Fix Excel LAMBDA Recursion Limits When Parsing Nested JSON

A user on a popular Excel engineering forum recently posted: “I’m using the FILTERXML hack to parse JSON strings by replacing braces with tags. I wrote a recursive LAMBDA to traverse nested paths, but once my JSON hits a certain depth, I get a #NUM! error. It seems I’ve hit a recursion limit. How do I parse deep structures without crashing the stack?”

This is a common bottleneck. Excel’s calculation engine has a hard limit on recursion depth (typically around 80 to 100 levels depending on memory). When you are drilling into deeply nested JSON or XML structures, a recursive function calls itself for every node, quickly hitting that ceiling.

Why Recursion Fails and What to Use Instead

While recursion is elegant, it is memory-intensive in Excel formulas. To handle deep nesting, you need a method that uses iteration rather than recursion, or better yet, a tool designed for data transformation.

Method	Complexity	Depth Limit	Software Version Compatibility
Recursive LAMBDA	High	~80 Levels	Excel 365 / Excel 2021
REDUCE / SCAN	Medium	~1,000,000	Excel 365 (Tested 2024)
Power Query	Low	Unlimited	Excel 2016 or newer (Windows)
Python in Excel	Medium	No Limit	Excel 365 (Beta/Current Channel)

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Recommended Approach 1: Power Query (The Robust Fix)

Power Query is the professional choice for parsing JSON. It doesn’t use the worksheet’s recursion stack, making it immune to the #NUM! error caused by deep nesting.

1. Select the cell containing your JSON string or the range of data.
2. Go to Data > From Table/Range.
3. In the Power Query Editor, right-click the column header and select Transform > JSON.
4. Click the “Expand” icon (two arrows pointing apart) on the column header to drill into the nested records.
5. Repeat the expansion for each nested level. Power Query handles the “looping” logic behind the scenes.
6. Click Home > Close & Load to return the flattened data to a new sheet.

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Recommended Approach 2: Using REDUCE to Avoid Recursion

If you must stay within a worksheet formula, swap your recursive LAMBDA for the REDUCE function. REDUCE iterates through an array without adding new layers to the call stack.

Tested on Excel for Microsoft 365 (Version 2405) as of 2024.

Suppose you have a path string like level1/level2/level3 and you want to traverse it. Instead of calling a function inside itself, you use the path as the array to iterate over.

=LAMBDA(json_xml, path_string,
    LET(
        path_array, TEXTSPLIT(path_string, "/"),
        result, REDUCE(json_xml, path_array,
            LAMBDA(current_node, segment,
                FILTERXML(current_node, "//" & segment)
            )
        ),
        result
    )
)

Why this works: REDUCE starts with your initial XML. For every segment in your path, it performs a single FILTERXML operation and passes the result to the next step. The “stack” never gets deeper; it just moves forward.

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Adapting to Your Data: Common Traps

1. What if my JSON has arrays, not just objects?

FILTERXML is notoriously picky about arrays. If your JSON-to-XML conversion results in multiple nodes with the same name, FILTERXML will return an array of values. If your formula isn’t wrapped in INDEX or a similar selection tool, the formula may return a #SPILL! error or fail during the REDUCE iteration. Always ensure your segment logic targets a specific index if you expect multiples.

2. Is FILTERXML available on all platforms?

No. FILTERXML is a legacy function available only on Excel for Windows. If you are sharing this file with Mac users or using Excel Online, the formula will return a #NAME? error. In these cases, you must use Power Query or the newer TEXTBEFORE/TEXTAFTER string manipulation functions to manually carve out data.

3. Can I automate this for thousands of rows?

If you have a column of 10,000 JSON strings, a LAMBDA (even a non-recursive one) can be slow because FILTERXML is re-calculated frequently. In this scenario, Power Query is significantly faster because it loads the data into memory once, processes the entire batch, and outputs a static table.

Summary Checklist for Deep Parsing

• Check Depth: If nesting is >50 levels, avoid recursive LAMBDAs.
• Choose Tool: Use Power Query for Windows-based, large-scale tasks.
• Iterate: Use REDUCE or SCAN if you need a formula-based solution that stays in the cell.
• Sanitize: Ensure your JSON-to-XML conversion handles special characters (like &) which break XML parsers.