I would say this one is the "official": https://developer.mozilla.org/en/JavaScript
You also have the ECMAScript Language Specification, 5.1 Edition (or as a PDF, the definitive specification).
And from Microsoft: JavaScript Language Reference: "This documentation explains the Microsoft implementation of JavaScript, which is compliant with the ECMAScript 5th Edition language specification. It also provides additional features that are not included in the Ecma Standards."
