Developer shares four-layer method to unit-test Google Apps Script with Vitest

A developer on DEV Community has outlined a practical approach to unit-testing Google Apps Script, a platform that lacks a built-in test runner and whose global services like SpreadsheetApp are unavailable in Node.js environments. The method separates code into layers based on whether each part depends on Google services, allowing pure logic functions to be tested directly with Vitest without any mocks. For functions that must interact with Google services, the approach recommends injecting those services as parameters so fake versions can be substituted during testing. The strategy was motivated by a real-world tax rounding error that went undetected for two weeks because the project had no tests. The author also provides working code examples and notes that pure logic typically accounts for 50–60% of a project's codebase, making it the highest-priority layer to test first.