This catches the error, saves the error message, and allows us to ensure the error references `sum`

```js
it("automatically adds declared variables to the most immediate \
    context available.", function(){
  ExecutionContextExample04 = function(){
    function add(num1, num2) {
      var sum = num1 + num2;
      return sum;
    }
    
    var result = add(10, 20);  //30
    return(sum);  //causes an error since sum is not a valid variable
  };
  
  var errorMessage;
  
  try{
    ExecutionContextExample04();
  }catch(e){
    errorMessage = e.message;
  }
  
  expect(errorMessage).toContain("sum");
});
```