Convert JavaScript Uint8Array object into Scala Object

Question

A TypeScript application sends a Uint8Array object through an HTTP POST request to a Scala Play application.

How to convert the Uint8Array object into a Scala object in the Play application?

For example, the TypeScript code sends the following object:

{stuff: new Uint8Array([0, 1, 2])}

Inside the Scala Play controller:

case class StuffData(stuff: List[Byte])

implicit val reads: Reads[StuffData] = Json.reads[StuffData]

def processStuff  = Action.async(parse.json[StuffData]) { request =>
  val stuffData = request.body
  println(stuffData)
}

This does not work... the error message from Play is:

For request 'POST /send-stuff' 
[Json validation error List((obj.key,List(JsonValidationError(List(error.expected.jsarray),WrappedArray()))))]

UPDATE:

To be able to get on the Scala side exactly the same byte array as was on the JavaScript side (i.e. a byte for a byte, in the original order), the current code in the accepted answer should be updated to something like:

  implicit val stuffReader = new Reads[StuffData] {
    def reads(js: JsValue): JsResult[StuffData] = {
      JsSuccess(StuffData(
        (js \ "stuff").as[Map[String, Int]].toList.sortBy(_._1.toInt).map(_._2.toByte)
      ))
    }
  }

Now, the reader populates a

case class StuffData(stuff: List[Byte])

with the same order as the original JavaScript Uint8Array.

It's possible to convert all the Ints into Bytes without losing information, because we know that all the Ints are in the range [0..255].

What JSON data is sent by your TS code? What is received by the Play app? — Gaël J
– Gaël J, Commented Feb 11, 2023 at 19:49

Stanislav Kovalenko · Accepted Answer · 2023-02-12 16:27:51Z

1

By default, Unit8Array is encoded in JSON as {"0":1,"1":2,"2":3,"3":4}, so you can decode it in Scala as a Map or write your custom reader, that can translate this object into an array type. Or you could make changes from the other side, instead of using Uint8Array you can use an array or a custom stringify function that makes expected JSON.

In my opinion, the easiest one is writing the custom reader. Custom reader example:


  implicit val stuffReader = new Reads[StuffData] {
    def reads(js: JsValue): JsResult[StuffData] = {
      JsSuccess(StuffData(
        (js \ "stuff").as[Map[String, Int]].toList.map(_._2)
      ))
    }
  }

edited Feb 12, 2023 at 16:27

answered Feb 11, 2023 at 18:57

Stanislav Kovalenko

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26 Comments

rapt Over a year ago

If you look inside JavaScript's Unit8Array, it's more complicated than what you wrote. I don't think writing my own converter is the way to go here.

Stanislav Kovalenko Over a year ago

I saw the object in the web inspector. Sure it's the hardest option, I just wanted to be clear about all options that I have found out. In my opinion, the easiest one is writing the custom reader.

rapt Over a year ago

Regarding your original comment before the edit, note that if the JavaScript object is something like {stuff: [0, 1, 2]}, then the code works.

Stanislav Kovalenko Over a year ago

Yep, you are right. First, when I wrote my first answer I thought that this is an array. But after my little research about Unit8Array type, I think I got your problem.

rapt Over a year ago

You are right, the problem was trying to put an Int inside a Byte, which stops working for integers > 127. I'm glad it's less complicated than I first thought and that we've found an easy way to do it. Thank you for the ideas!

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