I feel the need to have a little rant about MooTools and ES7 and the whole 'Array.contains' hoo-hah.
When MooTools came out in 2006, the most popular framework was Prototype. As the name suggests, it extended prototypes, as did MooTools. People still referred to making websites with JavaScript as 'DHTML', there was no trim method on strings, there wasn't even a forEach method on arrays. JavaScript was a crippled language. IE6 ruled the waves.
MooTools, Prototype, Dojo, Base2 - they made the language usable, even fun, to work with. By using an incredible feature of JavaScript, prototypical inheritance, we were able to add features to the language that made it palatable.
Be it simple methods like number.toInt, string.trim, array.forEach, or familiar programming constructs such as Class, MooTools and its ilk took JavaScript from something impossible to work with to something that you could properly use to build awesome sites, and even apps - Microsoft, IE and desktop ruled everything, and the concept of a 'we