Fix async callback bug in XLSX export with sourcefilename option and add comprehensive test coverage

[Copilot]

Description

The sourcefilename option for updating existing Excel files had a critical bug: the callback was invoked before async file loading completed, causing files not to be created and test failures.

Changes

Fixed async callback timing in src/833xlsx.js:

• Moved callback invocation inside the loadBinaryFile async callback for the sourcefilename path
• Non-sourcefilename path remains synchronous as before

Added comprehensive test coverage in test/test107-B.js (8 tests total):

• Loading existing Excel files with sourcefilename parameter
• Positioning data at specific cells with range parameter (e.g., "B3", "E5", "AA5")
• Headers on/off, sheet creation, and file overwriting scenarios
• Content verification: Each test reads back the generated XLSX files using SELECT FROM XLSX() and validates using assert.deepEqual():
  • Data is written at the correct positions
  • Original data is preserved when using sourcefilename
  • Headers are correctly included/excluded based on the headers option
  • Multiple sheets can coexist in the same workbook
  • Multi-letter column positions (like AA) work correctly
• Async/Promise flow validation: Added 2 async/await tests to confirm:
  • Promise-based operations work correctly with alasql.promise()
  • Async flow for sourcefilename and range operations
  • Promise-based flow for multi-sheet creation

Usage

// Update existing spreadsheet at specific location (callback style)
alasql('SELECT HOUR(NOW()), MINUTE(NOW()), SECOND(NOW()) \
    INTO XLSX("output.xlsx", { \
        sourcefilename: "template.xlsx", \
        sheetid: "Sheet1", \
        range: "B3", \
        headers: false \
    })', [], function(res) {
    // Callback invoked after file is written
});

// Using promises/async-await
await alasql.promise(
    'SELECT * INTO XLSX("output.xlsx", { \
        sourcefilename: "template.xlsx", \
        sheetid: "Sheet1", \
        range: "B3", \
        headers: false \
    }) FROM ?',
    [data]
);

The feature now correctly:

• Loads the source file asynchronously
• Writes data starting at the specified cell
• Invokes callbacks after completion
• Preserves existing data in the source file
• Supports both callback and promise-based async patterns

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update existing Excel spreadsheet with INTO XLSX()</issue_title>
<issue_description>I had a specific question related to the export to excel (xlsx).

I am working with AngularJS and looking to use your library to export data from an application into a spreadsheet. I want to SELECT INTO a spreadsheet that already exists and starting at a specific row and column. In the examples I have found and the attempt that I have tried, I can only export to a new file and the export always starts at A1.

Is there a way to do what I have explained?
</issue_description>

<agent_instructions>name the test file test/test107-B.js</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@agershun I have added two new experimental flags to INTO XLSX function: - sourcefilename - your basic file (if filename and sourcefilename are the same, the basic file will be overwritten) - range - left upper corner of range

This is an example:

    alasql('SELECT HOUR(NOW()), MINUTE(NOW()), SECOND(NOW()) \
        INTO XLSX("test251.xlsx",{sourcefilename:"test251.xlsx", \
          sheetid:"test2", range:"B3", headers:false})',[],function(res){
    });

@mathiasrw > I have added two new experimental flags

Shuold we include this in the wiki and close the issue or is it too experimental yet?

@mathiasrw Nothing new @mathiasrw Please make a new issue if its not related to the existing one. Its a known bug. Please let us know if you find a solution.

• Fixes Update existing Excel spreadsheet with INTO XLSX() #107

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