FAQ: The State Hook - Separate Hooks for Separate States

Frequently Asked Questions React.js FAQ
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This community-built FAQ covers the “Separate Hooks for Separate States” exercise from the lesson “The State Hook”.

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This exercise can be found in the following Codecademy content:

Learn React

FAQs on the exercise Separate Hooks for Separate States

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4

Why React use const instead to let to declare hooks state. Isn’t const value supposed not be change?
(const [state, setState] = useState;)

5

I don’t understand how we’re supposed to learn effectively when there are such basic mistakes like this. Unless I’m missing something here, the below code snippet, which is direct from the lesson’s material, isn’t even syntactically correct. Can’t even run it without refactoring to correct syntax errors.

function Subject() {
  const [state, setState] = useState({
    currentGrade: 'B',
    classmates: ['Hasan', 'Sam', 'Emma'],
    classDetails: {topic: 'Math', teacher: 'Ms. Barry', room: 201};
    exams: [{unit: 1, score: 91}, {unit: 2, score: 88}]);
  });

Refactored code below:

function Subject() {
    const [state, setState] = useState({
        currentGrade: 'B',
        classmates: ['Hasan', 'Sam', 'Emma'],
        classDetails: {
            topic: 'Math',
            teacher: 'Ms. Barry',
            room: 201
        },
        exams: [
            {unit: 1, score: 91},
            {unit: 2, score: 88}
        ]
    })
}

If it wasn’t mandatory for me to complete this course in order to complete my studies, I would’ve gone elsewhere for this study material weeks ago. :face_exhaling:

1 Like
6

This a very good question! I don’t know why it hasn’t gotten a reply so far.

About const, you’re right. They are constant values, so they cannot be changed… within the same scope :bulb: . If you look closely, we always call the setState() function to update this value for us, right? But how is that possible then?

Well, we’ve learned in the previous lessons that React re-renders Components whenever they change. So in this case, when the Component is re-rendered it is not the same scope where const state was first defined. Actually, the next time const [state, setState] = useState(initialValue) is called this initial value will be ignored since state isn’t empty anymore. It would be something like this:

First render:

  1. state receives the initial value
  2. an event is triggered and the handler function calls setState
  3. Component is then re-rendered :warning:

Second render:

  1. state ignores the initial value, since it has been updated by setState.
  2. state has been updated but not in the same function component scope, since the component has been re-rendered

Maybe knowing the actual implementation of useState() could clarify things even more, but I hope it is somehow clear that the key point is the scope of the function. :smiley: