fix(xlsx): write newer functions with their _xlfn. prefix, and LET/LAMBDA names with _xlpm. by ronistrom · Pull Request #608 · productdevbook/hucre

ronistrom · 2026-10-20T18:48:54Z

Closes #597

What was wrong

Functions added after Excel 2007 are stored in the file under a prefixed name, for example _xlfn.XLOOKUP, _xlfn.IFS, and _xlfn._xlws.FILTER / _xlfn._xlws.SORT for the two worksheet-only ones ([MS-XLSX] §2.2.2). Excel adds the prefix when it saves and hides it in the formula bar, so callers write the bare name. hucre wrote that bare name into <f>, and a file that says XLOOKUP( names no function a reader knows, so the cell computes #NAME?.

The reproduction from the issue:

const formulas = ['XLOOKUP("b",A1:A3,B1:B3)', 'IFS(A1>1,"big",TRUE,"small")', 'TEXTJOIN(",",TRUE,A1:A3)', "FILTER(A1:A3,B1:B3>1)", "SORT(A1:A3)"]
await writeXlsx({ sheets: [{ name: "Sheet1", rows: [["a", 1], ["b", 2], ["c", 3]], cells: new Map(formulas.map((formula, i) => [`${i},3`, { formula }])) }] })

<!-- before -->
<c r="D1"><f>XLOOKUP("b",A1:A3,B1:B3)</f></c>
<c r="D2"><f>IFS(A1&gt;1,"big",TRUE,"small")</f></c>
<c r="D3"><f>TEXTJOIN(",",TRUE,A1:A3)</f></c>
<c r="D4"><f>FILTER(A1:A3,B1:B3&gt;1)</f></c>
<c r="D5"><f>SORT(A1:A3)</f></c>
<!-- after -->
<c r="D1"><f>_xlfn.XLOOKUP("b",A1:A3,B1:B3)</f></c>
<c r="D2"><f>_xlfn.IFS(A1&gt;1,"big",TRUE,"small")</f></c>
<c r="D3"><f>_xlfn.TEXTJOIN(",",TRUE,A1:A3)</f></c>
<c r="D4"><f>_xlfn._xlws.FILTER(A1:A3,B1:B3&gt;1)</f></c>
<c r="D5"><f>_xlfn._xlws.SORT(A1:A3)</f></c>

Why the tests didn't catch it

The tests compared the writer's output with what they had passed in, so they checked that the formula text survived. None of them checked that the text was in the form Excel stores. formulas-advanced.test.ts went further and pinned the bug: it wrote SORT(A2:A10) and UNIQUE(A1:A5) and asserted the bare names in the XML and on read-back. Those three assertions now expect the prefixed form.

What landed

A new src/xlsx/formula-prefix.ts with addFuturePrefixes(formula) and a map from function name to prefix. The names come from the [MS-XLSX] Functions table (linked in the file). FILTER and SORT map to _xlfn._xlws., and everything else maps to _xlfn.. Four names are added that the spec does not list yet but Excel prefixes: XMATCH, ARRAYTOTEXT, VALUETOTEXT and IMAGE. XlsxWriter's future-function table has all four. Every name is in one of those two lists.
- ECMA.CEILING, ISO.CEILING, NETWORKDAYS.INTL and WORKDAY.INTL are left bare, because the spec lists them without a prefix.
- _xlfn._xlws.PY, _xlfn.SINGLE and _xlfn.ANCHORARRAY are left out, because they aren't names a caller types.
What the scanner touches:
- It prefixes a name only when the name is a call: directly followed by (, and not after !.
- It skips "…" string literals (with "" escapes), '…' quoted sheet names (with '' escapes) and […] structured references or external-book indexes.
- Dots count as name characters, so _xlfn.XLOOKUP and _xlfn._xlws.SORT scan as one unknown name and are left alone. That makes the pass idempotent: formulas read from Excel's files, which already carry the prefix, write back unchanged.
- Matching ignores case and keeps the caller's spelling (xlookup( becomes _xlfn.xlookup().
- SORTBY gets _xlfn.SORTBY, not SORT's _xlws..
- A formula with no ( returns at once, because this runs on the per-cell write path.
Where it runs: once in serializeCell, before the <f> branches. That covers normal, shared and array formulas, and both streaming writers, because they serialize through the same function. It also runs on the other places hucre stores a formula in the same grammar:
- conditional-formatting <formula>
- data-validation formula1 and formula2
- table totalsRowFormula
- defined names
Defined names and table totals. The plan was to leave defined names alone. I prefix them for these reasons:
- <definedName> holds a formula, not only a range, and a named LAMBDA or LET is exactly where these functions show up.
- Excel writes the prefix there too.
- The usual contents (Sheet1!$A$1:$B$5, print areas, quoted sheet names) have no call in them, so they pass through unchanged.
- The table writer has no calculated-column formula, only totalsRowFormula, which gets the same treatment for the same reason.
Read-back changes. The reader keeps formula text as the file has it, so a formula written as XLOOKUP(…) now reads back as _xlfn.XLOOKUP(…). It's the same formula, but a strict equality check on a round trip will see the difference. Writing the read-back text again does not prefix it twice. docs/PARITY.md, the Cell.formula doc and the NamedRange.range doc now say so.
_xlpm. on the names LET and LAMBDA bind. The issue suggested leaving this out. Checking in Excel showed it can't be left out: with only _xlfn., Excel asks to repair the file and removes the LET formula and the named LAMBDA (see "Checked in Excel"). A bound name is stored with the prefix both where it is bound and everywhere it is used: _xlfn.LET(_xlpm.x,2,_xlpm.x*3). Which names to prefix depends on scope, which a scan for NAME( can't see. So a formula that mentions LET or LAMBDA goes through a parse of its argument lists, and every other formula keeps the plain scan. Each rule matches what Excel 16.114 for Mac stored when the bare formula was typed in:
- A LET name is in scope from the next value onwards, not in its own value.
- LAMBDA parameters are in scope in the body only, so the arguments of a LAMBDA invoked on the spot, LAMBDA(x,x*2)(x), stay bare.
- An optional [b] parameter is bound as _xlop.b and used as _xlpm.b.
- A LET-bound LAMBDA called by name is written _xlpm.f(2).
- A use takes the spelling of its binding: LET(Ab,2,aB) is stored with _xlpm.Ab twice.
- Inside a scope that binds a, A:A becomes _xlpm.a:_xlpm.a, but $A$1, A$1 and $A1 stay cell references.
- Strings, quoted sheet names, sheet-qualified names, table references and already-prefixed text are left alone, so Excel's own stored form writes back unchanged.
PARITY.md and the Cell.formula / NamedRange.range docs now describe this instead of saying the names are written as given.

Tests: test/formula-future-prefix.test.ts adds 23 tests.

_xlfn. (16 tests). 13 fail against main. The other three pass on main because they check what must not change: future functions the spec stores bare, already-prefixed formulas, and names that aren't calls. The three corrected assertions in formulas-advanced.test.ts fail against main too.
_xlpm. (7 tests). Six fail without the _xlpm. commit.

bun run test passes, with 241 files and 10,721 tests.

After review, a third commit adds ten functions that Excel 16.114 for Mac saved with _xlfn. when typed in bare, though neither [MS-XLSX] nor XlsxWriter lists them yet: DETECTLANGUAGE, GROUPBY, PERCENTOF, PIVOTBY, REGEXEXTRACT, REGEXREPLACE, REGEXTEST, STOCKHISTORY, TRANSLATE and TRIMRANGE. A file written with five of them opens cleanly and computes.

ENCODEURL is left out. Neither source lists it, and Excel for Mac, which doesn't have the function, saves it bare.
One prefix is still missing. A function passed as an argument, as in GROUPBY(A2:A3,B2:B3,SUM), is stored as _xleta.SUM. Written bare, Excel computes #NAME?; with _xleta.SUM hand-edited in, it computes. Adding it needs Excel's whole function list, to tell such a name apart from a defined name. PARITY.md says so and points to a LAMBDA instead (LAMBDA(x,SUM(x))), which is written in its stored form.

The new test fails without that commit. bun run test passes, with 241 files and 10,723 tests.

Checked in Excel

In Excel for Mac.

The issue's five formulas. The file main writes is worse than #NAME?. Excel asks to repair it, and its repair log says it removed formulas from sheet1.xml. After the repair, D1–D3 show #NAME?, and D4 and D5 (FILTER, SORT) are empty. This branch's file opens without a prompt and shows D1 2, D2 small, D3 a,b,c, D4 b and D5 a. FILTER and SORT show only their first value because no dynamic-array metadata is written; that is separate from this PR.
Defined names. Names holding XLOOKUP(…) and TEXTJOIN(…), written by this branch, work: cells =Lookup and =Joined show 2 and a,b,c.
LAMBDA and LET needed _xlpm., which is why it is in this PR. I wrote a defined name Double as LAMBDA(x,x*2) and a cell LET(x,2,x*3). With only _xlfn. (_xlfn.LAMBDA(x,x*2), _xlfn.LET(x,2,x*3)), Excel asks to repair the file, removes the named range and the formula, and =Double(B2) shows #NAME?. main's output fails the same way. The same file with _xlpm.x hand-edited in opens cleanly and computes 2 and 6. With the _xlpm. commit, a file the branch writes also opens cleanly in Excel 16.114 for Mac, with every formula and defined name kept and the expected values. That file has a named LAMBDA, an optional parameter and a LET-bound call.
Table total rows. These could not be checked through this branch alone. Any custom totalsRowFormula hucre writes makes Excel ask for repair, on main as well, even SUM(Table1[Qty])*2 with no newer function in it, because hucre does not write the total row's cells. That is A table's total row is empty in Excel, and a custom total makes Excel repair the file #604, a separate bug. With A table's total row is empty in Excel, and a custom total makes Excel repair the file #604's branch merged in, a custom XLOOKUP total row opens cleanly and shows 3: the total-row cell gets _xlfn. from this PR's cell-formula pass, and <totalsRowFormula> from this PR's table-writer change.

Not verified

Table total rows on this branch alone, as above: they need A table's total row is empty in Excel, and a custom total makes Excel repair the file #604.
LibreOffice and Windows Excel. Not opened.

Functions added after Excel 2007 are stored in the file under a prefixed name: _xlfn.XLOOKUP, _xlfn.IFS, _xlfn.TEXTJOIN, and _xlfn._xlws.FILTER / _xlfn._xlws.SORT. Excel adds the prefix when it saves and hides it in the formula bar, so callers write the bare name. hucre wrote that name into <f> as given, and a file that says XLOOKUP( names no function a reader knows, so the cell computes #NAME?. A new addFuturePrefixes() in src/xlsx/formula-prefix.ts holds the future-function list from [MS-XLSX] 2.2.2 (plus XMATCH, ARRAYTOTEXT, VALUETOTEXT and IMAGE, which Excel also prefixes) and adds the prefix to a name only when it is a call: followed by "(", outside string literals, quoted sheet names and [...] structured references, and not after "!". Already-prefixed text scans as a different name, so formulas read from Excel's files write back unchanged and the pass is idempotent. Matching ignores case, and SORTBY gets _xlfn., not SORT's _xlfn._xlws. It runs once in serializeCell before the <f> branches, which covers normal, shared and array formulas and both streaming writers, and on the other places a formula is stored in the same grammar: conditional formatting <formula>, data validation formula1/formula2, table totalsRowFormula and defined names. A formula written bare now reads back in its prefixed form; docs/PARITY.md and the Cell.formula and NamedRange.range docs say so. _xlpm. on LET/LAMBDA parameters is not added. Closes productdevbook#597

The names LET and LAMBDA bind are stored with a prefix of their own, where they are bound and wherever they are used: _xlfn.LET(_xlpm.x,2,_xlpm.x*3). With only _xlfn. added, Excel for Mac asked to repair the file and removed the formula; a named LAMBDA went with it, and =Double(B2) computed #NAME?. Which names to prefix depends on scope, so a formula that mentions LET or LAMBDA now goes through a parse of its argument lists instead of the plain scan. It does what Excel 16.114 for Mac writes when the bare formula is typed in, checked case by case: - a LET name is in scope from the next value onwards, not in its own value; LAMBDA parameters only in the body, so the arguments of a LAMBDA invoked on the spot stay outside it - an optional [b] is bound as _xlop.b and used as _xlpm.b - a LET-bound LAMBDA called by name is _xlpm.f(2) - a use takes its binding's spelling (LET(Ab,2,aB) -> _xlpm.Ab) - with a bound, A:A is _xlpm.a:_xlpm.a; $A$1, A$1 and $A1 stay cells Strings, quoted sheet names, sheet-qualified names, table references and already-prefixed text are left as they are, so Excel's own stored form writes back unchanged. Formulas without LET or LAMBDA take the old scan. A file written this way, with a named LAMBDA, an optional parameter and a LET-bound call, opens in Excel 16.114 for Mac with every formula and defined name intact and the expected values. Refs productdevbook#597

coderabbitai · 2026-10-20T18:49:14Z

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📒 Files selected for processing: 3

docs/PARITY.md
src/xlsx/formula-prefix.ts
test/formula-future-prefix.test.ts

📝 Walkthrough

The XLSX writer now adds required future-function prefixes to formulas across several workbook features. It also prefixes LET/LAMBDA bindings and references. Documentation and tests describe and verify these stored formula forms.

Changes: Formula prefixing

Layer / File(s)	Summary
Formula scanning and scoped name handling `src/xlsx/formula-prefix.ts`, `src/_types.ts`, `docs/PARITY.md`, `test/formula-future-prefix.test.ts`	Adds formula scanning for future functions and scoped LET/LAMBDA names. Documents formula storage rules and tests binding prefixes, references, and nested scopes.
Formula prefixes in XLSX serialization `src/xlsx/worksheet-writer.ts`, `src/xlsx/table-writer.ts`, `src/xlsx/workbook-writer.ts`, `test/formula-future-prefix.test.ts`, `test/formulas-advanced.test.ts`	Applies prefixing to cell, data-validation, conditional-formatting, table-total, and defined-name formulas before XML serialization. Tests cover stored formulas, streaming output, and read/write expectations.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~60 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant WorksheetWriter
  participant addFuturePrefixes
  participant XLSXFormulaXML
  Caller->>WorksheetWriter: Provide formula
  WorksheetWriter->>addFuturePrefixes: Normalize formula prefixes
  addFuturePrefixes-->>WorksheetWriter: Return prefixed formula
  WorksheetWriter->>XLSXFormulaXML: Serialize formula

Suggested reviewers: productdevbook

Merge Risk: 🔵 Low · up to 23ff9

Formulas using three newer functions may be saved without the prefixes Excel expects. Add those names before merging, or accept this bounded compatibility gap.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 68.18% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 22 functions across 7 files. (1 skipped: …	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: writing newer Excel functions with _xlfn. prefixes and LET/LAMBDA names with _xlpm. prefixes.
Linked Issues check		Issue `#597` requires stored future-function prefixes while callers can use bare names. `src/xlsx/formula-prefix.ts` maps recognized calls to `_xlfn.` or `_xlfn._xlws.`, skips strings, quoted names, bra…
Out of Scope Changes check		The changed source files, tests, and documentation remain connected to formula serialization. The additional targets—conditional formatting, data validation, table totals, and defined names—store form…

Full details: Linked Issues check

Explanation

Issue #597 requires stored future-function prefixes while callers can use bare names. src/xlsx/formula-prefix.ts maps recognized calls to _xlfn. or _xlfn._xlws., skips strings, quoted names, bracketed references, sheet-qualified names, and existing prefixes, and preserves caller spelling. The shared serialization path applies the transformation to normal, shared, array, and streaming cell formulas. Tests cover the five issue formulas, idempotence, and streaming output. The implementation also covers formula-bearing defined names, validation rules, conditional formatting, and table totals. Scoped _xlpm. handling covers LET and LAMBDA bindings.

Full details: Out of Scope Changes check

Explanation

The changed source files, tests, and documentation remain connected to formula serialization. The additional targets—conditional formatting, data validation, table totals, and defined names—store formula text and require the same prefix rules. LET/LAMBDA scope handling supports valid storage of the same future-function formulas. The tests verify these related paths and no unrelated change is shown.

Full details: Docstring Coverage

Explanation

Docstring coverage is 68.18% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 22 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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Inline comments:
Review comments at @src/xlsx/formula-prefix.ts:
- Around line 55-212: Add ENCODEURL, GROUPBY, and TRIMRANGE to the XLFN_NAMES
list, preserving the existing alphabetical ordering. Do not add the other
unverified function names.

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📒 Files selected for processing: 8

docs/PARITY.md
src/_types.ts
src/xlsx/formula-prefix.ts
src/xlsx/table-writer.ts
src/xlsx/workbook-writer.ts
src/xlsx/worksheet-writer.ts
test/formula-future-prefix.test.ts
test/formulas-advanced.test.ts

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Review asked for ENCODEURL, GROUPBY and TRIMRANGE. Typing the bare formulas into Excel 16.114 for Mac and reading the saved file showed ten functions it stores with `_xlfn.` that neither [MS-XLSX] nor XlsxWriter lists yet: DETECTLANGUAGE, GROUPBY, PERCENTOF, PIVOTBY, REGEXEXTRACT, REGEXREPLACE, REGEXTEST, STOCKHISTORY, TRANSLATE and TRIMRANGE. They are added. A file written with five of them opens cleanly in Excel and computes. ENCODEURL is not: neither list has it, and Excel for Mac, which lacks the function, stores it bare. Prefixing a name Excel stores bare would break a formula that works. A function passed as an argument, `GROUPBY(…,SUM)`, is stored `_xleta.SUM`; written bare it computes #NAME?. That prefix is not added here — it needs Excel's whole function list to tell from a defined name — and PARITY.md says so, with LAMBDA as the way round it. Refs productdevbook#597

ronistrom and others added 2 commits October 20, 2026 21:20

coderabbitai Bot reviewed Oct 20, 2026

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ronistrom mentioned this pull request Oct 20, 2026

A function passed as an argument needs _xleta., so GROUPBY(…,SUM) computes #NAME? #617

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fix(xlsx): write newer functions with their _xlfn. prefix, and LET/LAMBDA names with _xlpm. - #608

fix(xlsx): write newer functions with their _xlfn. prefix, and LET/LAMBDA names with _xlpm.#608
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