Is there a reflection-based way (not `kotlinx.serialization`) in Kotlin/JS to parse JSON into Kotlin...

# javascript

[Joffrey]

Is there a reflection-based way (not kotlinx.serialization) in Kotlin/JS to parse JSON into Kotlin data classes?

@Test
    fun jsonTest() {
        data class Person(val name: String)

        val expected = Person("Bob")
        val parsedPerson = JSON.parse<Person>("""{"name":"Bob"}""")

        assertEquals(expected, parsedPerson)
    }

This test gives:

AssertionError: Expected <Person(name=Bob)>, actual <[object Object]>.

[diesieben07]

Reflection on JS is very limited, the answer is most likely no at this point.

[Roman Artemev [JB]]

The problem is that kotlinx.serialization guarantees type safety and restores class prototype proprely when basic JSON.parse constructs annonymous object with the similar layout. Could you please provide some details why kotlinx.serialization doesn’t fit your requirements?

[Joffrey]

I'm trying to work on a multiplatform library that needs to parse JSON into user-defined classes at some point. I can't force users of the library to annotate their code and run an annotation processor, so I can't really expect serializers to be available on user-defined classes 😕

[Joffrey]

Do you have a solution to this problem? I believe any multiplatform library (or even Kotlin/JS libraries I guess) that needs to parse JSON will run into this at some point

[Joffrey]

Isn't it misleading for JSON.parse() to return T? If we can't ensure the instance returned will be of type T , shouldn't we just return some sort of JS object type?

[Roman Artemev [JB]]

Unfortunatly there is no recomended solution besides ktx.serialization. Prorably you could try to set prototype manualy.

[Roman Artemev [JB]]

I think this might help

val o = js("{}")
o.__proto__ = js("Object.create(O.prtotype)")

[Roman Artemev [JB]]

Or even move data from json’s object into manually created

[Joffrey]

I'm sorry but I don't get how this works 😕 how does equals work in JS btw?

[Joffrey]

I can't move data because I don't know the available properties of the class (T::class.members is not available in Kotlin/JS)

[Roman Artemev [JB]]

Which equals? When you write == in kotlin?

[Joffrey]

Yes

[Roman Artemev [JB]]

It depends on, but generally it just invokes Any.equals

[Joffrey]

I mean how does that translate to JS?

[Roman Artemev [JB]]

There are lot of cases. In case of general objects a == b becomes a.equals(b)

[Roman Artemev [JB]]

Under the hood equals usually checks for instanceof

[Joffrey]

So the generated equals() on data classes checks for instanceof and this is based on JS prototype when compiled to JS? (I'm trying to understand how/why my sample code fails)

[Roman Artemev [JB]]

Yes. If you interesting especcialy in data class equals there are general alhorithm

[Roman Artemev [JB]]

For data class Dat(val start: String, val end: String)
equals in js going to be look like

Dat.prototype.equals = function (other) {
    return this === other || (other !== null && (typeof other === 'object' && (Object.getPrototypeOf(this) === Object.getPrototypeOf(other) && (Kotlin.equals(this.start, other.start) && Kotlin.equals(this.end, other.end)))));
  };

[Joffrey]

I understand how == mapping to equals() work, how data classes generate equals(), and how equals() work in general in the JVM world and how Kotlin translates that to the JVM world. What I don't understand is what equals() does in the compiled JS code. Because I can access properties of the object returned by JSON.parse(), and they are equal to the ones of the expected instance, but equals() fails

[Joffrey]

Oh great, thanks a lot

[Joffrey]

So it does check for prototype, and this is the part that fails in my case. Also, all the methods like copy() cannot be called on the object returned by JSON.parse(), but I only see the failure at runtime

[Roman Artemev [JB]]

So that the reason why we recommend to use ktx.serialization to parse json

[Joffrey]

Yes, I understand, I thought Kotlin would allow me to get away from JS, but JS still comes back to bite me 😄

[Roman Artemev [JB]]

Yes, that true. We are limited with platforms restrictions.

[Joffrey]

I guess I'll have to make the users provide the conversion function, but I'll provide built-in kotlinx.serialization support to simplify the API if they're willing to use it

[Roman Artemev [JB]]

Yes, that could be a good solutions I think

[Dave]

I had something working that did this for version 1.3.x
not yet updated for 1.5, IF you are still interested I can give details (I know its an old thread)