.map returns collection while .forEach simply iterates over items. So whn you want to produce collection of different shape you should user .map. Alternatively if you want to modify existing collection you should user .forEach

The forEach() method is used to loop through each element of an array or object. The forEach() method takes a callback function as an argument. The callback function is invoked for each element of the array or object.
The map() method is used to loop through each element of an array or object. The map() method takes a callback function as an argument. The callback function is invoked for each element of the array or object.