build.gradle

group 'com.example'
version '1.0-SNAPSHOT'

buildscript {
    ext.kotlin_version = '1.3.0'
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

apply plugin: 'kotlin2js'

repositories {
    mavenCentral()
    maven { url "http://dl.bintray.com/kotlin/kotlinx" }
//    maven { url "https://dl.bintray.com/kotlin/kotlin-eap" }
    maven { url "https://dl.bintray.com/kotlin/ktor" }
}

dependencies {
    compile "org.jetbrains.kotlin:kotlin-stdlib-js:$kotlin_version"
    compile "io.ktor:ktor-client-js:1.0.0-beta-3"
//    compile "io.ktor:ktor-client-core:1.0.0-beta-3"
//    compile "io.ktor:ktor-client:1.0.0-beta-3"
//    compile "org.jetbrains.kotlinx:kotlinx-coroutines-core-js:1.0.1"
}

compileKotlin2Js {
    kotlinOptions.outputFile = "${projectDir}/web/output.js"
    kotlinOptions.moduleKind = "commonjs"
    kotlinOptions.sourceMap = true
    kotlinOptions.languageVersion = "1.3"
    kotlinOptions.sourceMapEmbedSources = "always"
//    kotlinOptions.metaInfo = false
}

task assembleWeb(type: Sync) {
    configurations.compile.each { File file ->
        from(zipTree(file.absolutePath), {
            includeEmptyDirs = false
            include { fileTreeElement ->
                def path = fileTreeElement.path
                path.endsWith(".js") && (path.startsWith("META-INF/resources/") ||
                        !path.startsWith("META-INF/"))
            }
        })
    }
    from compileKotlin2Js.destinationDir
    into "${projectDir}/web"

    dependsOn classes
}
assemble.dependsOn assembleWeb

main.kt

import io.ktor.client.HttpClient
import io.ktor.client.request.get

external fun require(module:String):dynamic

data class IndexRequest(val title: String, val message: String)

suspend fun main(args: Array<String>) {
    println("Hello JavaScript!")

    println(args)

    val twig = require("twig")
    twig.cache(false)
    val express = require("express")
    val app = express()

//    app.set("view engine", "ejs")
    app.set("view engine", "twig")

    val options: dynamic = object{}
    options["allow_async"] = true
    options["strict_variables"] = false

    app.set("twig options", options)

    val client = HttpClient()
    val wiki = client.get<String>("https://en.wikipedia.org/wiki/Main_Page")
    println(wiki)

//    app.get("/") { _, res ->
//        res.type("text/plain")
//        res.send("i am a beautiful butterfly")
//    }

    app.get("/") { req, res ->
        res.render("index", IndexRequest("hello", "message"))
//        res.render("index")
    }

    app.listen(3000) {
        println("Listening on port 3000")
    }
}

package.json

{
  "name": "example",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "dependencies": {
    "express": "^4.16.4",
    "kotlin": "^1.3.0",
    "kotlinx-coroutines-core": "^1.0.1",
    "twig": "^1.12.0"
  }
}

/build.gradle
/package.json
/src/main/kotlin/main.kt

yarn install && gradle build && gradle assemble
NODE_PATH=web node web/output.js

If you change module kind to umd, leave only the http client call in main() {} and make this /index.html file - it get's further and is stopped by cors - if you load this in a web browser.
`

<script type="text/javascript" src="web/kotlin.js"></script>

<script type="text/javascript" src="web/kotlinx-atomicfu.js"></script>
<script type="text/javascript" src="web/kotlinx-io-js.js"></script>
<script type="text/javascript" src="web/kotlinx-coroutines-core.js"></script>
<script type="text/javascript" src="web/kotlinx-coroutines-io-js.js"></script>

<script type="text/javascript" src="web/ktor-utils-js.js"></script>

<script type="text/javascript" src="web/ktor-http-js.js"></script>

<script type="text/javascript" src="web/ktor-utils-js.js"></script>
<script type="text/javascript" src="web/ktor-client-core-js.js"></script>

<script type="text/javascript" src="web/output.js"></script>

`