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To pick the right tool

LAMBDA vs JS: which to use and when

A mental guide to decide whether to ask for LAMBDA or the JS sandbox. Spoiler: it depends on whether the logic is a pure formula or needs control flow.

schedule 4 minutes read
  1. Step 1

    The rule of thumb

    If you can write it as an Excel formula, use LAMBDA. If you need a loop, regex, or to process text character by character, use JS. LAMBDA lives inside the file (portable to desktop Excel). JS lives in the app (re-runnable from the Functions panel).

  2. Step 2

    LAMBDA shines on pure logic

    LAMBDA cases: applying VAT, calculating commissions, capped discounts, fixed currency conversion, range-based scoring, lookups with conditions. Anything you'd solve in Excel with nested IF, MIN, MAX, VLOOKUP, LET, etc. — but as a single reusable function.

    💡 Tip: Example: <code>=LAMBDA(score, IF(score >= 90, "A", IF(score >= 70, "B", IF(score >= 50, "C", "D"))))</code> registered as <code>Grade</code>.
  3. Step 3

    JS shines when there's control flow

    JS cases: counting unique words in a cell, extracting emails from free-text, parsing dates in mixed formats, deduping with custom rules, normalizing names (Juan / juan / JUAN → Juan), Levenshtein distance, custom-rule grouping. Anything you'd solve in VBA with a For Each loop.

    💡 Tip: Example: <code>data.map(r => (r[0] || "").toLowerCase().match(/\b\w+\b/g) ?? [])</code>.
  4. Step 4

    Quick mental table

    Iterate rows + transform a column → JS (data.map). Reusable formula with a few parameters → LAMBDA. Filter + project columns → JS (data.filter().map()). Conditional logic in a single cell → LAMBDA. Regex / parsing → JS. Match / lookup across sheets → native lookup_column tool, not LAMBDA or JS.

  5. Step 5

    Different persistence

    LAMBDA is saved as a workbook Named Range → if you export the .xlsx, it travels with it. Another user in desktop Excel sees the same function. JS scripts are stored in the SaaS database, tied to the project. If you export the .xlsx, the scripts don't travel — but they remain available in the project's Functions panel.

    💡 Tip: For things that must exist in the final file (client deliverable), prefer LAMBDA. For internal flow automation, JS is fine.
  6. Step 6

    Combine the two

    Nothing stops you from using both in the same project. For instance: use a JS script to clean and dedup names in column A, then a LAMBDA Saludo to generate ="Hello " & A2. The JS preps the data, the LAMBDA formats it.

  7. Step 7

    When in doubt, ask the AI

    The assistant knows this decision and picks for you. If you say "make X work", the AI chooses LAMBDA or JS based on what fits, explains it in the reply, and saves it to Functions so you can see it. To force one: "...using LAMBDA" or "...with JS".

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