async-through

Readable stream that ensures that onend is only called once no ondata items are pending, thus supporting async operations inside ondata.

var asyncThru = require('async-through')

var from = require('from');
var stream = asyncThru(ondata, onend);

function ondata(data) {
  setTimeout(function () {
    stream.queue(data * 2 + '\n');
    // this === stream, so the below also works:
    // this.queue(data * 2 + '\n');
  }, 200 * data);
}

function onend() {
  // this === stream, so the below also works:
  // this.queue(null);
  stream.queue(null);
}

from([1, 3, 4, 5])
  .pipe(stream)
  .on('end', function () { console.log('stream ended'); })
  .pipe(process.stdout);

2
6
8
10
stream ended

onend is optional

var from = require('from');

// onend is optional
var stream = asyncThru(ondata);

function ondata(data) {
  setTimeout(function () {
    stream.queue(data * 2 + '\n');
  }, 200 * data);
}

from([1, 3, 4, 5])
  .pipe(stream)
  .on('end', function () { console.log('stream ended'); })
  .pipe(process.stdout);

Installation

npm install async-through

API

asyncThrough(ondata, onend)

/**
 * Returns a stream that works just like a normal through stream except that it keeps track of pending items that are
 * being processed inside the `ondata` function.
 * This allows the processing code to be async yet ensures that `onend` doesn't get called until no items are pending.
 * 
 * @name exports
 * @function
 * @param ondata_ {Function} function (data) { .. } to process data item and queue the result
 * @param onend_ {Function} (optional) function () { .. }
 * @return {Stream} async-through stream
 */

Limitations

At this point a one to one relationship of incoming items to outgoing items is assumed. Therefore for each incoming item you can only queue one processed item.

This limitation get fixed in future versions.

License

MIT