Read XMLHTTPRequest as Json in Scala.js

Question

I´m using Scala.js and want to read a JSON i get from my backend.
My problem is, i don't know how to work with my response as json. All examples i found use JSON.toJson(xhr.responseText) but this only works if i get one String (right?)
Also i dont want to parse the JSON in Objects (in this example Users) I use the Json lib from the Play-Framework.
Example for Json:

[
    {
        "name": "User1",
        "age": 18
    },
    {
        "name": "User2",
        "age": 18
    },
    {
        "name": "User3",
        "age": 18
    }
]

My Code

 val xhr = new dom.XMLHttpRequest()
    xhr.open("GET", backend + "/ROUTE")
    xhr.responseType="json"
    xhr.onload = { (e: dom.Event) =>
       println(xhr.response) 
       //What i want
       // for (user<-response) println(user("age"),user("name")) 
    }
    xhr.send()

The output is

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

I tried things like

val js=Json.obj("users"->xhr.response)

and so on.

I guess i have a misunderstanding how exactly

 xhr.responseType="json"

works but can't figure it out.

I know how i would do it in "normal" Play json("name")

cbley · Accepted Answer · 2020-06-12 10:18:09Z

When you are using responseType = "json" you instruct the Browser to interpret the response as a normal Javascript Object.

In scalasjs terms, this would be a value of type js.Object too (see https://www.scala-js.org/doc/interoperability/types.html).

More specifically, when you are receiving a JSON Array, you could expect a js.Array[T] in scalajs:

import scala.scalajs.js

...

req.onload = { e: dom.Event =>
  if (js.Array.isArray(req.response)) {
    val array =  req.response.asInstanceOf[js.Array[js.Dynamic]]

    for (user <- array) {
      println(user.name)
    }
  }
}

Instead of using responseType = "json", you might consider doing it like this:

req.onload = { e: dom.Event =>
  val r = js.JSON.parse(req.responseText)

  r match {
    case jsonlist: js.Array[js.Dynamic] =>
      for (user <- jsonlist) {
        println(user)
       }
    }
  }
}

However, if you want to use play-json, you would not want to set responseType = "json" at all, just transfer it as a String and call Json.parse on it:

val json = Json.parse(req.responseText)

println((json \ 1 \ "name").as[String])

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