Create a JavaScript API from Java


Before trying this example, make sure you have the MMM CLI (Command Line Interface) installed.

It is also recommended to follow the Getting Started page and/or the Simple Application example before.

In this example a JavaScript API is exposed from Java. This can be useful when a specific API must be defined in JavaScript or when adapting an existing Java API to a JavaScript API.

Create a class MyApiHostObject (src/main/java/com/mycompany/

public class MyApiHostObject extends JsObject {

        public MyApiHostObject(Object thisBinding) {

                this.put("count", new DataPropertyDescriptor(JsRuntime.createFunction(new JsClosure() {
                        public Object invoke(@Nullable Object thisBinding, int argsLength, Object... arguments) {
                                String data = (String) arguments[0];
                                return Integer.valueOf(data.length());


This class defines a JavaScript object using the MicroEJ JavaScript API by extending the class JsObject. It also defines a count method which accepts a String parameter and returns its length.

Then in the Java Main class of the application, add the glue to expose the MyApi object to the JavaScript code and init the JavaScript engine:

public static void main(String[] args) throws Exception {
        // Add the "MyApi" function in the JavaScript global object
        JsRuntime.JS_GLOBAL_OBJECT.put("MyApi", JsRuntime.createFunction(new JsClosure() {
                public Object invoke(Object thisBinding, int argsLength, Object... arguments) {
                        return new MyApiHostObject(thisBinding);
        }), false);

        // Init the JavaScript code
        // Start the JavaScript engine;

You can now call the new API from the JavaScript code:

var myApi = new MyApi();
print(myApi.count("Hello World!"));

Finally, build and execute the application with the MMM CLI:

$ mmm build
$ mmm run

The length of the string Hello World! (12) should be displayed.

For more information about communication between Java and JavaScript please refer to the Communication Between Java and JS page.