prefer-flat-map

Prefer flatMap over a map followed by a one-level flatten

Mapping an array and then flattening one level allocates an intermediate array of mapped values.
Instead, you SHOULD call flatMap to map and flatten in one operation.

- Category: performance
- Level: warning
- Fix: automatic
- Scope: module

Reported

function pairs(values: int32[]): int32[] {
    return values.map((value) => [value, value]).flat();
}
Accepted

function pairs(values: int32[]): int32[] {
    return values.flatMap((value) => [value, value]);
}
Prior art

- Clippy · mapflatten
- eslint-plugin-unicorn · prefer-array-flat-map

language/linter/src/rules/performance/preferflatmap.rs:8