My takeaway from reading stuff is that this is generally set to the object the function is defined on. If you’re saying foo.go(), then inside go() this will be set to foo. If you don’t know what object you called a function on, say because you have a naked go() in your code, then this is probably bound to the global window object.
The usual this behaviour can be overridden in various ways. Both call and apply let you invoke a function and explicitly provide a value for this. D3’s each() function does this so that inside each, this is bound to the DOM node for the specific element being manipulated. There’s also a new ECMA function bind that lets you create a function with a new value automatically used for this.
I’m still a bit fuzzy on the mechanics of how this is assigned, in particular when you copy a function from object A to object B. In general this seems to be set dynamically at the last moment before the function is invoked, so it generally picks up the value A or B depending on how the function was invoked.