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Monio (mō'ne-yo) is an async-capable IO Monad (including "do" style) for JS, with several companion monads thrown in.

See It In Action


Monio balances the power of monads -- often dismissed by the non-FP programmer as academic and convoluted -- while pragmatically embracing the reality of the vast majority of JS programs: paradigm mixture (some OO, some FP, and probably a lot of imperative procedural code).

The driving inspiration behind Monio is the IO monad -- useful for managing side-effects -- that additionally supports "do-style" syntax with JS-ergonomic asynchrony (based on promises) in the style of familiar async..await code. IOs are lazy, so their operations are not triggered until the run(..) method is called.

Monio's IO is a transformer over promises, which means that when promises are produced in an IO, they are automatically unwrapped; of course, that means subsequent IO operations are deferred. If any IO in a chain produces a promise, run(..)'s result will be "lifted" to a promise that resolves when the entire IO chain is complete. Otherwise, the IO instance and its run(..) call will operate synchronously and immediately produce the result.

Monio intentionally chooses to model asynchrony over promises instead of Task monads, because of its goal of balancing FP with pragmatic and idomatic non-FP JS. However, there's nothing that should prevent you from using a Task monad with Monio if you prefer.

IO's "do-style" syntax is specified with the do(..) method (automatically lifts the IO to promise-producing asynchrony), which accepts JS generators (including "async generators": async function *whatever(){ .. }). yield is used for chaining IOs (which can produce promises to defer), whereas await is for explicitly deferring on a promise that's not already wrapped in an IO. The resulting style of code should be more broadly approachable for JS developers, while still benefitting from monads.

IO's do(..) is JS-ergonomic for exception handling. Uncaught JS exceptions become promise rejections, and IO-produced promise rejections are try..catch'able. IO also supports modeling exception handling through Either monads: doEither(..) transforms uncaught exceptions into Either:Left values, and recognizes IO-produced Either:Left values as try..catch'able exceptions.

Monio's IO is also a Reader monad, which carries side-effect read environments alongside IO operations.

Monio includes several supporting monads/helpers in addition to IO:

  • Maybe (including Just and Nothing)

  • Either

  • Monio-specific AsyncEither (same promise-transforming behavior as IO)

  • IOEventStream(..): creates an IO instance that produces an "event stream" -- an async-iterable consumable with a for await..of loop -- from an event emitter (ie, a DOM element, or a Node EventEmitter instance)

For the FP savvy

Monio's IO models a function e => IO a (Promise b c), which is strong enough to capture (optional) environment passing, side effects, async, and error handling without the pain of composing each type separately.

Typically IO does not take an argument, but given one, it acts like an effectful Reader. In addition, it can model sync or async functions so the inner Promise becomes optional.

In that way, you can think of it as ReaderT (IOT (Promise|Identity a b)) where Promise gets swapped for Identity if you're not doing async.

Monio's IO is like a JS-style ZIO/RIO where we have all the functionality we need wrapped up in 1 monad.

Using Monio

To use monads/helpers from Monio, first import them:

  • CJS programs/modules in Node:

    var { Maybe, IO } = require("monio");
    // or:
    var Just = require("monio/just");
  • ESM in Node:

    import { Maybe, IO } from "monio";
    // or:
    import Just from "monio/just";

    Note: As of v0.20.0, the previously required ESM import specifier segment /esm in Monio import paths has been deprecated (and will eventually be removed), in favor of unified import specifier paths via Node Conditional Exports. For ESM import statements, always use the specifier style "monio" or "monio/just", instead of "monio/esm" and "monio/esm/just", respectively.

  • ESM in browser:

    import { Maybe, IO } from "/path/to/monio/dist/esm/index.mjs";
    // or:
    import Just from "/path/to/monio/dist/esm/just.mjs";
  • UMD in browser:

    <script src="/path/to/monio/dist/umd/bundle.js"></script>
    <!-- or -->
    <script src="/path/to/monio/dist/umd/just.js"></script>

Once the monads are imported into your module/program, instances are created from functions (no new constructors necessary):

var helloWorld = Just("Hello World");

// Just("Hello World");
// true

Monio's monads can of course be used together in various expected ways:

var helloWorld = Just("Hello World");
var greeting = Maybe(helloWorld);
var log = str => IO(() => console.log(str));

var main = *main(){
    var msg = => `${ m }!!`);

    // Uncomment this line to swap in an empty maybe
    // msg = Maybe.from(null);

    yield msg.fold(IO.of,log);
// Hello World!!


A test suite is included in this repository, as well as the npm package distribution. The default test behavior runs the test suite using the files in src/.

  1. The tests are run with QUnit.

  2. To run the test utility with npm:

    npm test
  3. To run the test utility directly without npm:


Test Coverage

Coverage Status

If you have NYC (Istanbul) already installed on your system (requires v14.1+), you can use it to check the test coverage:

npm run coverage

Then open up coverage/lcov-report/index.html in a browser to view the report.

Note: The npm script coverage:report is only intended for use by project maintainers. It sends coverage reports to Coveralls.


All code and documentation are (c) 2021 Kyle Simpson and released under the MIT License. A copy of the MIT License is also included.

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