Sum an array of numbers or objects

sum.ts

type KeysMatchingType<T, U> = NonNullable<{
    [K in keyof T]: [U] extends [T[K]] ? T[K] extends U ? K : never : never;
}[keyof T]>;

function sum(values: number[]): number;
function sum<T>(values: T[], prop: KeysMatchingType<T, number>): number;
function sum<T>(values: T[], prop?: KeysMatchingType<T, number>): number {
  if (values.length === 0) {
    return 0;
  }

  const numberArray = typeof values[0] === 'number' ? 
        (values as number[]) : 
  		values.map(obj => +obj[prop as keyof T]);

  return numberArray.reduce((acc, curr) => acc + curr, 0);
}

Got the KeysMatchingType from this StackOverflow thread.