Office Scripts Expert Skill

When to Use

• Writing or reviewing Office Scripts (TypeScript for Excel Online)
• Debugging runtime errors in Office Scripts
• Setting up testable architecture for Office Scripts projects
• Understanding ExcelScript API patterns and limitations
• Migrating VBA macros to Office Scripts
• Optimizing scripts to stay within 120-second timeout

Prerequisites

• Microsoft 365 subscription with Excel Online access
• Office Scripts enabled for the tenant (admin setting)
• Node.js + npm (for local testing with Vitest)
• TypeScript knowledge (Office Scripts use a TypeScript subset)

Overview

Office Scripts are TypeScript-based automation scripts for Excel on the web (Microsoft 365). They are fundamentally different from VBA and Power Automate:

Aspect	Office Scripts	VBA	Power Automate
Language	TypeScript (subset)	VBA	Low-code / expressions
Runtime	Server-side (Excel Online)	Client-side (Desktop)	Cloud service
Module system	None (no import/export)	Modules	Connectors
External libs	Not available	COM references	Connectors
Timeout	120 seconds	None	30 min (premium)
Testing	Indirect (lib extraction)	Manual	Manual

Architecture Pattern: lib/ Extraction + Inline

Because Office Scripts cannot use import, the project uses a dual-file approach:

office_scripts/
  lib/                      # Canonical, testable source (ES modules)
    types.ts                # Interface definitions
    rounding.ts             # Rounding functions (export)
    prep-calculator.ts      # Prep calculation logic (export)
    ingredient-calculator.ts # Ingredient calculation logic (export)
    order-calculator.ts     # Order lot calculation (export)
    csv-parser.ts           # CSV parsing and header normalization (export)
  __tests__/                # Vitest unit tests
    rounding.test.ts
    prep-calculator.test.ts
    ingredient-calculator.test.ts
    order-calculator.test.ts
    csv-parser.test.ts
  CalculateRequirements.ts  # Main script (lib/* functions INLINED)
  ImportCsvData.ts          # Import script (lib/* functions INLINED)

Key rule: Edit lib/*.ts first, run tests, then copy the updated functions into the main scripts with // INLINED: lib/*.ts markers.

6 Critical Platform Constraints (P1-P6)

ID	Constraint	Impact	Workaround
P1	No import/export	Cannot share code between scripts	Inline with markers
P2	No external libraries	No lodash, date-fns, etc.	Write from scratch
P3	Map/Set for..of may fail	Runtime errors on iteration	Array.from().forEach or indexed loop
P4	120-second timeout	Large datasets may fail	Batch processing, minimize API calls
P5	console.log limited	No persistent logging	Use cell writes for debug output
P6	TypeScript subset	Some TS features unavailable	Test in Office Scripts editor

See: references/platform_limitations.md

Reference Guide

Load these files based on your current task:

Situation	Reference to Load
Writing ExcelScript API code (read/write cells, sheets)	references/excel_api_patterns.md
Debugging or reviewing existing code	references/common_bug_patterns.md
Setting up or running tests	references/testing_strategy.md
Understanding runtime constraints	references/platform_limitations.md
Before deploying / code review	assets/implementation_checklist.md

Quick Reference Table

Task	Pattern	Reference
Read sheet data	getUsedRange().getValues() + header index lookup	excel_api_patterns.md #1
Write results to sheet	getRangeByIndexes().setValues()	excel_api_patterns.md #2
Protect/unprotect sheet	unprotect() -> write -> protect()	excel_api_patterns.md #3
Parse CSV input	Dual-mode: single-cell text vs cell grid	excel_api_patterns.md #7
Section-based sheet parsing	currentSection + currentHeaders state machine	excel_api_patterns.md #6
Clear data without destroying headers	Pre-locate section positions, clear data rows only	excel_api_patterns.md #8
Convert Excel dates	excelDateToJsDate(serial)	excel_api_patterns.md #4
Sheet name with spaces in formulas	Quote: 'Sheet Name'!A1	excel_api_patterns.md #5
Iterate Map/Set	Array.from(map.entries()) + indexed for loop	platform_limitations.md P3
Rounding to match Python	roundHalfEven2 / roundUp2 / roundUp4 / roundDown4	excel_api_patterns.md #9
Test pure logic	lib/*.ts with Vitest	testing_strategy.md
Pre-deploy review	Implementation checklist	implementation_checklist.md

Workflow

1. Understand requirements — Identify what data to read/write, which sheets, and any automation triggers
2. Check platform constraints — Review P1-P6 constraints; confirm no external libraries needed
3. Design with lib/ extraction — Write pure logic in lib/*.ts for testability
4. Implement and test locally — Use Vitest to test lib/*.ts functions
5. Inline into main script — Copy tested functions into .ts script with // INLINED: markers
6. Test in Excel Online — Run the script in the Office Scripts editor
7. Review with checklist — Use assets/implementation_checklist.md before deployment

Resources

Type	Path	Purpose
Reference	references/platform_limitations.md	6 critical platform constraints
Reference	references/excel_api_patterns.md	ExcelScript API patterns and examples
Reference	references/common_bug_patterns.md	13 real-world bug patterns
Reference	references/testing_strategy.md	lib extraction + Vitest setup
Asset	assets/implementation_checklist.md	Pre-deployment review checklist

Output

This is an advisory/knowledge skill. Output is conversational guidance — no files are generated. The skill provides:

• Code patterns and examples for Office Scripts development
• Debugging assistance and bug pattern identification
• Architecture recommendations for testable code
• API usage guidance based on reference documentation