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

Disallows a spread element in an array literal that is assigned to an identifier or property, or initializes a variable, when that expression executes once per iteration. Repeatedly rebuilding an array with [...result, item] allocates and copies the growing prefix on every iteration, producing quadratic work. A built-in per-element iteration callback is treated as a loop body.

The rule targets only bound array literals. Spread passed as a call argument, such as result.push(...items), is a related but distinct O(n) pattern — spreading into an argument list rather than into an array literal bound to a variable or property — and is intentionally out of scope; nested or unbound literals are likewise excluded.

Per-iteration detection resolves through LoopContext, not a function-boundary walk: a spread inside a .forEach()/.map() callback allocates a fresh array once per element, identically to one written with a loop keyword, so the callback counts as a loop body. CallIdentity is not used here, unlike sibling rules that target a method call — a SpreadElement inside an ArrayExpression is pure syntax, not a resolved call, so there is no callee signature to resolve identity from.

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

✗ Incorrect

let result: string[] = [];
for (const item of items) {
  result = [...result, item];
}

let result: string[] = [];
items.forEach((item) => {
  result = [...result, item];
});

✓ Correct

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

const merged = [...first, ...second];