Design Patterns — the ones worth knowing

Patterns are vocabulary, not goals. Reach for one only when the problem it solves is actually present.

Strategy

Problem: an algorithm varies independently of its caller. Use when: multiple interchangeable behaviors selected at runtime (pricing rules, compression, sort orders). Overkill when: there is only one strategy. Modern alternative: a first-class function or lambda.

Factory Method

Problem: construction is non-trivial or must be swappable. Use when: the concrete type depends on input or config. Overkill when: new Thing() works and always will. Modern alternative: a constructor function plus DI at the composition root.