Having learned JavaScript mainly by doing, it was quite late that I figured out some important details. Please comment if you think or know that I'm wrong.

The arguments array as well as the caller instance are universal variables inside every JavaScript function. The names are self-explanatory.
An anonymous function inside another function keeps the variables in scope as in:

// javascript
var somefunction = function(arg1) {
  var avar1 = 10;  
  object.onclick = function() {
    alert('arg1=' + arg1 + ' avar1=' + avar1);
// "arg1=Foo avar1=10" when object is clicked on

So this means that the scope of the function defines what variables can be accessed within it, even after the function call has finished.

Another part I always found confusing in JavaScript is the "this"-keyword, but that might have to be the topic of another post.