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This project is part of the @thi.ng/umbrella monorepo.


Transducer-based Finite State Machine transformer. This is a support package for @thi.ng/transducers.

This package provides a single function, a general purpose Finite State Machine transducer, which acts as useful & lightweight mechanism to provide context-sensitive processing capabilities as part of a transducer transformation pipeline.


BETA - possibly breaking changes forthcoming

Search or submit any issues for this package

This package might be merged with (or deprecated by) the newer @thi.ng/fsm package.

Related packages

  • @thi.ng/fsm - Composable primitives for building declarative, transducer based Finite-State Machines & matchers for arbitrary data streams
  • @thi.ng/sax - Transducer-based, SAX-like, non-validating, speedy & tiny XML parser


yarn add @thi.ng/transducers-fsm
// ES module
<script type="module" src="https://unpkg.com/@thi.ng/transducers-fsm?module" crossorigin></script>

// UMD
<script src="https://unpkg.com/@thi.ng/transducers-fsm/lib/index.umd.js" crossorigin></script>

Package sizes (gzipped, pre-treeshake): ESM: 217 bytes / CJS: 270 bytes / UMD: 385 bytes


Usage examples

For a real world example, the @thi.ng/sax package provides a SAX-like XML parser transducer, built around the FSM provided here.

3-state FSM

The following example defines a simple FSM with 3 states:

  • skip
  • take
  • done

The FSM always starts in the skip state.

The FSM alternates between skipping or consuming (passing through) 5 inputs as long as each input is < 20. Once an input is >= 20, the FSM switches into the done state, which has been declared as a terminal state and once entered will cause processing to terminate (also see API description further below).

import { fsm } from '@thi.ng/transducers-fsm'
import * as tx from '@thi.ng/transducers'
import { isOdd } from '@thi.ng/checks'

const testFSM = fsm({

    // initial state initializer
    // (called before processing 1st input)
    init: () => ({ state: "skip", count: 0 }),

    // terminal state ID
    terminate: "done",

    // individual state handlers
    states: {
        // skip state
        skip: (state, x) => {
            if (x < 20) {
                if (++state.count > 5) {
                    state.state = "take";
                    state.count = 1;
                    return [x];
            } else {
                state.state = "done";

        // take state
        take: (state, x) => {
            if (x < 20) {
                if (++state.count > 5) {
                    state.state = "skip";
                    state.count = 1;
                } else {
                    return [x];
            } else {
                state.state = "done";

        // terminal state, ignore inputs
        done: () => { },

[...tx.iterator(testFSM, tx.range(100))]
// [ 5, 6, 7, 8, 9, 15, 16, 17, 18, 19 ]

// Use FSM as part of composed transducers...

[...tx.iterator(tx.comp(tx.takeNth(2), testFSM), tx.range(100))]
// [ 10, 12, 14, 16, 18 ]

            tx.mapcat((x) => x.split(/[,\s]+/g)),
            tx.map((x) => parseInt(x)),
        ["9,8,7,6", "14 1 0 17 15 16", "19,23,12,42,4"]
// [ 1, 17, 15 ]


Generated API docs

fsm transducer

fsm<T extends FSMState, A, B>(opts: FSMOpts<T, A, B[]>): Transducer<A, B>

Finite State Machine transducer. Takes an FSM configuration object and returns a transducer, which processes inputs using the provided state handler functions, which in turn can produce any number of outputs per consumed input.

Before processing the first input, the FSM state is initialized by calling the user provided init() function, which MUST return a state object with at least a state key, whose value is used for dynamic (i.e. stateful) dispatch during input processing. This state object is passed with each input value to the current state handler, which is expected to mutate this object, e.g. to cause state changes based on given inputs.

If a state handler needs to "emit" results for downstream processing, it can return an array of values. Any such values are passed on (individually, not as array) to the next reducer in the chain. If a state handler returns null or undefined, further downstream processing of the current input is skipped.

Regardless of return value, if a state handler has caused a state change to the configured terminate state, processing is terminated (by calling ensureReduced()) and no further inputs will be consumed.


Karsten Schmidt


© 2018 - 2020 Karsten Schmidt // Apache Software License 2.0

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