If you are using Maven or Gradle, add the following dependency to the dependencies section of your project descriptor to access the Vert.x Core API and enable the JavaScript support:

- Maven (in your `pom.xml`):

    <dependency>
     <groupId>io.vertx</groupId>
     <artifactId>vertx-core</artifactId>
     <version>3.4.2</version>
    </dependency>
    <dependency>
      <groupId>io.vertx</groupId>
      <artifactId>vertx-lang-js</artifactId>
      <version>3.4.2</version>
    </dependency>

- Gradle (in your `build.gradle` file):

    compile "io.vertx:vertx-core:3.4.2"
    compile "io.vertx:vertx-lang-js:3.4.2"

Important | The JavaScript support is based on Nashorn, and so is constrained by Nashorn feature set and limits. It does not support EcmaScript 6, because Nashorn does not support it. In addition, native NPM modules cannot be used as they are compiled for Node.JS. NPMs developed in EcmaScript 6 are also not usable.