# A Design Patterns Checklist — Design Patterns

Source: https://www.skillbyai.com/en/design-patterns/a-check

> Questions to ask before reaching for a pattern.

## Before, during and after

Before: what exactly varies or is likely to change, and is that change real or speculative? Is there a language feature (functions, modules, generators, union types) that solves it more simply? During: does the pattern's name match its intent, so the next reader recognises it? Are dependencies explicit and injected rather than global? Is the abstraction small and focused? After: is the code easier to test and change than the simple version, and would you remove the pattern if the variation disappeared? Patterns are tools for managing change; measure them by how much change they make easier.

## The checklist

Use it in design reviews and pull requests.

```text
[ ] the problem is real: at least two concrete variants or a known upcoming change
[ ] a simpler language feature (function, module, options object, union) was considered
[ ] the pattern name matches its intent and is used in naming or comments
[ ] dependencies are injected at a composition root, not reached through globals
[ ] interfaces are small; no god objects or "manager" classes doing everything
[ ] behaviour depending on lifecycle is modelled as an explicit state machine
[ ] third-party APIs are wrapped behind adapters
[ ] cross-cutting concerns use decorators or middleware with a documented order
[ ] the design is easier to test (fakes, in-memory implementations)
[ ] you would remove the abstraction if the variation went away
```

## Refactor towards patterns

Most good pattern usage appears during refactoring, when the code shows you where variation lives. Write simple code first and let the pattern emerge.

**Quiz:** Which is the best reason to introduce a pattern?

- [ ] It makes the codebase look more professional
- [ ] The pattern appears in a well-known book
- [x] There is real, observed variation or change that the pattern makes easier to handle
- [ ] Every class should implement at least one pattern

*Answer:* There is real, observed variation or change that the pattern makes easier to handle. Patterns earn their place by easing change.