Apple Javascript (Script Editor) Documentation
Apple has not updated their scripting documentation for JavaScript. The most recent documentation appears in the OS X 10.11 Release Notes - JavaScript for Automation.

There is extensive documentation for AppleScript in the AppleScript Language Guide.

MacStories has published a tutorial Getting Started with JavaScript for Automation on Yosemite.

Both this tutorial and Ash Ryan's Script Editor on Mac now lets you use JavaScript for automation mention a WWDC 2014: “JavaScript for Automation” which is probably the best place to learn more.