[ronistrom]

Closes #599

What was wrong

A formula cell's cached result goes into formulaResult as a plain string, whether the file stored it as an error (t="e") or as text (t="str"). #DIV/0! is a valid text result too, so the two cells below read back the same:

<c r="A1" t="e"><f>1/0</f><v>#DIV/0!</v></c>
<c r="B1" t="str"><f>"#DIV/0!"</f><v>#DIV/0!</v></c>
<c r="C1" t="e"><v>#N/A</v></c>

// before
A1 {"value":"#DIV/0!","type":"formula","formula":"1/0","formulaResult":"#DIV/0!"}
B1 {"value":"#DIV/0!","type":"formula","formula":"\"#DIV/0!\"","formulaResult":"#DIV/0!"}
C1 {"value":"#N/A","type":"error"}
// after
A1 {"value":"#DIV/0!","type":"formula","formula":"1/0","formulaResult":"#DIV/0!","formulaResultType":"error"}
B1 {"value":"#DIV/0!","type":"formula","formula":"\"#DIV/0!\"","formulaResult":"#DIV/0!"}
C1 {"value":"#N/A","type":"error"}

The writer emitted every string result as t="str", so readXlsx → writeXlsx turned the error into text:

<!-- before -->
<c r="A1" t="str"><f>1/0</f><v>#DIV/0!</v></c>
<!-- after -->
<c r="A1" t="e"><f>1/0</f><v>#DIV/0!</v></c>

B1 and C1 are written the same before and after. ECMA-376 Part 1 §18.18.11 ST_CellType keeps e (error) and str (formula string) separate.

Why the tests didn't catch it

The round-trip test in formula-result-types.test.ts (from #497) only checked that the text #DIV/0! appeared somewhere in the rewritten sheet. It never checked the cell's t. Every read-side assertion compared formulaResult to the string, and that string is the same for both cell types.

What landed

• A new field, Cell.formulaResultType?: "error". The reader sets it only on a formula cell whose cached result is t="e". Every other cell, including a hard-coded error with no formula, has no such field. Nothing that already exists changes value.
• The writer emits t="e" for a string formulaResult when formulaResultType === "error", and t="str" otherwise. It never guesses from the token, because a formula can return the text "#DIV/0!". If formulaResult is not a string, the writer ignores the field, since an error token is always text. The doc comment says so, and a test pins it.
• cloneCell, the worker serializer and deserializer, and the FULL_CELL fixture in clone-sheet-coverage.test.ts all carry the field.
• docs/PARITY.md (under "Cached formula results") now says the error and text results stay distinct.
• writeXlsxStream is unchanged. StreamStyledCell has no way to mark a streamed formula's result as an error, so string results there are still written as t="str". PARITY.md says so. Supporting it would mean adding a field to StreamStyledCell, which I left out of this PR.

Tests: three of the five new tests fail against main: the read case, the read → write round trip, and the authored { formulaResultType: "error" } write. The tightened #497 round-trip assertion fails too. The other two new tests pass on main: one checks that no other result type gets the field, the other that a non-string result ignores it. Adding the field to FULL_CELL also makes six clone-sheet-coverage tests fail at runtime on main, because cloneCell and the worker drop the field. With the field in the type, tsc would reject the fixture there as well. On Node 24.22.0, lint, format check and both typechecks pass. The full vitest run had 10,701 of 10,703 tests passing across 240 files. The two failures were timeouts in html-import.test.ts and real-world.test.ts while the machine was heavily loaded (load average around 29). The first run hit a similar timeout in property-roundtrip.test.ts. All three files pass when run alone (71 tests).

After review: a cell a shared formula covers (<f t="shared" si="0"/>, read as formula: "") lost its cached error, because the error arm tested formula for truthiness. It now tests !== undefined. Two more tests cover the read and the rewrite; both fail without that commit. bun run test passes, with 240 files and 10,705 tests.

Checked in Excel

In Excel for Mac.

• Writing. A1 =1/0, B1 ="#DIV/0!", and ISERROR of each in A2 and B2, written by main and by this branch. Both files open without a repair prompt and show #DIV/0!, #DIV/0!, TRUE and FALSE. That is expected: every hucre workbook sets fullCalcOnLoad="1", so Excel recalculates on open and never uses the cached result. The fix matters to readers that use the cache without recalculating.
• Reading a file Excel wrote. I entered A1 =1/0 and B1 ="#DIV/0!" in Excel and saved. Excel stored <c r="A1" t="e"><f>1/0</f><v>#DIV/0!</v></c> and <c r="B1" t="str"><f>"#DIV/0!"</f><v>#DIV/0!</v></c>, the same shapes this PR writes. Reading that file, main gives the two cells the same result. This branch gives A1 formulaResultType: "error" and leaves B1 without it.

Not verified

• A reader that uses cached results without recalculating, the case this fix is for. I did not try one.
• LibreOffice and Windows Excel. Not opened.

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Reviewing files that changed from the base of the PR and between 7ea38b1 and fd5c646.

📒 Files selected for processing: 2

• src/xlsx/worksheet.ts
• test/formula-result-types.test.ts

📝 Walkthrough

Formula cells with cached error results now retain metadata that distinguishes error tokens from text results. The XLSX reader sets this metadata, cell cloning and worker serialization preserve it, and the writer uses it to serialize formula errors as errors.

Changes: Formula result type preservation

Layer / File(s)	Summary
Define and read formula error metadata src/_types.ts, src/xlsx/worksheet.ts, test/formula-result-types.test.ts	Cell adds optional formulaResultType: "error". The XLSX reader sets it for formula cells with cached error tokens. Tests distinguish formula errors from text results and hard-coded errors.
Preserve metadata across cell operations src/sheet-ops.ts, src/worker.ts, test/clone-sheet-coverage.test.ts	Cell cloning and worker serialization preserve formulaResultType. The clone fixture represents a formula error with a string result and the error marker.
Write and verify formula result types src/xlsx/worksheet-writer.ts, test/formula-result-types.test.ts, docs/PARITY.md	The writer emits t="e" for string formula results marked as errors, and t="str" for unmarked string results. Tests cover authored values and read/write round trips. The parity documentation records the behavior and notes that streamed formula results remain text.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: productdevbook

Merge Risk: 🟡 Moderate · up to 7ea38

Cached formula errors on shared-formula follower cells are dropped when reading, so they are not preserved when the file is written back. Change the check to formula !== undefined and add a test before merging.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 7 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: preserving the distinction between formula error results and text results.
Linked Issues check		Issue #599 requires the read and write paths to preserve the distinction between formula error results (t="e") and formula text results (t="str"). worksheet.ts sets formulaResultType: "error" …
Out of Scope Changes check		The changes stay within issue #599. Type declarations, clone and worker propagation, documentation, fixtures, and regression tests support the new formula-result metadata. The tests also define the re…

Full details: Linked Issues check

Explanation

Issue #599 requires the read and write paths to preserve the distinction between formula error results (t="e") and formula text results (t="str"). worksheet.ts sets formulaResultType: "error" only for formula cells in the t="e" branch. worksheet-writer.ts emits t="e" only when that marker is present with a string formulaResult; it keeps unmarked strings as t="str". The added tests cover reading, read/write round trips, authored error results, hard-coded errors, and non-string results. Cell cloning and worker serialization preserve the marker.

Full details: Out of Scope Changes check

Explanation

The changes stay within issue #599. Type declarations, clone and worker propagation, documentation, fixtures, and regression tests support the new formula-result metadata. The tests also define the required behavior for text results and non-string results. No unrelated product behavior is demonstrated. The documented unchanged writeXlsxStream path does not add an out-of-scope change.

Full details: Docstring Coverage

Explanation

Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 7 files. (1 skipped: 1 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @src/xlsx/worksheet.ts:
- Line 1946: Update the formula check near the cell-building logic to use an
undefined check rather than a truthiness check, so shared-formula slave cells
with an empty formula still preserve cached error results and set
formulaResultType to "error". Add coverage for a shared-formula slave with
t="e".

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⚙️ Run configuration

• Configuration used: defaults
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📥 Commits

Reviewing files that changed from the base of the PR and between c991ad0 and 7ea38b1.

📒 Files selected for processing: 8

• docs/PARITY.md
• src/_types.ts
• src/sheet-ops.ts
• src/worker.ts
• src/xlsx/worksheet-writer.ts
• src/xlsx/worksheet.ts
• test/clone-sheet-coverage.test.ts
• test/formula-result-types.test.ts

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