Real-life: groupBy() totals by account

Group payments by account and sum each group.

Solve the task in the editor — no long theory.
Run the check and compare your output with the expected result.
Stuck? Open the solution and carry the approach into your own code.

Solution spoiler · click to reveal

const { from, groupBy, mergeMap, reduce, map } = Rx;

const payments = [
  { account: 'A', amount: 10 },
  { account: 'B', amount: 5 },
  { account: 'A', amount: 7 },
];

const result$ = from(payments).pipe(
  groupBy(payment => payment.account),
  mergeMap(group$ => group$.pipe(
    reduce((sum, payment) => sum + payment.amount, 0),
    map(sum => group$.key + ':' + sum)
  ))
);

result$.subscribe(value => console.log(value));

script.ts // TypeScript

CONSOLE · Console output

Hit Run to see the result...