Given the object below, I'm looking for a clean and efficient way to create an array from each item's "subItems" array.

```
const items = [{
    'a': 'a1',
    'b': 'b1',
    'subItems': [
        {'c': 'c1'},
        {'d': 'd1'}
    ]
  },
  {
    'e': 'e1',
    'subItems': [
      {'f': 'f1'},
      {'g': 'g1'}
    ]
  }
];

```

So for this example, the result I would need is:

```
const groupedSubItems = [{c: c1},{d:d1},{f:f1},{g:g1}];

```

Currently I'm just looping through and concatting all the arrays but it does not feel like the best way to handle this:

```
let groupedSubItems = [];
items.forEach(function(item) {
  if (item.subItems) {
    groupedSubItems = groupedSubItems.concat(item.subItems);
  }
});

```

Note: All subitems arrays will only be one level down as shown, with the same key, and may or may not exist.