- [Instructor] So when would you use a variant in the dependency of AP useful? As an example, we might use a value to help construct a URL dependent on user inputs. To demonstrate this we've added a dropdown to the app that allows the user to choose a course ID and the associated code to make the app go and get the course with the ID from the server and display it correctly. A value of zero effectively means display all courses. The value from the dropdown sets a state called course ID. And this is used to construct the query part of a URL in the useEffect hook on line 22. This query is then passed into the getCourses, function and use as part of the URL in the fetch call on line 15. When we supply a query value at the end of a restful URL, we return a single object. We need to call setCourses with this as the single element in an array shown on line 17, to ensure that the courses is set as an array. Otherwise we get an error when we call array functions on courses later in the code. If we get more than one course returned, these are implicitly put into an array. You can see that the application renders just fine. Although we have a warning message on the console more on that in a moment. Changing the value in the dropdown box to a particular number, will update the value of course ID, but useEffect is not fired again. That's because of the empty array dependency, meaning it will only fire after the initial render. As you can see, the app pre-rendered, but there was no change to the list of courses. That warning message shown in the console is all about the dependencies in the userEffect hook. At the moment, the userEffect hook is only fired once, even though it thinks it should fire again. When the state of courseID updated, that's why we're getting the warning. Clearly the calling of the useEffect hook should depend on the course ID state In this example. The affect should only fire. If this value has changed adding this into dependency, array will make this happen. When this value here is added the array, you can see that the warning has now disappeared, selecting a value from the dropdown, updates, the state of course ID and hence, the course is to display. Although removing the dependency array would have got the same result. The effect would have fired on every re render thus making up app less efficient. So it's good practice to put any dependencies we can find in the array for the effect.