@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 — a helper's binding must be unshadowed and its every reference a direct call, or the call graph is unprovable and the rule reports nothing. Calls through .call() and .apply() are outside this rule because direct-invocation-only rejects those invocations; arr.concat.call(...) also resolves to Function.prototype.call, not Array.prototype.concat, so the resolved-signature check this rule uses never matches it regardless.

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

A per-element callback is a loop body

A callback passed to .forEach(), .map() or another per-element iteration method counts as a loop body here, so chunks.forEach((chunk) => { result = result.concat(chunk); }) is reported exactly as the equivalent for loop is. The iteration method is identified through its resolved signature rather than its callee name, so an aliased or computed method reference resolves the same way.

✗ 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();