Apa tegese "single source of truth"?

Question

Accepted Answer

**Single source of truth** tegese saben piece of data disimpen ing **exactly one place**, lan kabeh sing butuh baca saka kana — tinimbang nyimpen copies akeh-akehe sing bisa drift out of sync.

## Masalah sing disingkal: duplicated, diverging state

```jsx
// ❌ the same data stored in two places → they can disagree
function Profile() {
  const [user, setUser] = useState({ name: "Ann" });
  const [displayName, setDisplayName] = useState("Ann"); // a COPY of user.name

function rename(newName) {
    setUser({ ...user, name: newName }); // updated user...
    // ...but forgot to update displayName → now they're out of sync! 🐛
  }
}
```

Kelone saben fakta urip ing rong state, saben update kudu eling-eling gawe owah loro — lan mesthi siji ketinggalan, ngasilake bugs ngendi UI nudoni nilai inconsistent.

## Solusi: one source, derive sing liyane

```jsx
// ✅ user.name is the single source; displayName is DERIVED, not stored
function Profile() {
  const [user, setUser] = useState({ name: "Ann" });
  const displayName = user.name.toUpperCase(); // computed on render — always in sync

function rename(newName) {
    setUser({ ...user, name: newName }); // one update, everything stays consistent
  }
}
```

Simpen nilai canonical sapisan; compute apa-apa derivable saka kana on the fly. Ora ana apa-apa sing kudu keep in sync.

## Ngendi principle iki berlaku

```text
✓ Don't copy props into state (the prop is the source)
✓ Don't store derived values (compute them: totals, filtered lists, formatted text)
✓ Global stores (Redux/Pinia): one store is the source for shared data
✓ Server data: cache it once (React Query) rather than copying into many components
✓ URL as source: filters/page in the URL, read from there
```

## Kenapa iku penting

Single source of truth iku foundational principle saka reliable state management — ngilangake entire class bugs sing disebabake duplicate data drift apart.

Iku *kenapa* kowe derive values tinimbang nyimpen kaya, *kenapa* kowe avoid copying props into state, lan *kenapa* global stores centralize shared data.

Kelone data nduweni one authoritative home lan kabeh baca saka kana, UI mu tetep consistent by construction.