JavaScript

I suggest you start by reading A re-introduction to JavaScript (JS Tutorial) - JavaScript | MDN which is a good start and introduction by Mozilla.

Mozilla also provide a good starting point at JavaScript | MDN which I have found very helpful, however JavaScript reference - JavaScript | MDN is ideal for me as the language reference.

Another good place to look stuff up and learn is JavaScript Tutorial.

If you are writing a lot of JavaScript then consider using something like Use JSDoc to generate documentation.

A helpful explanation of var and let is available at Why You Shouldn’t Use ‘var’ Anymore.

If you have a variable called myVar and want to know what type it is then typeof myVar is a handy technique but not that arrays will be show as being objects. However it is sometimes note that simple, so Fixing the JavaScript typeof operator – JavaScript, JavaScript… helps with a more advanced solution.

Language

When declaring variables use let, or if appropriate use const but do not use the older var.

See Expressions and operators - JavaScript | MDN for a good overview of operators.

There is a good introduction to JavaScript at The Modern JavaScript Tutorial.

Console

Almost everyone has done something like console.log('Here'), however we rarely think, "so what else can console do", well we should! Have a look at console - Web APIs | MDN because there is log, warn, error, group and table that can all be very useful.

Rhino

mozilla/rhino: Rhino is an open-source implementation of JavaScript written entirely in Java
Rhino - Mozilla | MDN
Download Rhino - Mozilla | MDN
Rhino documentation - Mozilla | MDN
Scripting Java - Mozilla | MDN