// Set a global variable, foo, equal to test
var foo = "test";

// Within an if block
if ( true ) {
    // Set foo equal to 'new test'
    // NOTE: This is still within the global scope!
    var foo = "new test";
}

// As we can see here, as foo is now equal to 'new test'
alert( foo == "new test" );

// Create a function that will modify the variable foo
function test() {
    var foo = "old test";
}

// However, when called, 'foo' remains within the scope
// of the function
test();

// Which is confirmed, as foo is still equal to 'new test'
alert( foo == "new test" );