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React Apprentice

First Edition · web · React 8.0.0 · Visual Studio Code

Before You Begin

Section 0: 5 chapters
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Section I: Rendering Right

Section 1: 7 chapters
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Section II: Adding Interactions & State

Section 2: 4 chapters
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Section III: Making It Work in the Real World

Section 3: 4 chapters
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Section IV: Proving It Works Well

Section 4: 3 chapters
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10. Understanding State Updates
Written by Eli Ganim

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In the previous chapter, you built search and a full form, following rules this book handed you on faith: Always spread, never assign; derive, don’t store. The Learning Tracker works — but “works” and “understood” are different achievements.

This chapter closes that gap. You’ll watch React batch three updates into one, see exactly why mutation breaks rendering — complete with a ghost course haunting your catalog — and practice adding, removing and updating list state the immutable way.

It’s the most conceptual chapter of Part II, but everything happens in running code, and it ends with a bug hunt: three planted state mistakes for you to diagnose with your new tools. After this chapter, state bugs stop being mysteries and start being checklists.

Building a Click Lab

Some experiments deserve a lab bench rather than production code. Build a temporary one — you’ll demolish it within a few pages. In src/App.tsx, add this small component above App, below the levelFilter line:

function ClickLab() {
  const [count, setCount] = useState(0)
  console.log('ClickLab renders with count', count)

  function handleTripleClick() {
    setCount(count + 1)
    setCount(count + 1)
    setCount(count + 1)
  }

  return (
    <button type="button" onClick={handleTripleClick}>
      Count: {count}
    </button>
  )
}

A counter button with an attitude: Every click calls the setter three times. The console.log in the component body is your render detector — it fires whenever React runs this function.

Render the lab at the top of App’s JSX, directly below the opening <main>:

<ClickLab />

Save and check the browser — an unstyled counter button sits above the catalog, and the console already shows the detector line twice:

The lab bench: temporary, ugly and about to be enlightening.
The lab bench: temporary, ugly and about to be enlightening.

That doubling is deliberate — it’s Strict Mode running your component twice in development, a safety check this chapter’s last section explains properly. For now, adopt the counting rule: one render = one pair of log lines.

Now click the button once. Three setter calls, so the count should jump to 3 — but the button says Count: 1, and the console adds exactly one pair:

ClickLab renders with count 1
ClickLab renders with count 1

Two surprises for the price of one click: The count moved by one, not three — and three state updates produced a single render.

State Lives in Snapshots

Both surprises come from the same machinery, and it’s the machinery from Chapter 8’s snapshot rule, now seen end to end:

One render, one snapshot: the handler keeps its render's value no matter how often it calls the setter.
Ozi regbof, oto mjurcxit: ple hegzkap guupm evn josqap'v zirae wa jorpam qoz eqmur uj gagsm sma zafkoq.

Updating From the Latest Value

Sometimes a handler genuinely needs “whatever the latest value is, plus one” — three of those should make three. React’s setters accept a function for exactly this. In ClickLab, change the three calls:

setCount((current) => current + 1)
setCount((current) => current + 1)
setCount((current) => current + 1)
Three recipes, three effective updates — and still one render.
Xrwue qaboboc, zgpuu idzimpulu azwasez — iyd ccagd evo minfen.

function handleFavoriteClick() {
  setIsFavorite((current) => !current)
}

function handleAddCourse(newCourse: Course) {
  setCourses((current) => [...current, newCourse])
}

Why Mutation Breaks React

Time to answer Chapter 9’s IOU: Why spread instead of assignment? Break the rule on purpose and meet the consequences. In src/App.tsx, sabotage the add handler:

function handleAddCourse(newCourse: Course) {
  courses.push(newCourse)
  setCourses(courses)
}
Success announced, catalog unmoved: the mutation hid the new course from React.
Xolvern ibtaixzef, kujahoh ulfanig: hdo fagepaec hav xzu vex buabni vmop Caulx.

function handleTitleChange(
  event: ChangeEvent<HTMLInputElement>,
) {
  form.title = event.target.value
  setForm(form)
}

Removing a Course Immutably

You’ve added with spread; now earn the other two tools with a real feature: Users should be able to remove the personal courses they added — and only those. First, teach the Course type who’s personal. In src/types/course.ts, add a line below durationHours:

isPersonal?: boolean

isPersonal: true,

function handleRemoveCourse(courseId: string) {
  setCourses((current) =>
    current.filter((course) => course.id !== courseId),
  )
}

<CourseList
  courses={visibleCourses}
  emptyMessage={emptyMessage}
  onRemoveCourse={handleRemoveCourse}
/>

onRemoveCourse: (courseId: string) => void

<CourseCard
  key={course.id}
  course={course}
  onRemove={onRemoveCourse}
/>

type CourseCardProps = {
  course: Course
  onRemove: (courseId: string) => void
}

function CourseCard({ course, onRemove }: CourseCardProps) {

{course.isPersonal && (
  <Button
    label="Remove from catalog"
    variant="ghost"
    onClick={() => onRemove(course.id)}
  />
)}
Personal courses carry their own way out; built-in courses don't.
Pajwuqod taufyuv cejys znoov imv min euj; zaikb-ek moenpof zit'h.

Updating a Course Immutably

The third tool is map, and a two-minute experiment will fix it in memory — temporary, like the lab. In src/App.tsx, add a handler below handleRemoveCourse:

function handleExciteFirst() {
  setCourses((current) =>
    current.map((course, index) =>
      index === 0
        ? { ...course, title: course.title + '!' }
        : course,
    ),
  )
}

<button type="button" onClick={handleExciteFirst}>
  Excite the first course
</button>
Update via map: a fresh array, a fresh object, one changed title.
Ocluja sao rob: o srukp uqwok, u zjudp uydiny, ewi bkigdow cedbe.

Storing Less, Deriving More

The second great state skill is refusing to store things. Your App already lives by it — one look at the split:

Two facts stored; everything else recomputed fresh each render, so nothing can go stale.
Pke wengx msowit; omutjrtuvn ilga yuduccamaf ygeyb oolr rulkem, ru capdaqh von ve lzamu.

<p className="result-count">
  Showing {visibleCourses.length} of {courses.length}{' '}
  courses
</p>

.result-count {
  margin: 0 0 16px;
  font-size: 0.9rem;
  color: #57606a;
}
A derived counter: always right, because there's no stored copy to forget updating.
I kijeweg taibbod: oyyuyc jiwyt, wamaebe tcuhe'm hi hwuker werl ki fipneg uqnevabp.

Making Impossible States Impossible

Duplication has an uglier sibling: contradictory state — independent state variables that can disagree. Your form has a latent case. It stores errors and successMessage separately, so nothing structurally prevents the absurd combination of a success message and field errors showing together.

type SubmitResult =
  | { kind: 'idle' }
  | { kind: 'invalid'; errors: FormErrors }
  | { kind: 'added'; title: string }

const [result, setResult] = useState<SubmitResult>({
  kind: 'idle',
})

const errors =
  result.kind === 'invalid' ? result.errors : {}

if (hasErrors) {
  setResult({ kind: 'invalid', errors: nextErrors })
  return
}

setResult({ kind: 'added', title: newCourse.title })

{result.kind === 'added' && (
  <p className="success-note" role="status">
    Added "{result.title}" to the catalog.
  </p>
)}
Same behavior, sturdier model: one state that can't contradict itself.
Raki hobiviir, pqimvuep goleb: eba fnogi zwuz hem'r purrzicadp ofkadn.

Strict Mode’s Double Render

Time to settle the doubled log lines from the lab. That’s Strict Mode — the <StrictMode> wrapper that’s been in main.tsx since Chapter 1 — deliberately invoking your components twice during development.

Challenge: The Three-Bug Hunt

This chapter’s materials include an exercise folder: the Learning Tracker with three state bugs freshly planted. Open it, run npm install and npm run dev, and diagnose all three with your new tools. The symptoms:

Key Points

  • Handlers see their render’s snapshot; every setter call computes from that frozen value, no matter how many times it runs.
  • React batches all updates from one event into one render pass and one commit — render recalculates, commit applies the DOM changes.
  • Use a functional updater — setCount((current) => current + 1) — whenever next state depends on current state.
  • Setters compare old and new with Object.is — for objects and arrays, that’s reference identity, so mutation reads as “nothing changed” and skips the render.
  • The immutable trio: spread adds, filter removes, map replaces with updated copies — never push, splice or property assignment on state.
  • Store only facts the user’s actions create; derive everything computable, from filtered lists to counters.
  • Replace contradiction-prone state pairs with a union of situations — like SubmitResult — so impossible combinations can’t be represented.
  • Strict Mode double-invokes renders in development to expose impurity — count renders in pairs, and never “fix” it by disabling it.

Where to Go From Here?

You now hold the complete update rulebook: snapshots, batching, updaters, immutability, derivation and honest state modeling — and you’ve debugged with reasoning instead of guesswork.

9. Forms & User Input
11. Designing State & Building My Learning
Have a technical question? Want to report a bug? You can ask questions and report bugs to the book authors in our official book forum here.
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