You can't do this, but you can instead do something like:

function saveItem(andClose) {
   if(andClose === undefined) {
      andClose = false;
   }
}

This is often shortened to something like:

function setName(name) {
  name = name || 'Bob';
}

Update

The above is true for ECMAScript <= 5. ES6 has proposed Default parameters. So the above could instead read:

function setName(name = 'Bob') {}