Idiomatic way to set default value in JavaScript

If myObj is a global it needs to reference the window object, otherwise it will throw an error if myObj is undefined.

myParam = myParam || (window.myObj ? window.myObj.mParam : 10);

or

myParam = myParam || (window.myObj && window.myObj.mParam) || 10;

This works as well:

myParam = myParam || ((typeof myObj !== "undefined") ? myObj.mParam : 10);