I’m trying to change the shape of Item to meet a complex interface NewItem by calling transformData(). However I’m getting the following error.

The error message is “Element implicitly has an ‘any’ type because expression of type ‘string’ can’t be used to index type ‘{}’.” Here’s a simplified version of my code that still produces the error:

interface Item {
  itemName: ItemName;
  quantity: number;
  itemBreakdown ? : [];
}

type ItemName = 'Dog' | 'Plant' | 'Unknown'

interface NewItem {
  animal: number;
  plant: number;
  food: number
  itemBreakdown ? : {
    catergory1 ? : [ItemDetails]
    catergory2 ? : [ItemDetails]
  };
  error ? : Error | Error[]
}

private transformData(items: Item[]): NewItem {
  const newItem: NewItem = {};

  items.reduce((acc, item) => {
    const { itemName, quantity } = item;
    // Error thrown by line below
    acc[this.mapItemName(itemName)] = quantity;
    return acc;
  }, newItem);

  return newItem;
}

private mapItemName(itemName: string): string {
  switch (itemName) {
    case 'Dog':
      return 'animal';
    case 'Plant':
      return 'plant';
    default:
      return 'unknown';
  }
}

TS7053: Element implicitly has an any type because expression of type string can't be used to index type NewItem
No index signature with a parameter of type string was found on type NewItem

I think I understand the issue but I’m not sure whats the right way to solve it.
The issue is that I’m passing in a key of type string to {}: NewItem, where NewItem has no index signature of key[string] : number.

I can’t assign key[string] : number to NewItem Interface because what about the other values?
I don’t want to set type as any either or turn off strict mode.

Whats the correct way to solve this without losing type safety?