Combine Functions into Transform

Goal

Multiple derived values from the same source come from one transform that produces an enriched record.

Before the refactoring

function base(reading)    { /* ... */ }
function taxable(reading) { /* ... */ }

After the refactoring

function enrich(reading) {
  return { ...reading, base: base(reading), taxable: taxable(reading) };
}

Savings

Derivations stay consistent (no two callers compute slightly different versions); cache invalidation becomes obvious.

Note

Building a transform up-front when only one derivation exists is BDUF — wait for the second derivation before introducing the transform.