through2-reduce's Introduction

through2-reduce

through2-reduce is a thin wrapper around through2 that works like Array.prototype.reduce but for streams.

This is a much less common use-case with streams, but it can occasionally be useful to do a Reduce function on a stream.

EXPERIMENTAL This is a bit of a bizarre one, so I wouldn't be surprised if there are some dangerous edge cases around flushing and pausing and such. Use at your own risk.

This stream will only ever emit a single chunk. For more traditional stream.Transform filters or transforms, consider through2 through2-filter or through2-map.

Also, if your stream never ends, Reduce will never end.

var reduce = require("through2-reduce")

var sum = reduce({objectMode: true}, function (previous, current) { return previous + current })

// vs. with through2:
function combine (value, encoding, callback) {
  if (this.total == undefined) {
    this.total = value
    return callback()
  }
  this.total += value
  return callback()
}
function flush (callback) {
  this.push(this.total)
  return callback()
}
var sum = through2({objectMode: true}, combine, flush)

// Then use your reduce: (e.g. source is an objectMode stream of numbers)
source.pipe(sum).pipe(sink)

// Works like `Array.prototype.reduce` meaning you can specify a function that
// takes up to three* arguments: fn(previous, current, index) AND you can specify
// an initial value
var mean = reduce({objectMode: true}, function (prev, curr, index) {
  return prev - (prev - curr) / (index + 1)
}, 0)

*Differences from Array.prototype.reduce:

No fourth array callback argument. That would require realizing the entire stream, which is generally counter-productive to stream operations.
Array.prototype.reduce doesn't modify the source Array, which is somewhat nonsensical when applied to streams.