Context
What can i do with JavaScript?
1. Add Interactivity to your web pages. With JS you can make your application accept input from the user and make something with it. That means you can make your app think, make decisions and act differently depending on the input it receives. This input can be anything, user preferences or even sensitive data that comes from a web form, cookies, whcih browser the user uses to access your app and many many more.
2. Communicate with the server. Send data to a server, thus you can save the information that you have gathered from the user into a safe place. If the user comes back you can retrieve this data and offer him based on that a more 'personalized' experience. Authenticate users, so they can perform actions to your application only after they have logged in.
3. Manipulate data, transform it, apply changes or make calculations and create an ending result.
4. Present this result to the end user. This is not exactly part of JS as a language but for now it is well connected to it.
Good to know:
i) High-level programming language
ii) Nothing to be confused with Java (entirely different languages)
iii) Uses C-like syntax (familiarities with c, c++, c# and Java, but not Ruby or Python).