I recommend Tern (https://packagecontrol.io/packages/tern_for_sublime). It provides excellent code completion and documentation. Install it through package control. I had to manually run npm install in the tern_for_sublime directory and restart Sublime to get it going.
Out of the box it is configured to provide completions based on the code in the current project, as well as standard library functions. You can add support for other environments through a .tern-project file in your project. A .tern-project file setup with completions for the browser and jQuery would look like this:
{
"libs":
"browser",
"jquery"
]
}
Hope this helps - couldn’t be more pleased with it myself.