In the old days a simple for loop would be enough. It still is, but way more code then needed when working with arrays.

When working with reduce we can create a sum for every item. In the example we have a TimeEntry interface with multiple attributes. We only care about numberOfHours. For every entry the callback is called, the second parameter is the default value:

interface TimeEntry {
  approvedAt?: string;
  rejectedAt?: string;
  numberOfHours: number;
}

...

get numberOfUnhandledHours() {
  return this.timeEntries
    .reduce((sum: number, b: TimeEntry) => b.numberOfHours, 0);
}

The nice effect here is that we can chain other methods like filter methods to it:

get numberOfUnhandledHours() {
  return this.timeEntries
    .filter((timeEntry: TimeEntry) => !timeEntry.approvedAt && !timeEntry.rejectedAt)
    .reduce((sum: number, b: TimeEntry) => sum + b.numberOfHours, 0);
}

This way we can add more complexity without have one if after another within a nested loop.