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Transducers for JavaScript.

Transducers are composable algorithmic transformations. They are independent from the context of their input and output sources and specify only the essence of the transformation in terms of an individual element. Because transducers are decoupled from input or output sources, they can be used in many different processes - collections, streams, channels, observables, etc. Transducers compose directly, without awareness of input or creation of intermediate aggregates.

If you are not familiar with transducers, check out Transducers Explained.

Install and Usage

$ npm install transduce


$ bower install transduce

Structured to allow creation of custom builds by loading only desired libs. For example, the base build contains only top-level core and transducers:

Explicit Require and Custom Bundles

Collected as a convenience for an aggregated API. Any function or transducer below can be bundled separately in browserify builds by requiring with path from transduce.

What does this mean? You can require the whole thing:

var tr = require('transduce')
tr.into([], [1,2,3,4,5,6])
// [1,2,3,4,5,6]
tr.into('', [1,2,3,4,5,6])
// '123456'
tr.into([], tr.filter(isEven), [1,2,3,4,5,6])
// [2,4,6]
tr.into([],, [[1,2],[3,4],[5,6]])
// [1,2,3,4,5,6])
tr.into({}, [[1,2],[3,4],[5,6]])
// {1:2,3:4,5:6}
var transducer = tr.compose(, tr.array.unshift(0),
tr.into([], transducer, [[1,2],[3,4],[5,6]])
// [1,2,3,4,5,6,7]

If you want to be reduce bundle size (or just like to be explicit), require packages with path from transduce.

  var core = require('transduce/core'),
      transducers = require('transduce/transducers'),
      array = require('transduce/array')
  var transducer = core.compose(, array.unshift(0),
  base.into([], transducer, [[1,2],[3,4],[5,6]])
  // [1,2,3,4,5,6,7]


Input Source

A source of values, normally a collection, coll. This library supports arrays, plain objects, strings, and anything that can be converted to iterators (see iterator function below).

Reducing Function

A two arity function, rf, appropriate for passing to reduce. The first argument is the accumulator and the second argument is the iteration value. When using transducers, the accumulator is normally a collection, but it is not required.

Initial Value

The initial accumulator value, init to use with Reduce.


An object that provides a reducing function, @@transducer/step, initial value function, @@transducer/init, and result extraction function, @@transducer/result. Combines the steps of reduce into a single object.


A function that folds over an input source to produce an Output Source. Accepts a Reducing Function or Transformer, xf, as the first argument, an optional initial value, init, as the second argument and an Input Source, coll as the third argument.

Also known as foldLeft or foldl in other languages and libraries.

The function begins with calling the Reducing Function of xf, @@transducer/step, with the initial accumulator, init, and the first item of Input Source, coll. The return value from the reducing function is used as the next accumulator with the next item in coll. The process repeats until either coll is exhausted or xf indicates early termination with reduced. Finally, the result extraction function of xf, @@transducer/result, is called with the final accumulator to perform potentially delayed actions and optionally convert the accumulator to the Output Source.

Reduce defines a Transducible Process.


A function, t, that accepts a transformer, xf, and returns a transformer. All transformations are defined in terms of transducers, independent of the Transducible Process. Can be composed directly to create new transducers.

Transducible Process

A process that begins with an initial value accumulator, steps through items of an input source and optionally transforming with transducer, t, and optionally completes with a result. Transduce is one transducible process. Transducible Processes can also be push based. See transduce-stream for an example. The same transducer can be used with any transducible process.


Supports the following functions:

// core
into: function(init, t?, coll?)
reduce: function(xf, init?, coll)
transduce: function(t, xf, init?, coll)
eduction: function(t, coll)
sequence: function(t, value)
compose: function(/*fns*/)
isReduced: function(value)
reduced: function(value, force?)
unreduced: function(value)
completing: function(rf, result?)
transformer: function(value)
iterable: function(value)
iterator: function(value)
transducer: function(step?, result?, init?)
protocols: {
  iterator: Symbol.iterator || '@@iterator'
  transducer: {
    init: '@@transducer/init',
    step: '@@transducer/step',
    result: '@@transducer/result',
    reduce: '@@transducer/reduce',
    reduced: '@@transducer/reduced',
    value: '@@transducer/value'
// transducers
map: function(f)
filter: function(predicate)
remove: function(predicate)
take: function(n)
takeWhile: function(predicate)
drop: function(n)
dropWhile: function(predicate)
cat: transducer
mapcat: function(f)
partitionAll: function(n)
partitionBy: function(f)
dedupe: function()
unique: function(f?)
tap: function(interceptor)
array {
  forEach: function(callback)
  find: function(predicate)
  push: function(/*args*/)
  unshift: function(/*args*/)
  every: function(predicate)
  some: function(predicate)
  contains: function(target)
  slice: function(begin?, end?)
  initial: function(n?)
  last: function(n?)
math {
  min: function(f?)
  max: function(f?)
string {
  split: function(separator, limit?)
  join: function(separator)
  nonEmpty: function()
  lines: function(limit?)
  chars: function(limit?)
  words: function(delimiter?, limit?)


Core functionality mixed into transduce directly or available by explictly requiring from transduce/core. The following are equivalent:

  • require('transduce').into
  • require('transduce/core').into
into(init, t?, coll?)

Returns a new collection appending all items into init by passing all items from source collection coll through the optional transducer t. Chooses transformer, xf from type of init. Can be array, object, string or have @@transformer. coll is converted to an iterator.

The function is automatically curried. If coll is not provided, returns a curried function using transformer from init and the same transformation can be used for multiple collections.

var tr = require('transduce')
// init, t and coll provided
tr.into([], tr.filter(isEven), [1,2,3,4,5,6]) // [2,4,6]
// init and coll, no t
tr.into([], [1,2,3,4,5,6]) // [1,2,3,4,5,6]
tr.into('hi ', [1,2,3,4,5,6]) // 'hi 123456'
// Curry on init
var toArray = tr.into([])
toArray([1,2,3]) // [1,2,3]
toArray(, [1,2,3]) // [2,3,4]
// Curry on init and t
var add1 =  into([],
var add1 =  toArray(
add1([1,2,3]) // [2,3,4])
// Iterator coll
toArray(range(1,4)) // [1,2,3])
var add2 = toArray(
add2(range(3)) // [2,3,4])
// Object coll
var toObject = tr.into({})
toObject([['a', 'b'], ['b', 'c']]) // {a: 'b', b: 'c'})
var part2 = toObject(tr.partitionAll(2))
part2(['a', 'b', 'b', 'c']) // {a: 'b', b: 'c'})
reduce(xf, init?, coll)

Reduces over a transformation. If xf is not a transformer, it is converted to one using completing. Arrays are special cased to reduce using for loop and to allow transducers using reduced. If coll has a '@@transducer/reduce' method, it is called with xf.step and init. Otherwise,coll is converted to an iterator. If the function is called with arity-2, the xf.init() is used as the init value.

transduce(t, xf, init?, coll)

Transduces over a transformation. The transducer t is initialized with xf and is passed to reduce. xf is converted to a transformer if it is not one already using completing. If the function is called with arity-3, the xf.init() is used as the init value.

eduction(t, coll)

Creates an iterable and reducible application of the collection coll transformed by transducert. The returned eduction will be iterable using sequence and have a ['@@transducer/reduce'](rf, init) method using transduce.

sequence(t, value)

Create an ES6 Iterable by transforming an input source using transducer t.


Simple function composition of arguments. Useful for composing (combining) transducers.


Is the value reduced? (signal for early termination)

reduced(value, force?)

Ensures the value is reduced (useful for early termination). If force is not provided or false, only wraps with Reduced value if not already isReduced. If force is true, always wraps value with Reduced value.


Ensure the value is not reduced (unwraps reduced values if necessary)

completing(rf, result?)

Lifts a reducing function, rf, into a transformer, xf. Uses identity if result function is not provided. The init function calls rf with no arguments.


Attempts to convert the parameter into a transformer. If cannot be converted, returns undefined. If defined, the return value will have init, step, result functions that can be used for transformation. Converts arrays, strings, objects, functions (completing) or anything that follows the transformer protocol into a transformer.

Objects support pairs or objects. If item is an array of length 2, uses first (0 index) as the key and the second (1 index) as the value. Otherwise iterates over own properties of items and merges values with same keys into the result object.

If value is undefined, returns a transformer that maintains the last value and does not buffer results. Ignores the accumulator and returns the input on every step. The init value will be undefined.


Returns the iterable for the parameter, returning if has an iterator protocol or wrapping in proper iterable if has a next method. Returns undefined if cannot create an iterator.

Converts arrays to iterables over each indexed item. Converts to functions to infinite iterables that always call function on next. Converts objects to iterables of key, value pairs.

The iterable is an object that has a function identified by protocols.iterator that should be called with no args and returns an iterator object that has a next method to cycle through the iterator.


Returns an iterator for the value, shorthand for calling iterable(value)[protocols.iterator]()


Returns true if a value fulfills the iterator protocol, also returning true for Array and String as these types both implement the iterator protocol in ES6.


Returns true if a value is an iterator, defined an object with a next function.


Symbol (or a string that acts as symbols) for @@iterator you can use to configure your custom objects.


String that acts as symbols for supporting the transducer protocol. Used to define transformers and reduced values.

transducer(step?, result?, init?)

Creates a transducer from a reducing function, step, result extraction function result and initial value function init. If any function is null or undefined, default is to forward directly to wrapped transformer.

The @@transducer/step function of the resulting transformer calls step(xfStep, result, input) bound to a transducer instance, where xfStep calls step on the wrapped transformer. The init and result functions behave similarly.

All functions are called with a bound to a transducer instance (this parameter) that contains init, step and result functions that forward to wrapped transformer. The context can be used to implement stateful transducers.

// Map from a step function
function map(callback) {
  return transducer(function(xfStep, value, input) {
    return xfStep(value, callback(input))
// using reduced
function takeWhile(p){
  return transducer(function(xfStep, value, input){
    return p(input) ? xfStep(value, input) : reduced(value)
// using context for stateful transducers
function drop(n){
  return transducer(function(xfStep, value, item){
    if(this.n === void 0) this.n = n
    return (--this.n < 0) ? xfStep(value, item) : value
// using custom result
function some(predicate) {
  return transducer(
    function(xfStep, value, input){
        this.found = true
        return reduced(xfStep(value, true))
      return value
    function(xfResult, value){
        value = this.xfStep(value, false)
      return xfResult(value)


Common transducers mixed into transduce directly or available by explictly requiring from transduce/transducers. The following are equivalent:

  • require('transduce').map
  • require('transduce/transducers).map

Transducer that steps all items after applying a mapping function f to each item.


Transducer that steps items which pass predicate test.


Transducer that removes all items that pass predicate.


Transducer that steps first n items and then terminates with reduced.


Transducer that take items until predicate returns true. Terminates with reduce when predicate returns true.


Transducer that drops first n items and steps remaining untouched.


Transducer that drops items until predicate returns true and steps remaining untouched.


Concatenating transducer. Reducing over every item in the transformation using provided transformer.


Transducer that applies a mappingFunction to each item, then concatenates the result of the mapping function. Same is compose(map(mappingFunction), cat)


Partitions the source into arrays of size n. When transformer completes, the transformer will be stepped with any remaining items. If n is not provided or size 0 it will collect all values and step with this value.


Partitions the source into sub arrays when the value of the function f changes equality. When transformer completes, the transformer will be stepped with any remaining items.


Removes consecutive duplicates from the transformation. Subsequent stepped values are checked for equality using ===.


Produce a duplicate-free version of the transformation. If f is passed, it will be called with each item and the return value for uniqueness check. Uniqueness is checked across all values already seen, and as such, the items (or computed checks) are buffered.


Transducer that invokes interceptor with each result and input, and then passes through input. The primary purpose of this method is to "tap into" a method chain, in order to perform operations on intermediate results within the chain. Executes interceptor with current result and input.


Transducer that places separator between each stepped value.


Use Array methods as Transducers. Treats each stepped item as an item in the array, and defines transducers that step items with the same contract as array methods.

Mixed into transduce.array or available by explictly requiring from transduce/array. The following are equivalent:

  • require('transduce').array.forEach
  • require('transduce/array').forEach

Passes every item through unchanged, but after executing callback(item, idx). Can be useful for "tapping into" composed transducer pipelines. The return value of the callback is ignored, item is passed unchanged.


Like filter, but terminates transducer pipeline with the result of the first item that passes the predicate test. Will always step either 0 (if not found) or 1 (if found) values.


Passes all items straight through until the result is requested. Once completed, steps every argument through the pipeline, before returning the result. This effectively pushes values on the end of the stream.


Before stepping the first item, steps all arguments through the pipeline, then passes every item through unchanged. This effectively unshifts values onto the beginning of the stream.


Checks to see if every item passes the predicate test. Steps a single item true or false. Early termination on false.


Checks to see if some item passes the predicate test. Steps a single item true or false. Early termination on true.


Does the stream contain the target value (target === item)? Steps a single item true or false. Early termination on true.

array.slice(begin?, end?)

Like array slice, but with transducers. Steps items between begin (inclusive) and end (exclusive). If either index is negative, indexes from end of transformation. If end is undefined, steps until result of transformation. If begin is undefined, begins at 0.

Note that if either index is negative, items will be buffered until completion.


Steps everything but the last entry. Passing n will step all values excluding the last N.

Note that no items will be sent and all items will be buffered until completion.


Step the last element. Passing n will step the last N values.

Note that no items will be sent until completion.


Mixed into transduce.math or available by explictly requiring from transduce/math. The following are equivalent:

  • require('transduce').math.min
  • require('transduce/math').min

Steps the minimum value on the result of the transformation. if f is provided, it is called with each item and the return value is used to compare values. Otherwise, the items are compared as numbers


Steps the maximum value on the result of the transformation. if f is provided, it is called with each item and the return value is used to compare values. Otherwise, the items are compared as numbers


Transduce over sequences of strings. Particularly useful with transduce-stream.

Treats every item as a substring, and splits across the entire transducer sequence. This allows functions to work with chunks sent through streams. When using transducers with streams, it is helpful to compose the transformation that you want with one of these functions to operate against a given line/word/etc.

Mixed into transduce.string or available by explictly requiring from transduce/string. The following are equivalent:

  • require('transduce').string.split
  • require('transduce/string').split
string.split(separator, limit?)

Works like ''.split but splits across entire sequence of items. Accepts separator (String or RegExp) and optional limit of words to send.


Buffers all items and joins results on transducer result.


Only steps items that are non empty strings (input.trim().length > 0).


Split chunks into lines and steps each line with optional limit (number of lines).


Split chunks into characters and steps each char with optional limit (number of chars).

string.words(delimiter?, limit?)

Split chunks into words using delimiter (default /\s+/) and steps each word with optional limit (number of words).


Extracted from underarm, which was created initially as a translation from Clojure. Now compatible with and inspired by protocols defined by transducers-js and transducers.js.



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