core.countby | Medplum
countBy() function
Counts iterable elements by a key function, returning a sparse map of key to count. Matches Lodash countBy semantics (no zero-fill for missing keys).
Signature:
export declare function countBy<T, K extends keyof any>(items: Iterable<T>, keyFn: (value: T) => K): Partial<Record<K, number>>;
Parameters
| Parameter | Type | Description |
|---|---|---|
| items | Iterable |
The items to count. |
| keyFn | (value: T) => K | The function to extract the key from each item. |
Returns:
Partial<Record<K, number>>
A map of key to count for keys that appear at least once.