Function sum

sum<T>(arr: T[], fn: (item: T) => number): number
Calculates the sum of all elements in the array based on the provided function.
Type Parameters
- T
Parameters
- arr: T[]
  Array of items to be summed.
- fn: (item: T) => number
  Summing function
Returns number
Usage
```
const total = sum([1, 2, 3, 4, 5], (num) => num);

console.log(total); // 15
```
- Defined in array.ts:40