The 4 Rules of Simple Design: A Practical Guide with TypeScript

Kent Beck introduced the 4 Rules of Simple Design as part of Extreme Programming in the late 1990s. Decades later, they remain one of the most elegant and practical frameworks for writing maintainable software. These rules define what "simple" truly means in code — not naive or dumbed-down, but intentionally minimal . The rules are ordered by priority. You never sacrifice a higher-priority rule to satisfy a lower one. Rule 1: Passes the Tests The code must work. It must do what it's supposed to do, and you must be able to prove it. A beautiful design that produces wrong results is worthless. This rule also implies that tests exist . Untested code is not "simple" — it's unknown. // ❌ No tests, no confidence function calculateDiscount ( price : number , tier : string ): number { if ( tier === " gold " ) return price * 0.8 ; if ( tier === " silver " ) return price * 0.9 ; return price ; } // ✅ Tested behavior — we know exactly what this does describe ( " calculateDiscount " , () => { it (