React Foundations·Components, state and data

State and events

State is a snapshot; updates schedule a new render instead of changing the old one.

Intermediate 13 min read Lesson 7 of 300 Challenge included

What you will be able to do

Use useState for values that change over time and affect rendering.
Never mutate objects or arrays in state; create new ones.
Lift state to the closest common parent when two siblings need it.

In plain English

Calling a state setter does not mutate the current value. It schedules a render with a new value. Use functional updates when the next value depends on the previous one, and keep derived values out of state.

Counter.jsxJSX

import { useState } from 'react'

export default function Counter() {
  const [count, setCount] = useState(0)

  return (
    <button onClick={() => setCount((current) => current + 1)}>
      Clicked {count} times
    </button>
  )
}

Your turn

Toggle a bookmark

Easy

Add a saved boolean and a button that toggles it. Show "Saved" or "Save" based on state.

JSX · starter

const [saved, setSaved] = useState(false)
// add a button

Check your understanding before moving on.

Knowledge checkQuestion 1

When should you use a functional state update?

Select an answer to continue

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Next: Effects and data fetching

Lesson 7 of 300 · 0% of course