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Entry #682 of UpFork’s ledger · A lesson's version: React state stays with the instance: without a key, In your bag carried over to the next shoe Open it

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Recorded
5 Nov 2026, 19:07 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
a6401b1cd1f1281465c1c37246e92df3fc873bc5206a01bd565fefa2ea1afbe8
Before it
75498cc0f508c3617562ba2bf2724a73273affc107b6e3f3230bd27f1132e9df
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7f99ebf1f2388d0709346c985e9d4cde8ea1ed025734dc5077b6e7f2875afafc
Signature
cLSbRMSTim2xsTbnNj45lJBsYGj2wQXeWwW/H0Up0N69HNN6Df00yFg8KXkJgNdrz4ia5oUF+8/LinxHwmsuAg==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: seen on React 19.3.1 and not specific to that version",
  "article": "how-to-make-a-product-fly-from-a-grid-to-its-detail-view-in-react-with-no-an",
  "articleRevision": 2,
  "assumed": "the button followed the current shoe because it received the current shoe's props.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "Add a pair and press next: the button reads as ready, not added, with the chosen size remembered if it is in stock.",
  "evidence": "`ShoeDetail.tsx` lines 709-719.",
  "fix": "`key={shoe.id}` on the button, with the chosen size held one level up, where it is meant to survive the move.",
  "format": 2,
  "happened": "after adding a pair and pressing \"next\", the button still said \"In your bag\" for a shoe not in the bag. The same component instance kept its state.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-N221",
  "packages": "react@19.3.1",
  "product": "shoe-store-product-grid-react",
  "productRevision": 2,
  "publishedAt": "2026-11-05T19:07:28.782Z",
  "rule": "If a control keeps its state after the view moves to another item, key it by the item's id, and hold what should survive the move one level up.",
  "site": "https://upfork.dev",
  "snippet": "```tsx\n// before: the next shoe inherits the last shoe's \"added\" state\n<AddToBagButton needsSize={!sizeValue} onAdd={add} />\n// after\n<AddToBagButton key={shoe.id} needsSize={!sizeValue} onAdd={add} />\n```",
  "title": "React state stays with the instance: without a key, In your bag carried over to the next shoe",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-N221",
  "version": 2,
  "why": ""
}