var foo = "bar";

function test() {
  console.debug("foo is (1st time)" + foo);
  if (false) {
    var foo = "baz";
  }
  console.debug("foo is (2nd time)" + foo);
}

Your console will show something like this:

foo is (1st time) undefined
foo is (2nd time) undefined

Which is a little weird since the local declaration of foo was never executed.  A quick search didn’t turn up any promising hits, but I imagine that there must be a preprocessing step that executes all variable declarations before the function is run.  The very similar function below (diffs in bold):

var foo = "bar";

function test() {
  console.debug("foo is (1st time)" + foo);
  if ("undefined" == typeof(foo)) {
    var foo = "baz";
  }
  console.debug("foo is (2nd time)" + foo);
}

Yields:

foo is (1st time) undefined
foo is (2nd time) baz

A more intentional use is probably (diffs also in bold):

var foo = "bar";

function test() {
  console.debug("foo is (1st time)" + foo);
  if ("undefined" == typeof(foo)) {
    foo = "baz";
  }
  console.debug("foo is (2nd time)" + foo);
}

Which has output:

foo is (1st time) bar
foo is (2nd time) bar

So this isn’t anything groundbreaking, or even weird.  It’s clearly defined¹, and discussed², but the consequences eluded me for about an hour last night.  Hopefully by writing this article, I’ll remember for next time.

1. http://docstore.mik.ua/orelly/webprog/jscript/ch04_03.htm#jscript4-CHP-4-SECT-3.1
2. http://stackoverflow.com/questions/1711173/declaration-for-variable-in-while-condition-in-javascript