I am connecting to an API which gives a rather large json payload. I need to add a key and value to the root object.Once I do ioutil.Readall from the package "net/http" the JSON is a byte array.
My goal is to just simply add to the structure and move on. As an example, the following the pretty similar to what I am doing: https://tutorialedge.net/golang/consuming-restful-api-with-go/
So how can I simply add to a JSON structure another element (key: value)?
If all you want to do is add a key and value to the root object and produce new JSON, and you don't care about having the data in a structure, you can unmarshal into map[string]interface{}, add your value, and then marshal again:
var m map[string]interface{}
err := json.Unmarshal(data, &m)
m["new_key"] = newValue
newData, err := json.Marshal(m)
(I'm not checking for errors, but you should do that of course.) Take a look at https://golang.org/pkg/encoding/json/ for more information about how to deal with JSON in Go.
Since you have a byte data, you need to parse it and store the result in a variable that has your json structure using json.Marshal.
Then after, to add a new key value pair, you can define a new struct with the key and its data type
var variable type1
// Unmarshal the byte data in a variable
json.Unmarshall(data, &variable)
// to add a new key value you can define a new type
type type2 struct {
type1
key type
}
// and you can add
variable2 := type2{variable, newValueToAdd}
While deserializing & reserializing is the more "correct" approach, it may be overkill for just adding a value at the root, which can be done with simple string manipulation (really byte slice manipulation, but the semantics are similar and it's arguably easier):
data := []byte(`{"foo": 1, "bar": true}`)
ins := []byte(`, "baz": "hello"`) // Note the leading comma.
closingBraceIdx := bytes.LastIndexByte(data, '}')
data = append(data[:closingBraceIdx], ins...)
data = append(data, '}')
This is more error-prone, because it is unaware of JSON syntax entirely, but it's safe enough for most cases and for large JSON documents it is more efficient than a parse, insert, and reserialize.
Playground example: https://play.golang.org/p/h8kL4Zzp7rq