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A through2 wrapper to emulate Array.prototype.reduce for streams.



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 ( == undefined) { = value
    return callback()
  } += value
  return callback()
function flush (callback) {
  return callback()
var sum = through2({objectMode: true}, combine, flush)
// Then use your reduce: (e.g. source is an objectMode stream of numbers) 
// 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.


reduce([options,] fn [,initial])

Create a Reduce instance

reduce.ctor([options,] fn [,initial])

Create a Reduce class

reduce.obj([options,] fn [,initial])

Create a Reduce instance that defaults to objectMode: true.

reduce.objCtor([options,] fn [,initial])

Just like ctor, but with objectMode: true defaulting to true.


  • wantStrings: Automatically call chunk.toString() for the super lazy.
  • all other through2 options