The size and complexity of JavaScript is part of its appeal. The core language itself, which doesn't include APIs such as the DOM, is small and simple. This makes it easy for Node.js to establish its own styles and conventions.

The new APIs provided by Node.js and the consistent approach to asynchronous programming wouldn't be possible in a more complex language with a larger pre-existing standard class library.

JavaScript was first built as a programming language for client-side functionality in the browser. This might not make it an obvious choice for general-purpose programming.

In fact, these two use cases do have something important in common. User interface code is naturally event-driven (for example, binding event handlers to button clicks). Node.js makes this a virtue by applying an event-driven approach to general-purpose programming.

JavaScript supports functions as first-class objects. This means it's easy to create functions dynamically and pass around references to them. This fits in well with the asynchronous, non-blocking approach of Node.js. In particular, it's easy to expose and use APIs based around callback functions.

JavaScript has received a lot of attention in the last several years as it has become more widely used for providing rich functionality on the Web. Browser vendors have put a huge amount of engineering effort into improving the performance of JavaScript. Node.js benefits from this directly via its use of Chrome's V8 engine.

The JavaScript language itself is undergoing some major changes for the better. The ECMAScript 2015 standard (previously known as ES6) represents the most significant revision of the language in its history. It introduces features that make the language more intuitive and less verbose. It also addresses flaws that JavaScript has been criticized for in the past, removing gotchas and making programs easier to reason about.