JavaScript as a language has been around for a long time. Originally designed as a language to support HTML within a single web browser, it inspired multiple clones of the language, each with its own implementations. Eventually, a global standard was introduced, allowing websites to support multiple browsers. The language defined in this standard is called ECMAScript.

Each JavaScript interpreter must deliver functions and features that conform to the ECMAScript standard. The ECMAScript standard that was published in 1999 was officially called ECMA-262, 3rd edition, but became known simply as ECMAScript 3. This version of JavaScript became widely adopted and formed the basis for the explosive popularity and growth of the internet as we know it.

With the popularity of the language, and the increase in usage outside of a web browser, the ECMAScript standard has been revised and updated a number of times. Unfortunately, the time it takes between proposing new language features and the ratification of a new standard to cover them can be rather lengthy. Even when a new version of the standard is published, web browsers only adopt these standards over time, and may also implement parts of the standard before others.

Before choosing which standard to adopt, therefore, it is important to understand which browsers, or more accurately, which runtime engine will need to be supported. To support these decisions, there are a number of reference sites that list support in what is known as a compatibility table.

Currently, there are three main versions of ECMAScript to choose from: ES3, ES5 and the newly ratified ES6. ES3 has been around for a long time, and pretty much any web browser will support it. ES5 is supported by most modern web browsers. ES6 is the latest version of the standard, and by far the biggest update to the language thus far. It introduces classes into the language for the first time, making object-oriented programming easier to implement.

The TypeScript compiler has a parameter that can switch between different versions of the ECMAScript standard. TypeScript currently supports ES3, ES5, and ES6. When the compiler runs over your TypeScript, it will generate compile errors if the code you are attempting to compile is not valid for that standard. The team at Microsoft has committed to follow the ECMAScript standards in any new versions of the TypeScript compiler, so, as and when new editions are adopted, the TypeScript language and compiler will follow suit.