Tern is a stand-alone code-analysis engine for JavaScript. It is intended to be used with a code editor plugin to enhance the editor's support for intelligent JavaScript editing. Features provided are:

• Autocompletion on variables and properties
• Function argument hints
• Querying the type of an expression
• Finding the definition of something
• Automatic refactoring

Vim 上用这货配上 syntastic 和 jslint，写 JS 实在是爽，基本上赶上 IDE 的功能了。 放一个官方的 demo: http://ternjs.net/doc/demo.html