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Wrapper for through stream to use async.queue under the hood.

It takes an asynchronous function and queue stream chunks. Then the queue executes the asynchronous function up to the concurrency threshold.

Any error will be emitted to opts.error_event event. By default, the stream will not stop on error; this can be configured via opts.stop_on_error.


  1. Install Node.js

  2. Run: npm install async-queue-stream


asyncqueue(write_fn [[, end_fn], options])


  • write_fn(data, callback) - an asnynchronous function that will be wrapped into a through stream

    data is the queued stream chunk.

    There are three ways to invoke callback:

    • callback(null, transformedData) - emit data
    • callback(error) - emit error
    • callback() - drop data (don't emit to next stream)
  • end_fn() - a function that will be invoked when no more data will be provided.

  • options - an object containing options

    • options.concurrency - concurrency argument to async.queue. Default: 1

    • options.error_event - event name used to emit the error from callback via the asynchronous function. Default: 'failure'

    • options.stop_on_error - boolean value for asyncqueue to stop queuing any more stream chunks if callback via the asynchronous function has emitted an error. Default: false

      Note: Any tasks already executed will be able to complete.


var es = require('event-stream');
var qasync = require('queue-async-stream');
// something that returns stream using queue-async-stream internally 
var plugin = function(filter, filter_func) {
    if(filter_func == void 0)
        filter_func = function(n) { return n; };
    return qasync(function (data, cb) {
        if(filter_func(data) == filter) {
            setTimeout(function() {
                return cb(new Error(filter+''));
            }, 1000);
        if(data === 2) {
            setTimeout(function() {
                return cb(null, data);
            }, 2000);
        console.log('caught in plugin: ' + data)
        return cb(null, data);
   }, {concurrency: 2});
        .on('failure', console.log)
    .pipe(es.through(function(n) {
        console.log('caught in es.through: ' + n);
        this.emit('data', n);
    caught in plugin: 1
    caught in es.through: 1
    [Error: 3]
    caught in plugin: 4
    caught in es.through: 4
    caught in plugin: 5
    caught in es.through: 5
    caught in es.through: 2
With stop_on_error:true, output is:
    caught in plugin: 1
    caught in es.through: 1
    [Error: 3]
    caught in es.through: 2

To Do

  1. Be able to pass opts/hooks to async.queue (e.g. drain, empty, etc)


MIT. See LICENSE file.