demonstrated in the following example:

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

When unmarshaling the JSON data into this struct, you can use a type switch or type assertion to determine the actual type of the Message field. This allows you to process the data accordingly.

Here's an example implementation:

<code class="go">func main() {
    inputs := []string{
        `{"message":"Message"}`,
        `{"message":["ERROR_CODE"]}`,
    }

    for _, input := range inputs {
        var r Response
        if err := json.Unmarshal([]byte(input), &r); err != nil {
            panic(err)
        }
        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>

Output:

Success, message: Message
Error, code: [ERROR_CODE]

By utilizing the interface{} approach, you gain flexibility in handling JSON data with varying types, simplifying your codebase.

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