Substrate

@studnicky/v8/chained-array-iteration ​

Disallows two or more built-in every, filter, find, flatMap, forEach, map, reduce, or some passes along one Array or ReadonlyArray call chain, including when non-iterating calls appear between them. It also detects an adjacent const temporary that holds one such call and has exactly one use as the receiver of the next. The rule resolves signatures, so same-named methods on unrelated fluent APIs are not reported.

For a 5,000,000-element array in Node v24, map(...).filter(...) takes 54.09 ms while a single reduce takes 26.76 ms: 2.02× slower, or 102% more time. Combine transformation and selection into one pass where the chain represents repeated traversal. An interposed call that meaningfully shrinks the receiver, such as .filter(...).slice(0, 10).map(...), is still reported even though the cost argument is weaker there; the reduce() remedy still applies.

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

✗ Incorrect ​

ts
const names = users.filter((user) => user.active).map((user) => user.name);
ts
const activeUsers = users.filter((user) => user.active);
const names = activeUsers.map((user) => user.name);

✓ Correct ​

ts
const names = users.reduce<string[]>((result, user) => {
  if (user.active) {
    result.push(user.name);
  }
  return result;
}, []);
ts
const visible = users.reduce<typeof users>((result, user) => {
  if (user.visible) {
    result.push(user);
  }
  return result;
}, []);