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JavaScript (JS) is a dynamic computer programming language. It is most commonly used as part of web browsers, whose implementations allow client-side scripts to interact with the user, control the browser, communicate asynchronously, and alter the document content that is displayed.
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Pros
Pro Massive ecosystem
JavaScript has one of the largest programming ecosystems, as shown by the being the most popular language for projects on GitHub. As there are so many projects written in JavaScript there are lots of libraries available to build off of and many of them are written to be easy to use and integrate into other projects.
There are also lots of resources available for learning JavaScript. Other than traditional tutorials, language learning sites such as Codecademy have JavaScript courses. The Mozilla Developer Database also serves as a great resource for learning about the standard libraries built into JavaScript.
Pro No installation required
If you run a web browser you already have JavaScript installed and can get started right away. Modern browsers such as Chrome also have very powerful programming consoles built into them.
Aside from the browser console, you can also use online Javascript playgrounds such as JS Bin and JS Fiddle. Even from a tablet.
Cons
Con Complex
JavaScript has a long litany of warts and gotchas. JavaScript will bite you if you're just a wee bit careless. You have to exercise a great deal of caution and awareness. You either have to know the entire 545-page ES6 spec to avoid them all, or use a linter to help restrict you from using the bad parts (and you still have to be familiar with the language), but beginners don't know these things. (Linters are also prone to time-wasting false positives.) This is a significant cognitive burden on even the experienced programmer (as if coding wasn’t hard enough already), but try learning to program in the first place on top of all of this and you'll understand that JavaScript is a terrible choice for the beginner.
Con The language itself is not very appealing to developers
JS is one of the most dreaded languages as it was designed for the purpose of becoming just an scripting language for a browser. It was never intended to take over as the leading technology in web development, thus the language has been streched past its own capabilities. A beginer should learn something else first, something that is better conceived and refined.