Google's in-house incubator, named Area 120, has built an app that makes it easy to learn Javascript, it's called Grasshopper. You will need to sign-in using a Google account, and the app will teach you through a series of exercises and quizzes how to program using Javascript.
For the description at the Google Play store:
Grasshopper is the best way to start your coding adventure with fun, quick games on your phone that teach you to write real JavaScript. Move through progressively challenging levels as you develop your abilities, then graduate with fundamental programming skills for your next step as a coder.
Visual puzzles develop your problem-solving skills and solidify coding concepts
Use industry-standard JavaScript with just a few taps on your phone
Real-time feedback guides you like a teacher
Collect achievements as you learn new skills
The app is currently available at the Google Play store and the Apple App Store.
Source: 9to5Google