javascript closure - how come I refer to an undeclared variable

Try to get my head around javascript closures as they are new to me.
I have a tutorial that indicates to use:

function warningMaker( obstacle ){
  function doAlert (obs) {

    alert("Beware! There have been "+obs+" sightings in the Cove today!");
  };
  return doAlert;
}

But I am confused about "obs". Does the parameter 'obstacle' automatically get passed to obs ?

A better example perhaps might be:

function warningMaker( obstacle ){
  return function (number,location) {
    alert("Beware! There have been " + 
          obstacle + 
          " sightings in the Cove today!\n" +
          number + " " + obstacle +
          "(s) spotted at the " +
          location + "!"
         );
  };

}

Answer

Either the example you've got is missing some lines or it isn't a proper example for closure. A better example (with simplified code) would be

function warningMaker(obstacle){
   return function() {
      alert("Look!" + obstacle);
   }
}

What the above does is, when the function is getting returned, the reference to the obstacle in the function body creates a closure so it will be in memory and will be used whenever called and it will be

warningMaker("Messi")(); // "Look! Messi"
warningMaker("CR7")(); // "Look! CR7"

Note that in the above, the function returned is being called. (I mean, the empty parentheses)