How to unmarshal an escaped JSON string - json

Question

I am using Sockjs with Go, but when the JavaScript client send json to the server it escapes it, and send's it as a []byte. I'm trying to figure out how to parse the json, so that i can read the data. but I get this error.
json: cannot unmarshal string into Go value of type main.Msg
How can I fix this? html.UnescapeString() has no effect.
val, err := session.ReadMessage()
if err != nil {
break
}
var msg Msg
err = json.Unmarshal(val, &msg)
fmt.Printf("%v", val)
fmt.Printf("%v", err)
type Msg struct {
Channel string
Name string
Msg string
}
//Output
"{\"channel\":\"buu\",\"name\":\"john\", \"msg\":\"doe\"}"
json: cannot unmarshal string into Go value of type main.Msg

Answer 1

You might want to use strconv.Unquote on your JSON string first :)
Here's an example, kindly provided by #gregghz:
package main
import (
"encoding/json"
"fmt"
"strconv"
)
type Msg struct {
Channel string
Name string
Msg string
}
func main() {
var msg Msg
var val []byte = []byte(`"{\"channel\":\"buu\",\"name\":\"john\", \"msg\":\"doe\"}"`)
s, _ := strconv.Unquote(string(val))
err := json.Unmarshal([]byte(s), &msg)
fmt.Println(s)
fmt.Println(err)
fmt.Println(msg.Channel, msg.Name, msg.Msg)
}

Answer 2

You need to fix this in the code that is generating the JSON.
When it turns out formatted like that, it is being JSON encoded twice. Fix that code that is doing the generating so that it only happens once.
Here's some JavaScript that shows what's going on.
// Start with an object
var object = {"channel":"buu","name":"john", "msg":"doe"};
// serialize once
var json = JSON.stringify(object); // {"channel":"buu","name":"john","msg":"doe"}
// serialize twice
json = JSON.stringify(json); // "{\"channel\":\"buu\",\"name\":\"john\",\"msg\":\"doe\"}"

Answer 3

Sometimes, strconv.Unquote doesn't work.
Heres an example shows the problem and my solution.
(The playground link: https://play.golang.org/p/Ap0cdBgiA05)
Thanks for #Crazy Train's "encodes twice" idea, I just decoded it twice ...
package main
import (
"encoding/json"
"fmt"
"strconv"
)
type Wrapper struct {
Data string
}
type Msg struct {
Photo string
}
func main() {
var wrapper Wrapper
var original = `"{\"photo\":\"https:\/\/www.abc.net\/v\/t1.0-1\/p320x320\/123.jpg\"}"`
_, err := strconv.Unquote(original)
fmt.Println(err)
var val []byte = []byte("{\"data\":"+original+"}")
fmt.Println(string(val))
err = json.Unmarshal([]byte(val), &wrapper)
fmt.Println(wrapper.Data)
var msg Msg
err = json.Unmarshal([]byte(wrapper.Data), &msg)
fmt.Println(msg.Photo)
}

Answer 4

As Crazy Train pointed out, it appears that your input is doubly escaped, thus causing the issue. One way to fix this is to make sure that the function session.ReadMessasge() returns proper output that is escaped appropriately. However, if that's not possible, you can always do what x3ro suggested and use the golang function strconv.Unquote.
Here's a playground example of it in action:
http://play.golang.org/p/GTishI0rwe

Answer 5

The data shown in the problem is stringified for some purposes, in some cases you can even have \n in your string representing break of line in your json.
Let's understand the easiest way to unmarshal/deserialize this kind of data using the following example:
Next line shows the data you get from your sources and want to derserialize
stringifiedData := "{\r\n \"a\": \"b\",\r\n \"c\": \"d\"\r\n}"
Now, remove all new lines first
stringifiedData = strings.ReplaceAll(stringifiedData, "\n", "")
Then remove all the extra quotes that are present in your string
stringifiedData = strings.ReplaceAll(stringifiedData, "\\"", "\"")
Let's now convert the string into a byte array
dataInBytes := []byte(stringifiedData)
Before doing unmarshal, let's define structure of our data
jsonObject := &struct{
A string `json:"a"`
C string `json:"c"`
}
Now, you can dersialize your values into jsonObject
json.Unmarshal(dataInBytes, jsonObject)}

Answer 6

You got into infamous escaped string pitfall from JavaScript. Quite often people face (as I did) the same issue in Go, when serializing JSON string with json.Marshal, e.g.:
in := `{"firstName":"John","lastName":"Dow"}`
bytes, err := json.Marshal(in)
json.Marshal escapes double quotes, producing the same issue when you try to unmarshal bytes into struct.
If you faced the issue in Go, have a look at How To Correctly Serialize JSON String In Golang post which describes the issue in details with solution to it.