How Can I Set Default Values for Missing JSON Fields in Go?

Customizing JSON Parsing Defaults in Go

When parsing JSON in Go, assigning default values to fields that are not present can enhance data manipulation. Here's how you can achieve this using the encoding/json package or leverage an external Go library.

Using encoding/json

The encoding/json package offers a straightforward solution. Instead of passing an empty struct to json.Unmarshal, you can initialize it with default values. For example:

type Test struct {
    A string "a"
    B string "b"
    C string
}

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

out := Test{}
err := json.Unmarshal(example, &out)
if err != nil {
    panic(err)
}
fmt.Printf("%+v", out)

Utilizing External Libraries

If the encoding/json package doesn't fully meet your needs, there are external Go libraries that provide enhanced JSON parsing functionality. One popular option is the go-json library, which allows you to specify default values for missing fields using struct tags.

import (
    "encoding/json"
    "github.com/go-json"
)

type Test struct {
    A string `json:"a,omitempty"`
    B string `json:"b,omitempty"`
    C string `json:"c,omitempty"`
}

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

out := Test{
    DefaultA: "a",
    DefaultB: "b",
    DefaultC: "c",
}

err := json.Unmarshal(example, &out)
if err != nil {
    panic(err)
}
fmt.Printf("%+v", out)

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