Separate Query from Modifier

Removes smells

Symptom

A function that both returns a value AND mutates state — `findMiscreant(people)` that returns the miscreant AND alerts them.

Goal

Functions either return a value or mutate state, never both — callers can compose them without surprise.

Before the refactoring

function findMiscreant(people) {
  for (const p of people) {
    if (p.isMiscreant) { alert(p); return p; }
  }
}

After the refactoring

function findMiscreant(people) { return people.find(p => p.isMiscreant); }
function alertMiscreant(people) {
  const m = findMiscreant(people);
  if (m) alert(m);
}

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

Pressure

Callers can't query without triggering the side effect; tests must work around the dual contract; reasoning about side effects becomes non-local.

Tradeoff

If the modification and the query truly cannot be separated (e.g. find-and-remove on a queue), the constraint is fundamental — leave the combined operation but document it.

Relief

Reasoning about side effects is local; tests target each shape independently.

Trap

Splitting a function whose query and modification truly cannot be separated — find-and-remove on a queue, atomic compare-and-swap — fragments operations that need atomicity.