how come I refer to an undeclared variable

This question already has an answer here:

JavaScript closures vs. anonymous functions 12 answers

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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)

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我认为你的意思是：

function warningMaker( obstacle ){
  var obs = obstacle; // otherwise it would never be able to associate this two variables automatically
  function doAlert () {
    alert("Beware! There have been "+obs+" sightings in the Cove today!");
  };
  return doAlert;
}

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