RxJS FlexMap Operator

by David Björkevik

An RxJS operator using a single callback function that can cover the functionality of the map and filter operators as well as a few other cases.

To control behavior a handler function is passed to the operator. This handler function will be called once for each incoming value. It will be passed two parameters, the first being the incoming value and the second is a "yield function" that is used to pass values downstream. The yield function may be called an arbitrary number of times or not at all, and even after the handler function returns.

Any exception thrown by the handler function will be passed on as an error down the pipe. Also, if the handler returns a promise, in case of rejection of this promise, the error state is passed down the pipe. This allows for async handler functions.

Also, if the upstream completes, any outstanding promise must be resolved before the completion event is continued downstream.

Any error state from upstream is passed along by this operator

Installation

npm install rxjs-flexmap

Example usage

Map-like behavior

import { of } from "rxjs";
import { flexMap } from "flexmap";

// ...

of(1, 2, 3, 4)
  .pipe(
    flexMap<number, number>((v: number, yld) => yld(v * 2))
  )
  .subscribe(v => console.log(v));

The above code will output

2
4
6
8

Filter-like behavior

import { of } from "rxjs";
import { flexMap } from "rxjs-flexmap";

// ...

of(1, 2, 3, 4)
  .pipe(
    flexMap<number, number>((v: number, yld) => v % 2 && yld(v))
  )
  .subscribe(v => console.log(v));

The above code will output

1
3

Duplicate every value

import { of } from "rxjs";
import { flexMap } from "rxjs-flexmap";

// ...

of(1, 2, 3)
  .pipe(
    flexMap<number, number>((v, yld) => { yld(v); yld(v); })
  )
  .subscribe(v => console.log(v));

The above code will output

1
1
2
2
3
3