Say someone has an idea for a new, very useful and cool method, for instance String.prototype.multiply. She does not see it in current javascript implementations. But maybe it's already implemented in some javascript/ajax libraries or maybe similar method (with a different name, but she does not know) is current de-facto standard or even is on its way to some future ECMAScript version already..
Her aim is to implement the new functionality in her js code right, with compatibility with newer javascript or 3rd party code in mind (she does not necessarily want to propose a new standard).
What is the best place to find it out / discuss?
I would say Google and SO to find it. To propose/discuss, head over to ECMAScript site's community: http://www.ecmascript.org/dev.php