The easiest way to create a "Hello World!" example is to install node.js and execute: npx n4js-cli init. Aftwards, follow the instructions of the init command. When opening an n4js-file in VSCode, VSCode will offer to install the n4js extension automatically.
N4JS is an extension of ECMAScript providing a sound type system. It provides a transpiler to translate N4JS files to plain ECMAScript. The N4JS IDE enables the authoring of JS and N4JS files, providing tool support analogous to that of best-of-breed Java IDEs, e.g., Eclipse JDT.