rxjs-concat-join

The rxjs-concat-join package provides the single RxJS utility operator concatJoin, which is used to simplify the process of issuing a series of sequential requests and collecting together the results.

It is similar to forkJoin but the requests are issued sequentially, and the results of earlier requests can be used in later requests.

Installation

ES6

npm install rxjs-concat-join

import { concatJoin } from "rxjs-concat-join";

CommonJS

npm install rxjs-concat-join

const { concatJoin } = require('rxjs-concat-join');

Usage

concatJoin can be viewed as a combination of concat, forkJoin and pipe.

Like concat it consists of a list of observables that are subscribed to in turn, e.g.:

concatJoin( of(1), of(2) );

Like forkJoin the result is an observable of an array of the last value of each inner observable:

concatJoin( of(1), of(2) ).subscribe(console.log);
// outputs: [1, 2]

A key feature of concatJoin is that the array of results is built up request-by-request, so that the results of earlier requests can be use in later requests using a factory function, e.g.:

concatJoin(
  of(1), 
  of(2), 
  ([,result2])=>of(result2+1)
).subscribe(console.log);
// outputs: [1, 2, 3] 

It is also possible to use the object notation available in forkJoin. Note however that properties of objects are not ordered, so instead we input a list of objects, while the outputs are merged into a single object:

concatJoin(
  {result1: of(1)}, 
  {result2: of(2)}, 
).subscribe(console.log);
// outputs: {result1: 1, result2: 2} 

And making use of the interim accumulated value:

concatJoin(
  {result1: of(1)}, 
  {result2: of(2)}, 
  {result3: ({result2})=>of(result2+1)}
).subscribe(console.log);
// outputs: {result1: 1, result2: 2, result3: 3} 

Finally, when using the object notation, each object is in fact passed to forkJoin. In this way a combination of parallel and sequential requests can be constructed:

concatJoin(
  {result1: of(1), result2: of(2)}, 
  {result3: ({result2})=>of(result2+1)}
).subscribe(console.log);
// outputs: {result1: 1, result2: 2, result3: 3} 
// Requests 1 & 2 are issued at the same time.  Request 3 is issued when both 1 & 2 have completed. 

Credits

rxjs-concat-join was developed by Untapped Solution Ltd