Kotlin To Javascript and Grails 3.3.2

Here is some sections of the build.gradle file we use for Grails 3.3.2 application, using the kotlin-gradle-plugin.

Both assemble and bootRun works fine. Assemble task generates the war file with javascript assets plugin being activated, and used after compileKotlin2Js task.

buildscript {
    ext.kotlin_version = "1.2.10"//'${kotlinVersion}'
    ext.web_dir = "grails-app/assets/javascripts/kt"
    /* . . . */
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        /* . . . */
        classpath "com.bertramlabs.plugins:asset-pipeline-gradle:2.14.7"
    }
}

/* . . . */
apply plugin: 'kotlin2js'
/* . . . */
apply plugin: "asset-pipeline"
/* . . . */

repositories {
    mavenLocal()
    jcenter()
    maven { url "http://repo.grails.org/grails/core" }
}

dependencies {
    assets "com.craigburke.angular:angular-template-asset-pipeline:2.3.0"
    assets "com.craigburke.angular:angular-annotate-asset-pipeline:2.4.0"
    /* . . . */
    runtime 'com.bertramlabs.plugins:asset-pipeline-grails:2.14.7'
    compile "org.jetbrains.kotlin:kotlin-stdlib-js:$kotlin_version"
}

assets {
    minifyJs = true
    minifyCss = true
    enableSourceMaps = false
    configOptions = [:] //useful for custom config on extension libraries
    minifyOptions = [
            optimizationLevel: 'SIMPLE',
            angularPass      : true
            // Can use @ngInject annotation for Angular Apps
    ]
    includes = []
    excludes = ['**/*.less'] //Example Exclude GLOB pattern
    packagePlugin = false //set to true if this is a library
    developmentRuntime = true
    jarTaskName = null
}

/* Kotlin javascript section +++ */
//sourceSets {
//    main.kotlin.srcDirs += "src/main/kotlin"
////    test.kotlin.srcDirs += "src/test/kotlin"
//}


//compileKotlin2Js {
//    kotlinOptions.outputFile = "${projectDir}/${web_dir}/MyApp.js"
//    kotlinOptions.moduleKind = "amd"
//    kotlinOptions.sourceMap = true
//}

task assembleWeb(type: Sync) {
    // Copy kotlin.js and kotlin-meta.js from jar into web directory
    configurations.compile.grep { File file ->
        file.name.startsWith("kotlin")
    }.each { File file ->
        copy {
            includeEmptyDirs = false
            def pff = zipTree(file)
            from pff
            include "*.js"
            into "${projectDir}/${web_dir}"
        }
    }
}

compileKotlin2Js.doLast() {
    copy {
        includeEmptyDirs = false
        from new File("build/classes/main")
        include "*.js"
        into "${web_dir}"
    }
}

assetCompile.dependsOn compileKotlin2Js
assetCompile.dependsOn assembleWeb
/* Kotlin javascript section --- */
/* . . . */

The important part is surrounded by "Kotlin Javascript Section" comments. 

You cannot set kotlinOptions.outputFile, because it will copy all the content of the "build/classes/main" directory into the assets directory (which is very annoying in case of Grails applications).

You have to copy "*.js" files from  "build/classes/main" directory into the assets folder you have chosen.

Usually, I would have open a bug report against this Gradle plugin for this kind of behavior, but I do not know Gradle deeply enough, and I am confused with the Intellij bug tracker system. 

Hope this helps someone!