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Working with Kotlin and JavaScript Modules

Last Updated 30 September 2016
A look at how to use Kotlin to interact with JavaScript modules.

In this tutorial we'll see how to

Configuring Modules with IntelliJ IDEA

Kotlin generate JavaScript code that is compatible with Asynchronous Module Definition (AMD), CommonJS and Universal Model Definition (UMD).

  • AMD is usually used on the client-side in the browser. The idea behind AMD is to load modules asynchronously, thus improving usability and performance.
  • CommonJS is the module system used on the server-side, and in particular with node.js. Node modules all abide by this definition. CommonJS modules can also be used in the browser via Browserify.
  • UMD tries to unify both models allowing these to be used either on the client or server.

We can configure the Kotlin compiler option to use any of these. The last option (UMD) will generate UMD and fallback to the other options if one is not available. Currently Kotlin compiler options are per IntelliJ IDEA project as opposed to a Kotlin module.

Kotlin Compiler Options

Configuring Modules when using Maven or Gradle

If using Maven or Gradle, we can also configure the module output format. For more information see JavaScript Modules.

Using AMD

When using AMD, we set the compiler option to use AMD. Once we do that, we can then reference any modules that we've defined as if they were regular AMD ones.

For instance, given

class Customer(val id: Int, val name: String, val email: String) {
    var isPreferred = false
    fun makePreferred() {
        isPreferred = true
    }
}

the following JavaScript code will be generated

define('customerBL', ['kotlin'], function (Kotlin) {
  'use strict';
  var _ = Kotlin.defineRootPackage(null, /** @lends _ */ {
    Customer: Kotlin.createClass(null, function Customer(id, name, email) {
      this.id = id;
      this.name = name;
      this.email = email;
      this.isPreferred = false;
    }, /** @lends _.Customer.prototype */ {
      makePreferred: function () {
        this.isPreferred = true;
      }
    })
  });
  Kotlin.defineModule('customerBL', _);
  return _;
});

Assuming we have the following project layout

Project Structure AMD

we could define our index.html to load require.js along with main.js as the value of the data-main attribute

<head>
    <meta charset="UTF-8">
    <title>Sample AMD</title>
    <script data-main="scripts/main"  src="scripts/require.js"></script>
</head>

The contents of our main.js would be:

requirejs.config({
    paths: {
        kotlin: 'out/lib/kotlin.js',
        customerBL: 'out/customerBL'
    }
});

requirejs(["customerBL"], function (customerBL) {
    console.log(customerBL)
});

With this, we can then access any of the functionality defined inside customerBL.

Using CommonJS

In order to use Kotlin with node.js, we need to set the compiler option to use CommonJS. Once we do that, the output of the application should be accessible using the node module system.

For instance, given

class Customer(val id: Int, val name: String, val email: String) {
    var isPreferred = false
    fun makePreferred() {
        isPreferred = true
    }
}

the following JavaScript code will be generated

module.exports = function (Kotlin) {
  'use strict';
  var _ = Kotlin.defineRootPackage(null, /** @lends _ */ {
    Customer: Kotlin.createClass(null, function Customer(id, name, email) {
      this.id = id;
      this.name = name;
      this.email = email;
      this.isPreferred = false;
    }, /** @lends _.Customer.prototype */ {
      makePreferred: function () {
        this.isPreferred = true;
      }
    })
  });
  Kotlin.defineModule('customerBL', _);
  return _;
}(require('kotlin'));

The last line is invoking the function itself and passing as argument kotlin, which refers to the standard library. This can be obtained in one of two ways:

Local reference

When compiling, the compiler always outputs the kotlin.js file. The easiest way to reference this without having to refer to paths, is to set the output library folder for the compiler options to node_modules. This way, Node will automatically pick it up as it does an exhaustive search for files under these folders

Node Modules

NPM Directory

The Kotlin standard library is available on npm and we can simply include it in our package.json as a dependency.

{
  "name": "node-demo",
  "description": "A sample of using Kotlin with node.js",
  "version": "0.0.1",
  "dependencies": {
    "kotlin": ">=1.0.4-eap-111"
  }
}

We can simply refer to any class or member function inside our node.js code by simply importing the module using require:



var customerBL = require('./scripts/customerBL')

var customer = new customerBL.Customer(1, "Jane Smith", "jane.smith@company.com")

console.dir(customer)
customer.makePreferred()
console.dir(customer)

In this case, we've set the output of our compilation to the scripts folder. On running the application we should see the following output:

Output CommonJS