@studnicky/v8/array-concat-outside-loops

Disallows .concat() calls in assignment inside for loop bodies. Array.concat creates a new array on every iteration, producing O(n²) allocations. Accumulate with .push() or pre-allocate the result array instead.

Fixable: No · Options: No · Suggested severity: error

✗ Incorrect

let result: number[] = [];
for (let i = 0; i < items.length; i++) {
  result = result.concat(items[i] ?? []);
}

let merged: string[] = [];
for (const batch of batches) {
  merged = merged.concat(batch);
}

✓ Correct

const result: number[] = [];
for (let i = 0; i < items.length; i++) {
  const item = items[i];
  if (item !== undefined) { result.push(item); }
}

// Flat concatenation outside the loop
const merged = batches.flat();