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In order to decode JSON that includes a universal interface as a field, you must first determine the specific data type that is being represented by the interface.

Once you have identified the data type, you can then use a type assertion in your Go code to convert the JSON data into the appropriate struct or interface type. For example, if your JSON data includes an interface named "myField" that could represent either a string or a number, you could use the following code to decode the JSON:

type myData struct {
    MyField interface{} `json:"myField"`
}

// Decode the JSON data into a myData struct
var data myData
err := json.Unmarshal(jsonData, &data)
if err != nil {
    // Handle error
}

// Check the type of the "myField" interface
switch val := data.MyField.(type) {
case string:
    // Handle string value
case float64:
    // Handle number value
default:
    // Handle unexpected type
}

By using a type assertion in this way, you can easily handle the various types that may be represented by the universal interface field in your JSON data.