JavaScript: Names, Versions, and Modes

JavaScript was created at Netscape in the early days of the web, and technically, “Java‐Script” is a trademark licensed from Sun Microsystems (now Oracle) used to describe Netscape’s (now Mozilla’s) implementation of the language. Netscape submitted the language for standardization to ECMA—the European Computer Manufacturer’s Association—and because of trademark issues, the standardized version of the language was stuck with the awkward name “ECMAScript.” In practice, everyone just calls the language JavaScript. This book uses the name “ECMAScript” and the abbreviation “ES” to refer to the language standard and to versions of that standard.

For most of the 2010s, version 5 of the ECMAScript standard has been supported by all web browsers. This book treats ES5 as the compatibility baseline and no longer discusses earlier versions of the language. ES6 was released in 2015 and added major new features—including class and module syntax—that changed JavaScript from a scripting language into a serious, general-purpose language suitable for large-scale software engineering. Since ES6, the ECMAScript specification has moved to a yearly release cadence, and versions of the language—ES2016, ES2017, ES2018, ES2019, and ES2020—are now identified by year of release.

As JavaScript evolved, the language designers attempted to correct flaws in the early (pre-ES5) versions. In order to maintain backward compatibility, it is not possible to remove legacy features, no matter how flawed. But in ES5 and later, programs can opt in to JavaScript’s strict mode in which a number of early language mistakes have been corrected. The mechanism for opting in is the “use strict” directive described in next session. That section also summarizes the differences between legacy JavaScript and strict JavaScript. In ES6 and later, the use of new language features often implicitly invokes strict mode. For example, if you use the ES6 class keyword or create an ES6 module, then all the code within the class or module is automatically strict, and the old, flawed features are not available in those contexts. This book will cover the legacy features of JavaScript but is careful to point out that they are not available in strict mode.

To be useful, every language must have a platform, or standard library, for performing things like basic input and output. The core JavaScript language defines a minimal API for working with numbers, text, arrays, sets, maps, and so on, but does not include any input or output functionality. Input and output (as well as more sophisticated features, such as networking, storage, and graphics) are the responsibility of the “host environment” within which JavaScript is embedded. The original host environment for JavaScript was a web browser, and this is still the most common execution environment for JavaScript code. The web browser environment allows JavaScript code to obtain input from the user’s mouse and keyboard and  by making HTTP requests. And it allows JavaScript code to display output to the user with HTML and CSS.

Since 2010, another host environment has been available for JavaScript code. Instead of constraining JavaScript to work with the APIs provided by a web browser, Node gives JavaScript access to the entire operating system, allowing JavaScript programs to read and write files, send and receive data over the network, and make and serve HTTP requests. Node is a popular choice for implementing web servers and also a convenient tool for writing simple utility scripts as an alternative to shell scripts.

Most of this book is focused on the JavaScript language itself. Next blog documents the JavaScript standard library, Next blog introduces the web browser host environment, and Next blog introduces the Node host environment. 

This blog covers low-level fundamentals first, and then builds on those to more advanced and higher-level abstractions. The chapters are intended to be read more or less in order. But learning a new programming language is never a linear process, and describing a language is not linear either: each language feature is related to other features, and this book is full of cross-references—sometimes backward and sometimes forward—to related material. This introductory chapter makes a quick first pass through the language, introducing key features that will make it easier to understand the in-depth treatment in the chapters that follow. If you are already a practicing Java‐Script programmer, you can probably skip this Blog.