How to specify default values when parsing JSON in Go

Issue

I want to parse a JSON object in Go, but want to specify default values for fields that are not given. For example, I have the struct type:

type Test struct {
    A string
    B string
    C string
}

The default values for A, B, and C, are “a”, “b”, and “c” respectively. This means that when I parse the json:

{"A": "1", "C": 3}

I want to get the struct:

Test{A: "1", B: "b", C: "3"}

Is this possible using the built-in package encoding/json? Otherwise, is there any Go library that has this functionality?

Solution

This is possible using encoding/json: when calling json.Unmarshal, you do not need to give it an empty struct, you can give it one with default values.

For your example:

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

out := Test{
    A: "default a",
    B: "default b",
    // default for C will be "", the empty value for a string
}
err := json.Unmarshal(example, &out) // <--
if err != nil {
    panic(err)
}
fmt.Printf("%+v", out)

Running this example returns {A:1 B:default b C:3}.

As you can see, json.Unmarshal(example, &out) unmarshals the JSON into out, overwriting the values specified in the JSON, but leaving the other fields unchanged.

Answered By – JW.

Answer Checked By – Terry (GoLangFix Volunteer)