How to sum up object properties with reduce and dynamic keys?

Issue

Need to create a method a method that takes array of objects and keys then sums up by the passed keys, attempts so far have been unsuccessful( this is pseudo code):

const calculateTotals = <T extends object, K extends keyof T>(items: T[], ...keys : K[]) => {
  let result;
      if (keys.length)
      {
          //if I needed to sum up ALL the properties this would suffice, how to plug-in passed keys here?  
          result = items.reduce((r, item) => (Object.entries(item).forEach(([key, value]) => r[key] = (r[key] || 0) + value), r), {});
    
      }
      return result;
}

Is it possible to achieve this?

Solution

There's a few changes you can make to implement your function:

1. Use keys.forEach instead of Object.entries
2. Don't use reduce and modify the accumulator. forEach works best here.
3. Change the type to have either T or K generic, but not both. Typescript struggles to maintain the relationship between two type variables.

const calculateTotals = <K extends PropertyKey>(items: Record<K, number>[], ...keys: K[]) => {
  let totals: Partial<Record<K, number>> = {};
  items.forEach((r) =>
    keys.forEach((key) =>
      totals[key] = (totals[key] ?? 0) + r[key]))
  return totals;
}

Answered By - Etienne Laurin