How to Set Default Values When Parsing JSON in Go?

Default Value Specification in JSON Parsing with Go

When parsing JSON data in Go using the encoding/json package, you may encounter the need to assign default values to fields that are absent in the input.

Approach Using the encoding/json Package

Yes, the built-in encoding/json package allows you to specify default values. Instead of initializing an empty struct for JSON unmarshaling, you can create a struct with default values and pass it to the unmarshal function.

Example:

type Test struct {
    A string
    B string
    C string
}

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

    out := Test{
        A: "default a",
        B: "default b",
    }

    if err := json.Unmarshal(example, &out); err != nil {
        panic(err)
    }

    fmt.Printf("%+v", out)
}

In this example, the values for A and B are specified as "default a" and "default b", respectively. When unmarshalling the JSON, it modifies only the values present in the input (A and C), leaving the others unchanged.

Output:

{A:1 B:default b C:3}

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