When to extract a function
September 8, 2026 · View on GitHub
- Reuse. If the same thing is needed in more than one place, give it its own function.
- Testability. If it needs a dedicated unit test, give it its own function.
- Don't overdo the test rule. Needing a test doesn't always mean needing a new function; it's often fine for one test to cover several things together.
- A function is a whole. The unit is "some stuff that combines into something meaningful," not an arbitrary slice of steps.
- Don't rename native code. Wrapping a single built-in call (e.g. a
sessionStoragecall) in a function just to give it a name isn't a reason on its own — it needs one of the reasons above. - No mixed responsibilities. A function tackling more than one responsibility is creating side effects; splitting it is how you remove them.
- One line is fine if it removes a side effect. Length is never the objection; mixed responsibility is.