[Home](./index.md) > [rc-js-util](./rc-js-util.md) > [mapConcat](./rc-js-util.mapconcat_1.md) ## mapConcat() function Used with maps that store arrays. Where an array exists for a given key the values will be appended to that array, otherwise a copy of values array will be inserted. **Signature:** ```typescript export declare function mapConcat(map: WeakMap, key: TKey, values: readonly TValue[]): void; ``` ## Parameters

Parameter	Type	Description
map	WeakMap<TKey, TValue\[\]>	The `Map` to check. May be modified.
key	TKey	The key to lookup in `map`.
values	readonly TValue\[\]	The values to concat.

**Returns:** void ## Remarks See [mapConcat()](./rc-js-util.mapconcat.md).