This example presents four JavaScript practice problems focused on array and object manipulation, with output shown only in the browser’s developer console. It demonstrates techniques such as iterating with `forEach()`, filtering array elements using `includes()` for substring matching and set difference, sorting an array of objects by a string property with a custom comparator, and checking numeric properties with the modulo operator. The code uses the D3.js v4 library for loading, but the exercises themselves rely on core JavaScript APIs, including `console.log()`, and include sample datasets of U.S. states with their regions and populations.