From Snowbillr's blog: "Lots of tutorials for Phaser assume all your game’s code will be in one JavaScript file and that all your dependencies will be downloaded by hand. In a real world development environment this would not be a feasible setup.
In a modern development environment, you’ll be using something like Yarn to manage your dependencies and Webpack to bundle your code. In some cases you might be using a tool to transpile your code.
So let’s go over what a modern Phaser development setup would look like. This tutorial is for people who understand some JavaScript, but are unfamiliar with how a full JavaScript project is typically set up."