Nice guide & nicely written! This example snippet actually generates a Javascript error (at least, in IE9): var foo = null; // foo is null, but bar

Nice guide & nicely written! This example snippet actually generates a Javascript error (at least, in IE9): var foo = null; // foo is null, but bar is undefined as it has not been declared if (foo == null && bar == null) { // still got in here } It will work if bar is declared but has value undefined, but not if bar is completely undeclared as shown.