# Eclipse Vert.x

As it should be clear at this point, Vert.x is the IO and default programming model used by ES4X. There are however some nice improvements to the standard Vert.x APIs (opens new window).

# Generated APIs

All APIs published to npm under the namespaces @vertx and @reactiverse are code generated. Code generation is an helper that allows these APIs to be used by JavaScript users in a format that feels familiar without compromising the application performance.

Interacting with the JVM all happens over the Java object. The most important bit is to pull a JVM class to JS:

// Import the java.lang.Math class to be usable
// as a JS type in the script
const Math = Java.type('javalang.Math');

Now one could just do this for all APIs but there are several limitations that ES4X tries to address:

  • Error Prone - One needs to know the exact Java APIs and Types in order to use them from JavaScript.
  • No way to define dependencies - If you need to use APIs from different modules, importing class by class cannot define dependencies between them.
  • No IDE support - The developer will need to know the API before using it and the IDE will not assist.

ES4X generator solves this by creating a npm module for each vertx module and type defintions for each class.

Each module will have the following files:

  • package.json - Defines dependencies between modules
  • index.js - commonjs API interfaces
  • index.mjs - ESM API interfaces
  • index.d.ts - Full type definitions for the API interfaces
  • enum.js - commonjs API enumerations
  • enum.mjs - ESM API enumerations
  • enum.d.ts - Full type definitions for the API enumerations
  • options.js - commonjs API data objects.
  • options.mjs - ESM API data objects.
  • options.d.ts - Full type definitions for the API data objects

All the index files will simplify importing of JVM classes by replacing, for example:

// without ES4X
const Router = Java.type('io.vertx.ext.web.Router');
// with
import { Router } from '@vertx/web';

This small change will make IDEs assist with development and package managers to download dependencies as needed. Finally all the .d.ts files will hint IDEs about types and give code completion support.

# Promise/Future

Vert.x has 2 types:

  • io.vertx.core.Future
  • io.vertx.core.Promise

Oddly enough, a Vert.x Promise is not the same as a JavaScript Future. A Vert.x Promise is the writable side of a Vert.x Future. In JavaScript terms:

  • Vert.x Future === JavaScript Promise Like (Thenable)
  • Vert.x Promise === JavaScript Executor Function

# async/await

async/await is supported without any need for a compilation step by GraalVM. ES4X adds an extra feature to Vert.x Future type. APIs that return a Vert.x Future can be used as a Thenable, this means that:

// using the Java API
  .onSuccess(server => {
    console.log('Server ready!')
  .onFailure(err => {
    console.log('Server startup failed!')

Can be used as a Thenable:

try {
  (async function futureTest1 () {
    let server = await vertx
        .requestHandler(req => {

    console.log('Server Ready!');
} catch (err) {
  console.log('Server startup failed!')


async/await works even with loops, which makes working with asynchronous code quite easy, even mixing JS and Java code.

# Type Conversions

Vert.x is coded in Java, however in JavaScript we don't need to worry about types as much as with Java. ES4X performs some automated conversions out of the box:

Java TypeScript
void void
boolean boolean
byte number
short number
int number
long number
float number
double number
char string
boolean[] boolean[]
byte[] number[]
short[] number[]
int[] number[]
long[] number[]
float[] number[]
double[] number[]
char[] string[]
java.lang.Void void
java.lang.Object any
java.lang.Boolean boolean
java.lang.Double number
java.lang.Float number
java.lang.Integer number
java.lang.Long number
java.lang.Short number
java.lang.Char string
java.lang.String string
java.lang.CharSequence string
java.lang.Boolean[] boolean[]
java.lang.Double[] number[]
java.lang.Float[] number[]
java.lang.Integer[] number[]
java.lang.Long[] number[]
java.lang.Short[] number[]
java.lang.Char[] string[]
java.lang.String[] string[]
java.lang.CharSequence[] string[]
java.lang.Object[] any[]
java.lang.Iterable any[]
java.util.function.BiConsumer <T extends any, U extends any>(arg0: T, arg1: U) => void
java.util.function.BiFunction <T extends any, U extends any, R extends any>(arg0: T, arg1: U) => R
java.util.function.BinaryOperator <T extends any>(arg0: T, arg1: T) => T
java.util.function.BiPredicate <T extends any, U extends any>(arg0: T, arg1: U) => boolean
java.util.function.Consumer <T extends any>(arg0: T) => void
java.util.function.Function <T extends any, R extends any>(arg0: T) => R
java.util.function.Predicate <T extends any>(arg0: T) => boolean
java.util.function.Supplier <T extends any>() => T
java.util.function.UnaryOperator <T extends any>(arg0: T) => T
java.time.Instant Date
java.time.LocalDate Date
java.time.LocalDateTime Date
java.time.ZonedDateTime Date
java.lang.Iterable<T> <T>[]
java.util.Collection<T> <T>[]
java.util.List<T> <T>[]
java.util.Map<K, V> { [key: <K>]: <V> }