Instantiate Javascript classes that expect "new" keyword on KotlinJS

Question

considering the following javascript code (partially taken from Apollo Server documentation), it creates an instance of ApolloServer and start it.


const {ApolloServer} = require('apollo-server')

const server = new ApolloServer({ ... });

server.listen().then(({ url }) => {
  console.log(`Server ready at ${url}`);
});

Now consider to replicate the same behaviour using KotlinJS. Firstly, Kotlin doesn't have the "new" keyword and calling ApolloServer() as expected, won't work but raise an error (TypeError: Class constructor ApolloServer cannot be invoked without 'new').

// We can banally represent part of the code above like:
external fun require(module: String): dynamic
val ApolloServer = require("apollo-server").ApolloServer

// ApolloServer is a js class

Declaring an external class like:

external open class ApolloServer() {
    open fun listen(vararg opts: Any): Promise<Any>
    operator fun invoke(): Any
}

and set it as ApolloServer type doesn't help.

How do we replicate "new ApolloServer()" call?

sorry I don't know Kotlin but if it has JS interop there has to be an idiomatic way to call `new`. One band-aid fix could be to implement `function createServer(...) { return new ApolloServer(...) }`? — Thank you, Apr 24 '20 at 23:46
I don't know about K/JS but you probably need to call it without the new keyword it'll instantiate a new instance. Because there's no `new` keyword in Kotlin (atleast what we do in Kotlin/JVM) — Animesh Sahu, Apr 25 '20 at 04:52
Thankyou: Potentially I could make other js files that contain factories like createServer and then import them. That was the same conclusion I thought initially, but there must be a way to avoid this practice. AnimeshSahu: I do use Kotlin in JVM too and started to use it with node.js. It's very powerful but it lacks of documentation. Many times I have to analyze the javascript output in order to understand how it works because there isn't much explained around. — Jac, Apr 25 '20 at 07:26
The only solution I found is the following one but it doesn't work as expected: https://discuss.kotlinlang.org/t/kotlin-js-doesnt-add-new-keyword-where-needed/14986 I'll try to make few experiments with that and share the solution. — Jac, Apr 25 '20 at 07:28

score 3 · Accepted Answer · answered Apr 26 '20 at 22:31

To solve this problem I found an interesting approach based on JsModule annotation. We need to create a Kotlin file that represent that javascript module we want to import, in my case "apollo-server".

@file:JsModule("apollo-server")
@file:JsNonModule
package com.package

import kotlin.js.Promise

external interface ServerInfo {
    var address: String
    var family: String
    var url: String
    var subscriptionsUrl: String
    var port: dynamic /* Number | String */
        get() = definedExternally
        set(value) = definedExternally
    var subscriptionsPath: String
    var server: Any
}

external open class ApolloServer(config: Any? /* ApolloServerExpressConfig & `T$0` */) : Any {
    open var httpServer: Any
    open var cors: Any
    open var onHealthCheck: Any
    open var createServerInfo: Any
    open fun applyMiddleware()
    open fun listen(vararg opts: Any): Promise<ServerInfo>
    open fun stop(): Promise<Unit>
}

With the above code we are basically describing what we expect to find in the apollo-server module and how to map it into Kotlin.

In our Kotlin main function we don't have to specify any require(...) but just use our ApolloServer class like:

    ApolloServer(null).listen().then {
       console.log(it)
    }

Using this approach Kotlin would transpile it correctly, using the new keyword in javascript.

Transpiled version extract:

  function main$lambda(it) {
    console.log(it);
    return Unit;
  }
  function main() {
    (new ApolloServer(null)).listen().then(main$lambda);
  }

This code is just an example, ApolloServer won't be initialized without a proper configuration, this case, for example, contains a nullable configuration.

Instantiate Javascript classes that expect "new" keyword on KotlinJS

1 Answers1