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Vert.x ES6 back to the future

On October 21th, 2015 we all rejoiced with the return from the past of Marty McFly with his flying car and so on, however in the Vert.x world we were quite sad that the JavaScript support we have was still using a technology released in December 2009. The support for ES5 is not something that we Vert.x team controls but something that is inherited from running on top of Nashorn.

With all these nostalgic thoughts on my mind I’ve decided to bring us back to the future and by future I mean, lets start using a modern JavaScript, or more correctly, lets start using ECMAScript 6.

It turned out to be quite simple to achieve this so I’ll pick the hello world example and write it in ES6 just to show how you can port your code to ES6 and still use the current Vert.x APIs. Note that Vert.x internals still are ES5 and have not been touched or modified to support any of ES6 features.


Traditionally your main.js file would reside in the root of your module (this is where NPM will look for it by default); however as we are going to transpile to ES5 you’ll want to put your index file in /src/main.js.

However, because we are transpiling to ES5, your package.json‘s main block should point to the transpiled index.js file in the /lib directory.

  "name": "vertx-es6",
  "version": "0.0.1",
  "private": true,

  "main": "lib/main.js",

  "scripts": {
    "build": "rm -Rf lib && ./node_modules/.bin/babel --out-dir lib src",
    "start": "./node_modules/.bin/vertx run lib/main.js"

  "dependencies": {
    "vertx3-full": "3.1.0",
    "babel-cli": "6.2.0",
    "babel-preset-es2015": "6.1.18"

As you can see, the main idea is to invoke the transpiler (Babel) when we are building our project, and run it using the generated files. This is slightly equivalent to a compilation process you would have using compiled language.


If you’re planning to deploy your package to npm either local or private you should be aware that npm will exclude anything listed on your .gitignore since we should ignore the generated code from git it need to inform npm to ignore that rule and keep the lib directory. The .gitignore should be something like:


And the .npmignore:


Hello fat arrows and let keywords

So all the heavy work has been done, in order to create our hello world we just need to code some ES6 in our src/main.js file:

var Router = require("vertx-web-js/router");
var server = vertx.createHttpServer();

var router = Router.router(vertx);

router.get("/").handler((ctx) => {

    let response = ctx.response();
    response.putHeader("content-type", "text/plain");

    response.end("Hello ES6 World!");


As you can see we’re using fat arrows instead of writing a function closure and scoped variables using let keyword. If you now compile your project:

npm run build

And then start it:

npm start

You have your first back to the future ES6 verticle!