Injecting runtime behavior into existing code without modifying source

I’ve been experimenting with a pattern for injecting behavior into existing code at runtime without modifying the original source. The approach separates execution into two layers: • Layer 1: original code (unchanged, byte-identical) • Layer 2: runtime wrappers that add behavior Implementation Each function is wrapped dynamically at runtime: function wrap(targetFn) { return (...args) => { // pre-execution behavior const result = targetFn(...args) // post-execution behavior return result } } Wrappers are composed based on behavior type: • interception → input validation / blocking • state → capture and persist outputs • execution → retry / failure handling • analysis → execution timing + anomaly detection ⸻ Key property The original file remains completely unchanged. All behavior is applied externally at execution time. ⸻ Tradeoffs This pattern behaves similarly to middleware or AOP, but with some differences: • operates post-build instead of at integration points • does not require mod