EcmaScript Language Support for Eclipse Vert.x

View the Project on GitHub reactiverse/es4x

GraalVM Support

ES4X has experimental GraalVM support. The same code will run either on Nashorn (JS Engine in JDK>=8) or GraalJS (if run on GraalVM).

There are benefits on using GraalJS namely the updated language support >=ES6 and support out of the box for generators, promises, etc….

The downsides are that the engine is not in 1 to 1 parity of features to the old Nashorn, for example, Nashorn allows the usage of property names to refer to getters and setters, while Graal is strict. For example:

class Hello {
  private String name;
  public String getName() {
    return name;
  public void setName(String name) { = name;

When using this Java Object from Nashorn one could get and set the name as:

var hello = new Hello();
// get the name
var name =;
// set the name = 'Paulo';

This is not valid in Graal and should be:

var hello = new Hello();
// get the name
var name = hello.getName();
// set the name

While Nashorn would not complain when using threads or being executed from different threads, GraalJS will not allow this, so you should ensure that you’re always executing in the right context. If there is a need to work with multiple threads, then consider looking at the Worker API.

Native Images

Currently you cannot generate native images with ES4X, the limitation is because the static analysis of the AOT compiler will not take in consideration the java code invoked from the script (so classes won’t be available), plus the fact that the compiler has no support for jvm interop at runtime.

REPL / Shell

Every ES4X project can run within a very simple REPL. This shell will take scripts line by line and evaluate the result, so there is currently no support for multiline scripts.

To start a REPL you can execute:

java -cp your_fatjar.jar io.reactiverse.es4x.Shell

If you need to pass arguments to your REPL, say for example, that you would like to have cluster support, then use the kebab notation in the arguments as:

java -cp your_fatjar.jar io.reactiverse.es4x.Shell -clustered

If the last argument is not in the kebab format it is assumed that is the startup script and will be executed on start:

java -cp your_fatjar.jar io.reactiverse.es4x.Shell -clustered ./index.js