📝 Disallow unnecessary use of Array#flatMap().
💼🚫 This rule is enabled in the ✅ recommended config. This rule is disabled in the ☑️ unopinionated config.
🔧💡 This rule is automatically fixable by the --fix CLI option and manually fixable by editor suggestions.
Array#flatMap() is useful when one input item can become multiple output items. When a callback only returns [item] or condition ? [item] : [], .map(), .filter(), or .filter().map() is clearer.
Method replacements only check simple arrow callbacks.
The rule also removes [item] wrappers from inline callbacks, including conditional and block-body returns, when syntax, primitive TypeScript annotations, or optional type information establish that item is not an array. Async callbacks, generators, and callbacks with explicit return types are ignored.
In TypeScript files, including Vue SFC <script> blocks with lang="ts" or lang="tsx", conditional callbacks like value ? [value] : [] are not rewritten to .filter() or .filter().map() because that can lose TypeScript control-flow narrowing and change the inferred type. Wrapper removal is suggested because it can narrow the inferred element type. Direct one-item callbacks like value => [value.id] are still reported.
// ❌
const ids = array.flatMap(value => [value.id]);
// ✅
const ids = array.map(value => value.id);// ❌
const ids = array.filter(value => value.active).flatMap(value => [value.id]);
// ✅
const ids = array.filter(value => value.active).map(value => value.id);// ❌
const active = array.flatMap(value => value.active ? [value] : []);
// ✅
const active = array.filter(value => value.active);// ❌
const ids = array.flatMap(value => value.active ? [value.id] : []);
// ✅
const ids = array.filter(value => value.active).map(value => value.id);// ✅
const descendants = array.flatMap(value => value.children);// ✅
const values = array.flatMap(value => [value, value * 2]);// ❌
const values = array.flatMap(() => Math.random() ? [1] : [2, 3]);
// ✅
const values = array.flatMap(() => Math.random() ? 1 : [2, 3]);declare const value: string;
// ❌
array.flatMap(() => condition ? [value] : []);
// ✅
array.flatMap(() => condition ? value : []);