Appendix D. Reintroducing JavaScript

This appendix covers

• Applying best practices when writing JavaScript
• Using JSON effectively to pass data
• Examining how to use callbacks and escaping callback hell
• Writing modular JavaScript with closures, patterns, and JavaScript classes
• Adopting functional programming principles

JavaScript is such a fundamental part of the MEAN stack (even if you’re writing the Angular part with TypeScript) that we’ll spend a little bit of time looking at it. We need to cover the bases because successful MEAN development depends on it. JavaScript is such a common language (uniquely, JavaScript has a runtime on almost every computer currently on the planet) that it seems that everybody knows some of it, partly because JavaScript is easy to start with and forgiving in the way it’s written. Unfortunately, this looseness and low barrier to entry can encourage bad habits, which can cause unexpected results.

The aim of this appendix isn’t to teach JavaScript from scratch; you should already know the basics. If you don’t know JavaScript at all, you may struggle and find it hard going. Like all things, JavaScript has a learning curve. On the other hand, not everybody needs to read this appendix in detail, particularly experienced JavaScript developers. If you’re lucky enough to count yourself as part of the experienced camp, it still may be worthwhile to skim this appendix in case you find something new here.

We don’t cover TypeScript, though we hope that chapters 8 through 12 cover it in enough detail for you to be comfortable with it.

One last thing before we get started in earnest. When you look around the internet for information around JavaScript, you’ll more than likely come across the appellations ES2015, ES2016, ES5, ES6, ES7, and so on.

ES5 is the version of JavaScript that has been available for the longest time, from the dim and distant past that includes the Firefox 4 web browser; the birth of Google Chrome; and the long, torturous death of the infamous Internet Explorer 6. Luckily, those days are long gone, but the specification still stands, and most browsers (mostly) adhere to it.

Officially, as of 2015, iterations of the JavaScript (or, if you prefer, ECMAScript [ES]) specification have been denoted by the year: ES2015, ES2016, and so on. Any reference to single-digit versioning post ES5, like ES6, is incorrect. Throughout this book, we’ve been careful to ensure that we named things correctly. Many authors across the internet haven’t been so diligent and continue to perpetuate the incorrect naming scheme.

As things stand today, most browsers adhere to most of the changes made in JavaScript as part of the ES2015 spec, with some browsers also providing some functionality for later iterations (2016, 2017, and so on). The pace of adoption and implementation is sometimes slower than we, as developers, would like, so transpilers such as Babel are available. JavaScript transpilers broadly take code written utilizing more modern ideas and convert it to a form that older browsers understand. They provide a bridge between old and new and between different languages. TypeScript, CoffeeScript, Elm, and ReasonML are all transpiled to JavaScript.