Javascript is used to make website interactive. But what makes that so cool and awesome? Javascript is not only helping developers build great front-ends in combination with HTML and CSS, but it is also found on mobile applications, and even on the back-end in the form of Node.js. The growth of Javascript came because of its great user experience features across platforms. There is not a required development environment that needs to be set up for beginners to start coding and testing, you can get started right from your browser, which is nice for everyone trying to learn it. Javascript is not a single purpose langauge.
It is so scalable and flexible that can be used for a lot of tasks like animation effects, logical algorithms, and even databases.
With great features, comes great complexity. While it is easy for most to learn the basics (which is true for almost every programming language), it is rather complex trying to achieve more advanced stuff. One way of looking at it is browsers. Your very same Javascript code may not behave the same way in different browser environments, and figuring out why your code works in Chrome but not in Firefox or other browsers may be boring or even frustrating sometimes.
Javascript is a dynamically untyped languages, which basically means the same thing can mean something different depending on the context. As projects grow larger, it may be hard to maintain and debug when errors occur (which is not that rare). It takes experience to learn the tricks to automated testing and other techniques used to avoid bugs.
