27. How to choose a flattening operator

Consolidation: the decision map

Now that you've seen all four flattening operators in action, the important skill is choosing the right one from the business rule. This lesson is a recognition exercise.

Cheat sheet

If you need to...	Operator
run tasks in parallel, order doesn't matter	`mergeMap`
preserve the order, queue them up	`concatMap`
cancel the stale one, take the latest	`switchMap`
ignore the new while busy	`exhaustMap`

The choosing principle

There's no "best" operator. There's the operator that exactly describes the behavior the business needs. Always answer one question: "what should happen if a new value arrives while the old one is still running?"

Your task

Inside the function in map, build a mapping object: parallel → mergeMap, queue → concatMap, latest → switchMap, busy → exhaustMap.
Return the string item.name + ': ' + operatorByStrategy[item.strategy].
Expected output: parallel uploads: mergeMap → ordered saves: concatMap → typeahead search: switchMap → login button: exhaustMap.

Solution spoiler · click to reveal

const { from, map } = Rx;

const scenarios = [
  { name: 'parallel uploads', strategy: 'parallel' },
  { name: 'ordered saves', strategy: 'queue' },
  { name: 'typeahead search', strategy: 'latest' },
  { name: 'login button', strategy: 'busy' },
];

const result$ = from(scenarios).pipe(
  map(item => {
    const operatorByStrategy = {
      parallel: 'mergeMap',
      queue: 'concatMap',
      latest: 'switchMap',
      busy: 'exhaustMap',
    };

    return item.name + ': ' + operatorByStrategy[item.strategy];
  })
);

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

script.ts // TypeScript

CONSOLE · Console output

Hit Run to see the result...