# flatMapFirst | selectSwitchFirst

Transform the items emitted by an Observable into Observables, and mirror those items emitted by the most-recently transformed Observable.

The `flatMapFirst` operator is similar to the `flatMap` and `concatMap` methods described above, however, rather than emitting all of the items emitted by all of the Observables that the operator generates by transforming items from the source `Observable`, `flatMapFirst` instead propagates the first `Observable` exclusively until it completes before it begins subscribes to the next `Observable`. Observables that come before the current `Observable` completes will be dropped and will not propagate.

## Arguments

1. `selector` *(`Function`)*:  A transform function to apply to each source element.  The callback has the following information:
   1. the value of the element
   2. the index of the element
   3. the Observable object being subscribed
2. `[thisArg]` *(`Any`)*: Object to use as `this` when executing the predicate.

## Returns

*(`Observable`)*: An Observable sequence that is the result of concatenating non-overlapping items emitted by an Observable of Observables.

## Example

```javascript
//Generate an event every 100 milliseconds
var source = Rx.Observable.generateWithRelativeTime(
   0,
   function(x) {return x < 5; },
   function(x) {return x + 1; },
   function(x) {return x; },
   function(x) {return 100; })
  .flatMapFirst(function(value) {
    //Observable takes 150 milliseconds to complete
    return Rx.Observable.timer(150).map(value);
  });

var subscription = source.subscribe(
  function (x) {
    console.log('Next: %d', x);
  },
  function (err) {
    console.log('Error: %s', err);
  },
  function () {
    console.log('Completed');
  });

// Next: 0
// Next: 2
// Next: 4
// Completed
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://soufatn.gitbook.io/rxjs-book/summary/observable/observable_instance_methods/flatmapfirst.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.