Set up a Kotlin/JS project

Edit pageLast modified: 18 March 2025

Kotlin/JS projects use Gradle as a build system. To let developers easily manage their Kotlin/JS projects, we offer the kotlin.multiplatform Gradle plugin that provides project configuration tools together with helper tasks for automating routines typical for JavaScript development.

The plugin downloads npm dependencies in the background using the npm or Yarn package managers and builds a JavaScript bundle from a Kotlin project using webpack. Dependency management and configuration adjustments can be done to a large part directly from the Gradle build file, with the option to override automatically generated configurations for full control.

You can apply the org.jetbrains.kotlin.multiplatform plugin to a Gradle project manually in the build.gradle(.kts) file:

plugins {
    kotlin("multiplatform") version "2.1.20"
}

The Kotlin Multiplatform Gradle plugin lets you manage aspects of your project in the kotlin {} block of the build script:

kotlin {
    // ...
}

Inside the kotlin {} block, you can manage the following aspects:

Execution environments

Kotlin/JS projects can target two different execution environments:

  • Browser for client-side scripting in browsers

  • Node.js for running JavaScript code outside of a browser, for example, for server-side scripting.

To define the target execution environment for a Kotlin/JS project, add the js {} block with browser {} or nodejs {} inside:

kotlin {
    js {
        browser {
        }
        binaries.executable()
    }
}

The instruction binaries.executable() explicitly instructs the Kotlin compiler to emit executable .js files. This is the default behavior when using the current Kotlin/JS compiler, but the instruction is explicitly required if you are working with the Kotlin/JS IR compiler, or have set kotlin.js.generate.executable.default=false in the gradle.properties file. In those cases, omitting binaries.executable() will cause the compiler to only generate Kotlin-internal library files, which can be used from other projects, but not run on their own.

tip

This is typically faster than creating executable files, and can be a possible optimization when dealing with non-leaf modules of your project.

The Kotlin Multiplatform plugin automatically configures its tasks for working with the selected environment. This includes downloading and installing the required environment and dependencies for running and testing the application. This allows developers to build, run, and test simple projects without additional configuration. For projects targeting Node.js, there is also an option to use an existing Node.js installation. Learn how to use pre-installed Node.js.

Support for ES2015 features

Kotlin provides an Experimental support for the following ES2015 features:

  • Modules that simplify your codebase and improve maintainability.

  • Classes that allow incorporating OOP principles, resulting in cleaner and more intuitive code.

  • Generators for compiling suspend functions that improve the final bundle size and help with debugging.

You can enable all the supported ES2015 features at once by adding the es2015 compilation target to your build.gradle(.kts) file:

tasks.withType<KotlinJsCompile>().configureEach {
    kotlinOptions {
        target = "es2015"
    }
}

Learn more about ES2015 (ECMAScript 2015, ES6) in the official documentation.

Dependencies

Like any other Gradle projects, Kotlin/JS projects support traditional Gradle dependency declarations in the dependencies {} block of the build script:

dependencies {
    implementation("org.example.myproject", "1.1.0")
}

The Kotlin Multiplatform Gradle plugin also supports dependency declarations for particular source sets in the kotlin {} block of the build script:

kotlin {
    sourceSets {
        val jsMain by getting {
            dependencies {
                implementation("org.example.myproject:1.1.0")
            }
        }
    }
}

note

Not all libraries available for the Kotlin programming language are available when targeting JavaScript: only libraries that include artifacts for Kotlin/JS can be used.

If the library you are adding has dependencies on packages from npm, Gradle will automatically resolve these transitive dependencies as well.

Kotlin standard libraries

The dependencies on the standard library are added automatically. The version of the standard library is the same as the version of the Kotlin Multiplatform plugin.

For multiplatform tests, the kotlin.test API is available. When you create a multiplatform project, you can add test dependencies to all the source sets by using a single dependency in commonTest:

kotlin {
    sourceSets {
        commonTest.dependencies {
            implementation(kotlin("test")) // Brings all the platform dependencies automatically
        }
    }
}

npm dependencies

In the JavaScript world, the most common way to manage dependencies is npm. It offers the biggest public repository of JavaScript modules.

The Kotlin Multiplatform Gradle plugin lets you declare npm dependencies in the Gradle build script, just how you declare any other dependencies.

To declare an npm dependency, pass its name and version to the npm() function inside a dependency declaration. You can also specify one or multiple version ranges based on npm's semver syntax.

dependencies {
    implementation(npm("react", "> 14.0.0 <=16.9.0"))
}

By default, the plugin uses a separate instance of the Yarn package manager to download and install npm dependencies. It works out of the box without additional configuration, but you can tune it to specific needs.

You can also work with npm dependencies directly using the npm package manager instead. To use npm as your package manager, in your gradle.properties file, set the following property:

kotlin.js.yarn=false

Besides regular dependencies, there are three more types of dependencies that can be used from the Gradle DSL. To learn more about when each type of dependency can best be used, have a look at the official documentation linked from npm:

Once an npm dependency is installed, you can use its API in your code as described in Calling JS from Kotlin.

run task

The Kotlin Multiplatform Gradle plugin provides a jsBrowserDevelopmentRun task that lets you run pure Kotlin/JS projects without additional configuration.

For running Kotlin/JS projects in the browser, this task is an alias for the browserDevelopmentRun task (which is also available in Kotlin multiplatform projects). It uses the webpack-dev-server to serve your JavaScript artifacts. If you want to customize the configuration used by webpack-dev-server, for example, adjust the port the server runs on, use the webpack configuration file.

For running Kotlin/JS projects targeting Node.js, use the jsBrowserDevelopmentRun task that is an alias for the nodeRun task.

To run a project, execute the standard lifecycle jsBrowserDevelopmentRun task, or the alias to which it corresponds:

./gradlew jsBrowserDevelopmentRun

To automatically trigger a re-build of your application after making changes to the source files, use the Gradle continuous build feature:

./gradlew jsBrowserDevelopmentRun --continuous

or

./gradlew jsBrowserDevelopmentRun -t

Once the build of your project has succeeded, the webpack-dev-server will automatically refresh the browser page.

test task

The Kotlin Multiplatform Gradle plugin automatically sets up a test infrastructure for projects. For browser projects, it downloads and installs the Karma test runner with other required dependencies; for Node.js projects, the Mocha test framework is used.

The plugin also provides useful testing features, for example:

  • Source maps generation

  • Test reports generation

  • Test run results in the console

For running browser tests, the plugin uses Headless Chrome by default. You can also choose another browser to run tests in, by adding the corresponding entries inside the useKarma {} block of the build script:

kotlin {
    js {
        browser {
            testTask {
                useKarma {
                    useIe()
                    useSafari()
                    useFirefox()
                    useChrome()
                    useChromeCanary()
                    useChromeHeadless()
                    usePhantomJS()
                    useOpera()
                }
            }
        }
        binaries.executable()
        // ...
    }
}

Alternatively, you can add test targets for browsers in the gradle.properties file:

kotlin.js.browser.karma.browsers=firefox,safari

This approach allows you to define a list of browsers for all modules, and then add specific browsers in the build scripts of particular modules.

Please note that the Kotlin Multiplatform Gradle plugin does not automatically install these browsers for you, but only uses those that are available in its execution environment. If you are executing Kotlin/JS tests on a continuous integration server, for example, make sure that the browsers you want to test against are installed.

If you want to skip tests, add the line enabled = false to the testTask {}:

kotlin {
    js {
        browser {
            testTask {
                enabled = false
            }
        }
        binaries.executable()
        // ...
    }
}

To run tests, execute the standard lifecycle check task:

./gradlew check

To specify environment variables used by your Node.js test runners (for example, to pass external information to your tests, or to fine-tune package resolution), use the environment() function with a key-value pair inside the testTask {} block in your build script:

kotlin {
    js {
        nodejs {
            testTask {
                environment("key", "value")
            }
        }
    }
}

Karma configuration

The Kotlin Multiplatform Gradle plugin automatically generates a Karma configuration file at build time which includes your settings from the kotlin.js.browser.testTask.useKarma {} block in your build.gradle(.kts). You can find the file at build/js/packages/projectName-test/karma.conf.js. To make adjustments to the configuration used by Karma, place your additional configuration files inside a directory called karma.config.d in the root of your project. All .js configuration files in this directory will be picked up and are automatically merged into the generated karma.conf.js at build time.

All Karma configuration abilities are well described in Karma's documentation.

webpack bundling

For browser targets, the Kotlin Multiplatform Gradle plugin uses the widely known webpack module bundler.

webpack version

The Kotlin Multiplatform plugin uses webpack 5.

If you have projects created with plugin versions earlier than 1.5.0, you can temporarily switch back to webpack 4 used in these versions by adding the following line to the project's gradle.properties:

kotlin.js.webpack.major.version=4

webpack task

The most common webpack adjustments can be made directly via the kotlin.js.browser.webpackTask {} configuration block in the Gradle build file:

  • outputFileName - the name of the webpacked output file. It will be generated in <projectDir>/build/dist/<targetName> after an execution of a webpack task. The default value is the project name.

  • output.libraryTarget - the module system for the webpacked output. Learn more about available module systems for Kotlin/JS projects. The default value is umd.

webpackTask {
    outputFileName = "mycustomfilename.js"
    output.libraryTarget = "commonjs2"
}

You can also configure common webpack settings to use in bundling, running, and testing tasks in the commonWebpackConfig {} block.

webpack configuration file

The Kotlin Multiplatform Gradle plugin automatically generates a standard webpack configuration file at the build time. It is located in build/js/packages/projectName/webpack.config.js.

If you want to make further adjustments to the webpack configuration, place your additional configuration files inside a directory called webpack.config.d in the root of your project. When building your project, all .js configuration files will automatically be merged into the build/js/packages/projectName/webpack.config.js file. For example, To add a new webpack loader, add the following to a .js file inside the webpack.config.d directory:

note

In this case, the configuration object is the config global object. You need to modify it in your script.

config.module.rules.push({
    test: /\.extension$/,
    loader: 'loader-name'
});

All webpack configuration capabilities are well described in its documentation.

Building executables

For building executable JavaScript artifacts through webpack, the Kotlin Multiplatform Gradle plugin contains the browserDevelopmentWebpack and browserProductionWebpack Gradle tasks.

  • browserDevelopmentWebpack creates development artifacts, which are larger in size, but take little time to create. As such, use the browserDevelopmentWebpack tasks during active development.

  • browserProductionWebpack applies dead code elimination to the generated artifacts and minifies the resulting JavaScript file, which takes more time, but generates executables that are smaller in size. As such, use the browserProductionWebpack task when preparing your project for production use.

Execute either of these tasks to obtain the respective artifacts for development or production. The generated files will be available in build/dist unless specified otherwise.

./gradlew browserProductionWebpack

Note that these tasks will only be available if your target is configured to generate executable files (via binaries.executable()).

CSS

The Kotlin Multiplatform Gradle plugin also provides support for webpack's CSS and style loaders. While all options can be changed by directly modifying the webpack configuration files that are used to build your project, the most commonly used settings are available directly from the build.gradle(.kts) file.

To turn on CSS support in your project, set the cssSupport.enabled option in the Gradle build file in the commonWebpackConfig {} block. This configuration is also enabled by default when creating a new project using the wizard.

browser {
    commonWebpackConfig {
        cssSupport {
            enabled.set(true)
        }
    }
}

Alternatively, you can add CSS support independently for webpackTask {}, runTask {}, and testTask {}:

browser {
    webpackTask {
        cssSupport {
            enabled.set(true)
        }
    }
    runTask {
        cssSupport {
            enabled.set(true)
        }
    }
    testTask {
        useKarma {
            // ...
            webpackConfig.cssSupport {
                enabled.set(true)
            }
        }
    }
}

Activating CSS support in your project helps prevent common errors that occur when trying to use style sheets from an unconfigured project, such as Module parse failed: Unexpected character '@' (14:0).

You can use cssSupport.mode to specify how encountered CSS should be handled. The following values are available:

  • "inline" (default): styles are added to the global <style> tag.

  • "extract": styles are extracted into a separate file. They can then be included from an HTML page.

  • "import": styles are processed as strings. This can be useful if you need access to the CSS from your code (such as val styles = require("main.css")).

To use different modes for the same project, use cssSupport.rules. Here, you can specify a list of KotlinWebpackCssRules, each of which defines a mode, as well as include and exclude patterns.

Node.js

For Kotlin/JS projects targeting Node.js, the plugin automatically downloads and installs the Node.js environment on the host. You can also use an existing Node.js instance if you have it.

Configuring Node.js settings

You can configure Node.js settings for each subproject, or set them for the project as a whole.

For example, to set the Node.js version for a specific subproject, add the following lines to its Gradle block in your build.gradle(.kts) file:

project.plugins.withType<org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsPlugin> {
    project.the<org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsEnvSpec>().version = "your Node.js version"
}

To set a version for the entire project, including all subprojects, apply the same code to the allProjects {} block:

allprojects {
    project.plugins.withType<org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsPlugin> {
        project.the<org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsEnvSpec>().version = "your Node.js version"
    }
}

note

Using the NodeJsRootPlugin class to configure Node.js setting for the entire project is deprecated and will eventually stop being supported.

Use pre-installed Node.js

If Node.js is already installed on the host where you build Kotlin/JS projects, you can configure the Kotlin Multiplatform Gradle plugin to use it instead of installing its own Node.js instance.

To use a pre-installed Node.js instance, add the following lines to your build.gradle(.kts) file:

project.plugins.withType(org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsPlugin) {
    // Set to `true` for default behavior
    project.extensions.getByType(org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsEnvSpec).download = false
}

Yarn

By default, to download and install your declared dependencies at build time, the plugin manages its own instance of the Yarn package manager. It works out of the box without additional configuration, but you can tune it or use Yarn already installed on your host.

Additional Yarn features: .yarnrc

To configure additional Yarn features, place a .yarnrc file in the root of your project. At build time, it gets picked up automatically.

For example, to use a custom registry for npm packages, add the following line to a file called .yarnrc in the project root:

registry "http://my.registry/api/npm/"

To learn more about .yarnrc, visit the official Yarn documentation.

Use pre-installed Yarn

If Yarn is already installed on the host where you build Kotlin/JS projects, you can configure the Kotlin Multiplatform Gradle plugin to use it instead of installing its own Yarn instance.

To use the pre-installed Yarn instance, add the following lines to build.gradle(.kts):

rootProject.plugins.withType<org.jetbrains.kotlin.gradle.targets.js.yarn.YarnPlugin> {
    rootProject.the<org.jetbrains.kotlin.gradle.targets.js.yarn.YarnRootExtension>().download = false
    // "true" for default behavior
}

Version locking via kotlin-js-store

note

Version locking via kotlin-js-store is available since Kotlin 1.6.10.

The kotlin-js-store directory in the project root is automatically generated by the Kotlin Multiplatform Gradle plugin to hold the yarn.lock file, which is necessary for version locking. The lockfile is entirely managed by the Yarn plugin and gets updated during the execution of the kotlinNpmInstall Gradle task.

To follow a recommended practice, commit kotlin-js-store and its contents to your version control system. It ensures that your application is being built with the exact same dependency tree on all machines.

If needed, you can change both directory and lockfile names in build.gradle(.kts):

rootProject.plugins.withType<org.jetbrains.kotlin.gradle.targets.js.yarn.YarnPlugin> {
    rootProject.the<org.jetbrains.kotlin.gradle.targets.js.yarn.YarnRootExtension>().lockFileDirectory =
        project.rootDir.resolve("my-kotlin-js-store")
    rootProject.the<org.jetbrains.kotlin.gradle.targets.js.yarn.YarnRootExtension>().lockFileName = "my-yarn.lock"
}

warning

Changing the name of the lockfile may cause dependency inspection tools to no longer pick up the file.

To learn more about yarn.lock, visit the official Yarn documentation.

Reporting that yarn.lock has been updated

Kotlin/JS provides Gradle settings that could notify you if the yarn.lock file has been updated. You can use these settings when you want to be notified if yarn.lock has been changed silently during the CI build process:

  • YarnLockMismatchReport, which specifies how changes to the yarn.lock file are reported. You can use one of the following values:

    • FAIL fails the corresponding Gradle task. This is the default.

    • WARNING writes the information about changes in the warning log.

    • NONE disables reporting.

  • reportNewYarnLock, which reports about the recently created yarn.lock file explicitly. By default, this option is disabled: it's a common practice to generate a new yarn.lock file at the first start. You can use this option to ensure that the file has been committed to your repository.

  • yarnLockAutoReplace, which replaces yarn.lock automatically every time the Gradle task is run.

To use these options, update build.gradle(.kts) as follows:

import org.jetbrains.kotlin.gradle.targets.js.yarn.YarnLockMismatchReport
import org.jetbrains.kotlin.gradle.targets.js.yarn.YarnRootExtension

rootProject.plugins.withType(org.jetbrains.kotlin.gradle.targets.js.yarn.YarnPlugin::class.java) {
    rootProject.the<YarnRootExtension>().yarnLockMismatchReport =
        YarnLockMismatchReport.WARNING // NONE | FAIL
    rootProject.the<YarnRootExtension>().reportNewYarnLock = false // true
    rootProject.the<YarnRootExtension>().yarnLockAutoReplace = false // true
}

Installing npm dependencies with --ignore-scripts by default

note

Installing npm dependencies with --ignore-scripts by default is available since Kotlin 1.6.10.

To reduce the likelihood of executing malicious code from compromised npm packages, the Kotlin Multiplatform Gradle plugin prevents the execution of lifecycle scripts during the installation of npm dependencies by default.

You can explicitly enable lifecycle scripts execution by adding the following lines to build.gradle(.kts):

rootProject.plugins.withType<org.jetbrains.kotlin.gradle.targets.js.yarn.YarnPlugin> {
    rootProject.the<org.jetbrains.kotlin.gradle.targets.js.yarn.YarnRootExtension>().ignoreScripts = false
}

Distribution target directory

By default, the results of a Kotlin/JS project build reside in the /build/dist/<targetName>/<binaryName> directory within the project root.

note

Prior to Kotlin 1.9.0, the default distribution target directory was /build/distributions.

To set another location for project distribution files, in your build script inside the browser {} block, add a distribution {} block and assign a value to the outputDirectory property by using the set() method. Once you run a project build task, Gradle will save the output bundle in this location together with project resources.

kotlin {
    js {
        browser {
            distribution {
                outputDirectory.set(projectDir.resolve("output"))
            }
        }
        binaries.executable()
        // ...
    }
}

Module name

To adjust the name for the JavaScript module (which is generated in build/js/packages/myModuleName), including the corresponding .js and .d.ts files, use the moduleName option:

js {
    moduleName = "myModuleName"
}

Note that this does not affect the webpacked output in build/dist.

package.json customization

The package.json file holds the metadata of a JavaScript package. Popular package registries such as npm require all published packages to have such a file. They use it to track and manage package publications.

The Kotlin Multiplatform Gradle plugin automatically generates package.json for Kotlin/JS projects during build time. By default, the file contains essential data: name, version, license, dependencies, and some other package attributes.

Aside from basic package attributes, package.json can define how a JavaScript project should behave, for example, identifying scripts that are available to run.

You can add custom entries to the project's package.json via the Gradle DSL. To add custom fields to your package.json, use the customField() function in the compilations packageJson block:

kotlin {
    js {
        compilations["main"].packageJson {
            customField("hello", mapOf("one" to 1, "two" to 2))
        }
    }
}

When you build the project, this code adds the following block to the package.json file:

"hello": {
    "one": 1,
    "two": 2
}

Learn more about writing package.json files for npm registry in the npm docs.