An additional wrinkle: recursive functions. If you've got a recursive function, you absolutely must use a function declaration to introduce the function identifier. Otherwise you inadvertently create a closure. Consider:

  var a = function(b) {
    if (b === 0) {
      return true;
    }
    return a(--b);
  };
  var b = a;
  a = function() { return false; };
  b(5);

As an alternative, you should go

  var a = function a2(b) {
    if (b === 0) {
      return true;
    }
    return a2(--b);
  };
  var b = a;
  a = function() { return false; };
  var a2 = function() { return false; };
  b(5);

Perhaps counterintuitively, a2 doesn't leak into the containing scope, which makes it safe to use within the function it declares.

Some older browsers fuck this up though, IIRC.