How to use the Google Closure Compiler to optimize Kotlin JS

First, you should declare an NPM dependency on the Closure Compiler (latest version):

dependencies {
    implementation(devNpm("google-closure-compiler", "20210808.0.0"))
}

Then, create a task that will run after webpack minification:

tasks.create<Exec>("compileWithClosure") {
    // browserProductionWebpack: production minified version by Webpack
    // kotlinNodeJsSetup: needed to execute Node scripts (':' because it is on the root project)
    dependsOn("browserProductionWebpack", ":kotlinNodeJsSetup")

    // Get the Node installation directory
    val kotlinNodeJsSetup = rootProject.tasks["kotlinNodeJsSetup"] as org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsSetupTask
    workingDir = File(kotlinNodeJsSetup.destination, "bin")

    // Execute the script
    commandLine(
        "node",
        "${File(rootProject.buildDir, "js/node_modules/google-closure-compiler/cli.js")}",
        "--js=${File(buildDir, "distributions/<your module name here>.js")}",
        "--js_output_file=${File(buildDir, "distributions/optimized.js")}",
        "-O=SIMPLE",
        "--env=BROWSER",
        "--warning_level=QUIET",
    )
}

Full list of arguments:

• -O=ADVANCED seems to break the Kotlin generated file (at least in my case with Kotlin JS 1.5.21 IR)
• -O=SIMPLE removes ~600kB from the binary (2.1MB → 1.5MB, using Kotlin React, KotlinX.Serialization & Ktor)
• --warning_level=QUIET because we are passing an already-minified file, console output is completely unreadable, and actually slows down compilation