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

Disallows Array.prototype.concat and ReadonlyArray.prototype.concat when the call executes once per iteration. With 200-element chunks, repeatedly assigning result = result.concat(chunk) takes 150.3 ms, while result.push(...chunk) takes 20.6 ms: 7.3× faster. The rule resolves the called signature, so computed and const-aliased access to the built-in is included while an unrelated method named concat is not.

It also reports a named function or a function held in a const when every provable call site is per-iteration. Calls through .call() and .apply() are outside this rule because direct-invocation-only rejects those invocations.

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

✗ Incorrect

let result: string[] = [];
for (const chunk of chunks) {
  result = result.concat(chunk);
}

let result: string[] = [];
const append = (chunk: readonly string[]): void => {
  result = result.concat(chunk);
};
for (const chunk of chunks) {
  append(chunk);
}

✓ Correct

const result: string[] = [];
for (const chunk of chunks) {
  result.push(...chunk);
}

const merged = chunks.flat();