TypeScript · performance

no-string-concat-in-loop

eslint:no-string-concat-in-loop

Disallow O(n^2) string building via `+=` on a string variable inside a loop; push parts to an array and `join` instead.

Why

Repeatedly rebuilding a growing string can copy all prior content on each iteration, making total work grow quadratically.

Collect each fragment in an array, then join the fragments after the loop.

Executed by this rule’s unit tests.

Before

src/render.ts · focus

let output = ''; for (const item of items) { output = `${output}${item}`; }

After

src/render.ts · focus

const parts = []; for (const item of items) { parts.push(item); } const output = parts.join("");

Only local identifiers initialized with a string or template literal and accumulated in a loop body are inspected.

noStringConcatInLoop