angular - RXJS Repeat query until a condition is met?

Question

I'm using an API that returns only a limited number of results, say 100.

I'd like to repeat a query until the return result set is < 100 which would mean I've got the last result.

So it would go like this:

1. Make query
2. Is result set less than limit? If so, do again and append results.
3. Once result set is less than limit, emit the final result.

Answer 1

You can use the expand operator for a simple "conditional repeat" behavior.

Just for the example, instead of a result set I changed the query to return a number. The following keep querying until the retrieved number is less than 100

const { defer, empty } = rxjs;
const { expand, toArray} = rxjs.operators;

const query$ = defer(async () =>  Math.floor(Math.random()*1000));
   
query$
  .pipe(
    expand(result => result < 100 ? empty() : query$),
    toArray()
  )
  .subscribe(console.log);

<script src="https://unpkg.com/rxjs/bundles/rxjs.umd.min.js"></script>