Split Loop

Removes smells

Symptom

A single loop iterating one collection but doing multiple unrelated jobs — accumulating a sum, finding a minimum, building a derived list, all interwoven in the loop body.

Goal

Each loop does one thing; mixed-purpose loops separate into named single-purpose passes.

Before the refactoring

let totalSalary = 0;
let youngest = Infinity;
for (const p of people) {
  if (p.age < youngest) youngest = p.age;
  totalSalary += p.salary;
}

After the refactoring

const totalSalary = people.reduce((s, p) => s + p.salary, 0);
const youngest    = Math.min(...people.map(p => p.age));

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

Pressure

Each line of the body could touch any of the loop's concerns; reasoning about one purpose requires holding all of them; bugs hide where concerns share an iteration variable.

Tradeoff

Two loops over the same collection are slower than one — only split when the doubled cost is dwarfed by the readability gain (which it usually is).

Relief

Each loop can then be replaced by a pipeline or extracted by name; bugs concentrate in one purpose at a time.

Trap

Splitting a loop whose concerns genuinely share state — running total AND previous value, where each iteration depends on the previous — fragments coupled logic into pieces that have to re-derive shared state.