demo · v143

Counter & group-by patterns

Three patterns that get one line shorter and one bug-class smaller with Map.prototype.getOrInsert. Word counter, group-by-key, and dedup counter. Each shows the old "has + get + set" dance vs the new getOrInsertComputed call.

Pick a pattern

before — manual

after — getOrInsertComputed

Input

Output

Snippet

// v143
function countWords(text) {
  const m = new Map();
  for (const w of text.split(/\s+/)) {
    m.set(w, (m.getOrInsert(w, 0)) + 1);
  }
  return m;
}

// Or, with the Computed variant for expensive defaults:
function groupBy(items, keyFn) {
  const m = new Map();
  for (const it of items) {
    m.getOrInsertComputed(keyFn(it), () => []).push(it);
  }
  return m;
}