String Concat In Loop (ARCH3070)

Key typescript.performance.string-concat-in-loop · Severity warning · Category performance

Accumulating a string with += inside a loop allocates a new string and copies the characters on every iteration, producing O(n^2) cost as input grows.

Incorrect

function join(items: string[]): string {
  let result = "";
  for (const item of items) {
    result += item + ",";
  }
  return result;
}

Correct

function join(items: string[]): string {
  const parts: string[] = [];
  for (const item of items) {
    parts.push(item);
  }
  return parts.join(",");
}

Fix

Collect the pieces in an array and join them once after the loop with join.