Combine Functions into Transform

Removes smells

Symptom

Multiple consumers compute the same derived values from the same source data, each independently re-deriving the result.

Goal

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

Before the refactoring

function baseCharge(reading)    { return reading.kwh * reading.tariff.baseRate; }
function taxableCharge(reading) { return reading.kwh * reading.tariff.taxRate; }
// every consumer recomputes:
const monthly  = baseCharge(reading) + taxableCharge(reading);
const discount = baseCharge(reading) * 0.95;

After the refactoring

function enrich(reading) {
  return {
    ...reading,
    baseCharge:    reading.kwh * reading.tariff.baseRate,
    taxableCharge: reading.kwh * reading.tariff.taxRate,
  };
}
const r = enrich(reading);
const monthly  = r.baseCharge + r.taxableCharge;
const discount = r.baseCharge * 0.95;

Example source: Illustrative example written for this site, not a quotation from any source.

Pressure

Derivations drift between consumers as they age; bugs hide where two consumers compute slightly different versions; performance suffers from redundant computation.

Tradeoff

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

Relief

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

Trap

Building a transform up-front when only one consumer derives anything — BDUF abstraction that adds an intermediate type without an active second user.