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 not that simple, so Fixing the JavaScript typeof operator – JavaScript, JavaScript… helps with a more advanced solution.