どうも追いかけてみたところ、JavaScript の古いドキュメントには "by reference" って文言があったようだ（ id:tkng:20070807:1186500367 とか参照）。https://developer.mozilla.org/ja/Core_JavaScript_1.5_Reference/Functions の翻訳元はそのバージョンと思われる
現在のバージョン https://developer.mozilla.org/en/JavaScript/Reference/Functions_and_function_scope を見ると
The parameters of a function call are the function's arguments. Arguments are passed to functions by value. If the function changes the value of an argument, this change is not reflected globally or in the calling function. However, object references are values, too, and they are special: if the function changes the referred object's properties, that change is visible outside the function, as shown in the following example:
（略）
"However, object references are values, too," ってはっきり書いてある
