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Lesson 1. Store a local state with useState 1. Creating and updating a local state

Let’s consider this component, containing a text input. Here we want to keep, as a local state, the value entered in the input and display it below the input:

  1. useState takes as a parameter the initial value of your state and returns an array with two elements: the current value of the state and a function to set the state's new value.
  2. To get the state's current value, use the first value returned by useState, here name.
  3. To update the state's value, call the function returned as second value by useState, here setName, with, as a parameter, the new value.

Updating the state

When a component defines a local state, the only way to update it is to use the setter returned as the second element by the call to useState.

This means that the following snippets are incorrect:

const [value, setValue] = useState(0)
value = 1
const [object, setObject] = useState({ key: 'aaa', value: 1 })
object.value = 1
const [array, setArray] = useState([1, 2, 3])

If you incorrectly update the state, you won’t get any error, but some weird behaviors will happen. For instance, the component will not re-render, and you will not see your updates in the browser.

This is why it can be very useful to be used to the spread syntax to create new arrays and objects from existing ones:

setObject({ ...object, value: 1 })
setArray([...array, 4])


Can you update this component so when the user clicks the button:

  • it adds the value of the input to the list, and
  • it clears the input?
Click here to see the solution!

To update the tasks state, we can use the spread operator (...) to create a new array from the existing one tasks, and call setTasks with this new array:

onClick={() => {
setTasks([...tasks, newTask])
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A complete course to help you become fluent with hooks.

  • 1
    Understand how hooks workWith several interactive examples.
  • 2
    Master common patternsDebouncing, contexts, reducers…
  • 3
    Solve recurring problemsAsynchronous effects, infinite loops…
  • 4
    Use hooks at their full potentialAnd create your own custom hooks!

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Does it cover JavaScript or TypeScript?

This course presents hooks with JavaScript because youdon’t need TypeScript to use them, so I preferred making itaccessible to developers who don’t use TypeScript.

But being a TypeScript-lover, I know it brings someadvantages, and you’ll find in the course a lesson dedicatedto improving your usage of hooks using TypeScript.

What will I find in the course?

The course contains 14 lessons, each divided into small sub-lessons, from the basics to solving tricky problems.

Here is the course’s table of contents:

  • 1. Store a local state with useState
  • 2. Trigger side effects with useEffect
  • 3. Custom hooks and classic async pattern
  • 4. What are hooks anyway?
  • 5. Debugging hooks
  • 6. Get the current state when dealing with async code
  • 7. Solving infinite loops when using useEffect
  • 8. Optimizing performance with useMemo and useCallback
  • 9. Debouncing a user input
  • 10. Using contexts to share values between components
  • 11. Simplify state update logic with reducers
  • 12. Custom hooks to use the browser’s APIs
  • 13. From render-props and high-order components to hooks
  • 14. Better hooks with TypeScript

Hi! I’m Sebastien Castiel 👋

I’m a French developer expatriated in Montreal.

I have used React for several years now and hooks for the last two years on several projects.

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