How to create GTM variable from dataLayer array?

If you have datalayer like that:

<script>
dataLayer.push({
  'ecommerce': {
     "transactionProducts":     [
      {
        "id": "5",
        "sku": "black-handbag",
        "price": 170,
        "priceexcludingtax": "0.00",
        "tax": "0.00",
        "taxrate": 0,
        "type": "bundle",
        "category": "",
        "quantity": 1
      },
      {
        "id": "3",
        "sku": "red-handbag",
        "price": 120,
        "priceexcludingtax": "0.00",
        "tax": "0.00",
        "taxrate": 0,
        "type": "bundle",
        "category": "",
        "quantity": 1
      }
    ]
  }
});
</script>

And you want to create variable which will return array of IDs ['5', '3']

Then you need to create two variables:

1) Name: transactionProducts

Type: Data Layer Variable

Data Layer Variable Name: ecommerce.transactionProducts

2) Name: transactionProductsIds

Type: Custom JavaScript

Custom JavaScript: function () { return {{transactionProducts}}.map(function(a) {return a.id;}); }

And then you can use your second variable transactionProductsIds to receive IDs