prefer-iterator-concat.md

prefer-iterator-concat

📝 Prefer Iterator.concat(…) over temporary spread arrays.

🚫 Disabled by default.

🔧💡 This rule is automatically fixable by the --fix CLI option and manually fixable by editor suggestions.

Prefer Iterator.concat(…) over temporary spread arrays.

Iterator.concat() creates one iterator from multiple iterables. In places that accept an iterable, this avoids materializing a temporary array.

Examples

// ❌
const set = new Set([...foo, ...bar]);

// ✅
const set = new Set(Iterator.concat(foo, bar));

// ❌
for (const item of [...foo, ...bar]);

// ✅
for (const item of Iterator.concat(foo, bar));

This rule only reports temporary arrays made entirely from spreads. Mixed arrays are intentionally ignored.

// ✅
new Set([first, ...rest]);

Iterator.concat() evaluates all its arguments first and then consumes the iterables lazily. The conversion is autofixed when the array is passed to new Map(…), new WeakMap(…), new Set(…), new WeakSet(…), a TypedArray constructor, Object.fromEntries(…), Array.from(…), or TypedArray.from(…) without a mapper function, since these only store the values, and no argument after the first has a side effect. Otherwise, it is offered as a suggestion, because replacing the spread array can change argument evaluation, mutation, callback timing, and error handling.

A spread of a string is ignored. Spreading a string works, but Iterator.concat() throws for a string primitive.

// ✅
const letters = new Set([...'abc', ...'xyz']);