📝 Prefer passing iterables directly to constructors instead of filling empty collections.
💼 This rule is enabled in the following configs: ✅ recommended, ☑️ unopinionated.
🔧💡 This rule is automatically fixable by the --fix CLI option and manually fixable by editor suggestions.
Some built-in constructors accept the values they should contain. Creating an empty collection and immediately filling it with a for…of loop is unnecessary when the constructor can consume the iterable directly.
This rule reports simple adjacent loops that fill an empty Set, WeakSet, Map, WeakMap, or URLSearchParams. It also reports new URLSearchParams(Object.entries(record)).
When a function that can run during the loop references the collection, the loop replacement is a suggestion, because the function could run while the constructor reads the source, before the collection is initialized.
URLSearchParams is only reported for Object.entries(record) because new URLSearchParams(record) can consume string-keyed records directly. The rule skips obvious non-record sources like arrays, strings, nullish values, primitive numbers, and constructor calls, including in the initializer of a const variable. Generic pair iterables are intentionally not reported for URLSearchParams#set() because duplicate names can make the loop behavior differ from constructor behavior.
FormData is intentionally unsupported because its constructor accepts a form and optional submitter, not arbitrary entries.
Exotic custom iterator behavior, symbol-keyed records hidden behind variables, and shadowed built-ins are intentionally unsupported. The rule skips object literals with computed properties and Map array literals that are not array pairs, directly or in the initializer of a const variable, but it does not try to prove whether other variables contain custom iterables or symbol keys.
// ❌
const set = new Set();
for (const item of items) {
set.add(item);
}
// ✅
const set = new Set(items);// ❌
const map = new Map();
for (const [key, value] of entries) {
map.set(key, value);
}
// ✅
const map = new Map(entries);// ❌
const searchParameters = new URLSearchParams();
for (const [key, value] of Object.entries(record)) {
searchParameters.set(key, value);
}
// ✅
const searchParameters = new URLSearchParams(record);// ❌
const searchParameters = new URLSearchParams(Object.entries(record));
// ✅
const searchParameters = new URLSearchParams(record);// ✅ because generic pair iterables can behave differently with duplicate names
const searchParameters = new URLSearchParams();
for (const [key, value] of entries) {
searchParameters.set(key, value);
}