{"record":{"id":"ef6f4dc8c3aeeb67","repo":"facebook/react","slug":"expected-a-previous-fiber-when-updating-an-existin","errorCode":null,"errorMessage":"Expected a previous Fiber when updating an existing root.","messagePattern":"Expected a previous Fiber when updating an existing root\\.","errorType":"exception","errorClass":"Error","httpStatus":null,"severity":"info","filePath":"packages/react-devtools-shared/src/backend/fiber/renderer.js","lineNumber":5100,"sourceCode":"        priorityLevel:\n          priorityLevel == null ? null : formatPriorityLevel(priorityLevel),\n        updaters: null,\n        // Initialize to null; if new enough React version is running,\n        // these values will be read during separate handlePostCommitFiberRoot() call.\n        effectDuration: null,\n        passiveEffectDuration: null,\n      };\n    }\n\n    const nextIsMounted = nextFiber.child !== null;\n    const prevWasMounted = prevFiber !== null && prevFiber.child !== null;\n    if (!prevWasMounted && nextIsMounted) {\n      // Mount a new root.\n      setRootPseudoKey(currentRoot.id, nextFiber);\n      mountFiberRecursively(nextFiber, false);\n    } else if (prevWasMounted && nextIsMounted) {\n      if (prevFiber === null) {\n        throw new Error(\n          'Expected a previous Fiber when updating an existing root.',\n        );\n      }\n      // Update an existing root.\n      updateFiberRecursively(rootInstance, nextFiber, prevFiber, false);\n    } else if (prevWasMounted && !nextIsMounted) {\n      // Unmount an existing root.\n      unmountInstanceRecursively(rootInstance);\n      removeRootPseudoKey(currentRoot.id);\n      rootToFiberInstanceMap.delete(root);\n    } else if (!prevWasMounted && !nextIsMounted) {\n      // We don't need this root anymore.\n      rootToFiberInstanceMap.delete(root);\n    }\n\n    if (isProfiling && isProfilingSupported) {\n      if (!shouldBailoutWithPendingOperations()) {\n        const commitProfilingMetadata = (","sourceCodeStart":5082,"sourceCodeEnd":5118,"githubUrl":"https://github.com/facebook/react/blob/eafeac097ba51e1eab809c07102126bd5f8e5425/packages/react-devtools-shared/src/backend/fiber/renderer.js#L5082-L5118","documentation":"Root update invariant in handleCommitFiberRoot: the branch is entered only when prevWasMounted is true, and prevWasMounted is computed as prevFiber !== null && prevFiber.child !== null, so prevFiber should never be null here. The guard exists largely to satisfy Flow's narrowing; if it ever throws, the backend's root bookkeeping (currentRoot.previousFiber pairing) is already corrupted.","triggerScenarios":"Reachable only when setRootPseudoKey/update bookkeeping loses the previous fiber while still marking the root as previously mounted - typically cascading after an earlier invariant failure in the same DevTools session.","commonSituations":"Appears after prior swallowed DevTools errors desynchronized rootToFiberInstanceMap/pseudo-key state; extremely rare on matched, current versions.","solutions":["Reload the inspected app to fully reset DevTools backend state","Update React DevTools and React to matching versions","Look for an earlier DevTools error in the console - this throw is usually a downstream symptom","Report both the first error and this one if they recur together on current versions"],"exampleFix":null,"handlingStrategy":"try-catch","validationCode":null,"typeGuard":null,"tryCatchPattern":"try {\n  backend.handleCommitFiberRoot(renderer, root);\n} catch (e) {\n  if (/Expected a previous Fiber when updating an existing root/.test(e.message)) {\n    reinitializeRootTracking(root); // defensive-check trip: state already corrupted\n  } else throw e;\n}","preventionTips":["Reload the app to reset root bookkeeping - this guard is near-unreachable and signals earlier corruption","Scan the console for a prior DevTools error; this throw is a downstream symptom","Keep React/DevTools versions matched to avoid the upstream desync","Report both errors together when they recur"],"tags":["react-devtools","invariant","root-fiber","internal","defensive-check"],"backgroundTag":"devtools-root-state-invariant","analyzedSha":"eafeac097ba51e1eab809c07102126bd5f8e5425","analyzedAt":"2026-08-21T22:01:08.818Z","schemaVersion":2},"datasetVersion":"2026-08-21T23:17:16.201Z"}