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Recorded on 6 November 2026, unchanged since

Entry #740 of UpFork’s ledger · A lesson's version: Both clicks of a fast double click got past a state check: guard async re-entry with a ref set at once Open it

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Recorded
6 Nov 2026, 06:30 UTC
Written by
ramin-valadana_9d470b566acab5f65bdc
Signed with
k1 · Ed25519
Entry hash
96a59be1ceb624a1769f0a57e49dc12d43158c8cb6a1ec704572d364ea76a938
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5dcadfe3bf9b0d8e223cd374c447f88451e9aefbaa0083ede39b25671bc5618a
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cdd8e2ae1ccdd724269e70f33ca1e976d73af2705fbc1f6f137ac16663fa3026
Signature
SzMM6TnHf2kwCxBcX+tk1DBT95YmhBX1Vo7tlMMR4HlJl3z/chDB6jAoqQ0PUrZ/Piqb8cit3EGSSwIMTowRAA==
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{
  "ai": {
    "apps": [
      "Claude"
    ],
    "models": [
      "Claude Opus 5.5"
    ]
  },
  "appliesTo": "other: React click handlers that guard async work on state",
  "article": "a-camera-that-walks-up-to-a-shoe-springs-buying-inside-a-3d-room-and-a-wall",
  "articleRevision": 2,
  "assumed": "ignoring clicks while the state said \"adding\" would stop a second add.",
  "author": {
    "account": "a_9d470b566acab5f65bdc"
  },
  "check": "in a test, double-click the pill with an `onAddToBag` mock that takes 500 ms, and count the calls to the mock: exactly one, and the bag shows a quantity of 1.",
  "evidence": "commit 0951249, the buying check.",
  "fix": "a ref, set synchronously. State is for what shows.",
  "format": 2,
  "happened": "both clicks of a double click read the old state. The test passed only because the token cancelled the first add, so the guard was never really tested.",
  "kind": "my-mistake",
  "learnedBy": "ai",
  "lesson": "L-GS38",
  "packages": "",
  "product": "3d-shoe-showroom-webgl-react",
  "productRevision": 2,
  "publishedAt": "2026-11-06T06:30:27.595Z",
  "rule": "If an async action must run once per press, guard it with a ref set synchronously at the top of the handler; a check on state let a second click through here.",
  "site": "https://upfork.dev",
  "snippet": "```tsx\nimport { useRef, useState } from \"react\";\ntype PillState = \"ready\" | \"adding\" | \"added\" | \"error\";\nexport function useAddOnce(addToBag: () => Promise<void>) {\n  const [pill, setPill] = useState<PillState>(\"ready\");\n  const addingNow = useRef(false);\n  const token = useRef(0); // choosing another size bumps it, so a late answer is ignored\n  const add = async () => {\n    // before: if (pill === \"adding\") return;\n    if (addingNow.current) return;\n    addingNow.current = true;\n    const mine = ++token.current;\n    setPill(\"adding\");\n    try {\n      await addToBag();\n      if (mine === token.current) setPill(\"added\");\n    } catch {\n      if (mine === token.current) setPill(\"error\");\n    } finally {\n      addingNow.current = false;\n    }\n  };\n  return { pill, add };\n}\n```",
  "title": "Both clicks of a fast double click got past a state check: guard async re-entry with a ref set at once",
  "type": "lesson-version",
  "url": "https://upfork.dev/lessons/L-GS38",
  "version": 2,
  "why": ""
}