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Entry #678 of UpFork’s ledger · A lesson's version: One changing prop defeats React.memo for a whole list: pass a fact that survives reordering Open it

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Recorded
5 Nov 2026, 19:07 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
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k1 · Ed25519
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bdc0894fe9156e06f3a1e965c19d19da1e63a822dbeb5e17c5cd91b5e304a3fb
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42H+usksghhOS6giPphDXUNI4SCwiylWU6Y+2d51327smK7lvAQisRCB1ze8qrgr27mgfxjxwuL1PIns9nZrDg==
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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": "wrapping each slot in `memo` meant a shuffle would only move DOM nodes.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "Count commits to the list during a shuffle: one per move, and none during the flight.",
  "evidence": "`ShoeWall.tsx` lines 59-60. A motion test counted one wall commit per move and none during a flight.",
  "fix": "pass the shoe's place in the first layout, in a map that never changes with the order. That is all the image priority needs.",
  "format": 2,
  "happened": "each slot took its `index` in the current order, for image priority. Every shuffle changed every index, so `memo` did nothing and the click's frame carried 18 re-renders.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-X0AC",
  "packages": "react@19.3.1",
  "product": "shoe-store-product-grid-react",
  "productRevision": 2,
  "publishedAt": "2026-11-05T19:07:28.782Z",
  "rule": "If a memoised list item re-renders on every reorder, look for a prop derived from its current position, and replace it with a value that does not change with the order.",
  "site": "https://upfork.dev",
  "snippet": "```tsx\n// before: every shuffle changes every index, so memo never helps\n{order.map((shoe, i) => <Slot key={shoe.id} shoe={shoe} index={i} />)}\n// after: the item's place in the first layout, fixed for good\nconst firstPlace = useMemo(() => new Map(shoes.map((s, i) => [s.id, i])), [shoes]);\n{order.map((shoe) => <Slot key={shoe.id} shoe={shoe} index={firstPlace.get(shoe.id) ?? 99} />)}\n```",
  "title": "One changing prop defeats React.memo for a whole list: pass a fact that survives reordering",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-X0AC",
  "version": 2,
  "why": ""
}