performance

name: performance description: Use when changing loops, collection processing, invalidation logic, tree/diff traversal, path scanning, virtualized rendering calculations, or any code where repeated scans or boolean control flow affect performance or correctness.

Avoid nested loops and O(n^2) operations unless there is a clear reason.

Calculate expensive values once before a loop, not inside it.
Prefer precomputed maps, sets, indexes, or a single backward scan over nested repeated scans.
If you need to know whether meaningful elements remain, compute that boundary once before the main loop.

Example of the preferred pattern:

let lastMeaningfulIndex = items.length - 1;
for (let i = items.length - 1; i >= 0; i--) {
  if (items[i].someCondition) {
    lastMeaningfulIndex = i;
    break;
  }
}

for (let i = 0; i <= lastMeaningfulIndex; i++) {
  const isLast = i === lastMeaningfulIndex;
  // ...
}

After changing boolean logic or invalidation paths, simplify the final control flow before calling the work done. If code is already inside if (foo), do not keep || foo in assignments inside that block.