CasperJS is scoped

In CasperJS, global variable cannot be accessed inside function.
The following code will return null

var globalVar;

function myFunction(){
  globalVar = 1;
  return globalVar;
}

echo(myFunction());

Output: null

The following code works

var globalVar;
function myFunction(){
  var localVar = 1;
  return localVar;
}

globalVar = myFunction();
echo(globalVar);

Output: 1