cosed

Fork of co that describes asynchronous tasks as data.

Why?

Instead of a generator activating side-effects it instead yields data objects that represent how side-effects ought to be executed. This pushes side-effects to co instead of the application itself.

Effectively this makes testing side-effects as easy as checking that each step in a generator returns the proper data structure.

This library was inspired by redux-saga and re-frame. Whenever I left the world of redux-saga and wanted to test my async/await/generator functions it would require mocking/intercepting HTTP requests which is a terrible developer experience after coming from describing side-effects as data.

Effects as Data talk by Richard Feldman

How?

cosed will work exactly like co with the exception that it can handle a new yielded value type: effect objects. An effect object looks something like this:

{
  "type": "CALL",
  "fn": [function],
  "args": ["list", "of", "arguments"]
}

task is an alias for the co function.

import { call, task } from 'cosed';

function* fetchBin() {
  const resp = yield call(fetch, 'http://httpbin.org/get');
  // sending an array makes `call` activate the function `json` on `resp` object
  // this is required because of the way fetch uses context to determine if the Body
  // promise has been used already.
  const data = yield call([resp, 'json']);
  return { ...data, extra: 'stuff' };
}

task(fetchBin)
  .then(console.log)
  .catch(console.error);

Check out the API section for more effects.

Testing

Taking the previous example, this is how you would test it:

const test = require('tape');

test('test fetchBin', (t) => {
  const gen = fetchBin();

  t.deepEqual(
    gen.next().value,
    call(fetch, 'http://httpbin.org/get'),
    'should make http request',
  );

  const respValue = { resp: 'value', json: 'hi' };
  t.deepEqual(
    gen.next(respValue).value,
    call([respValue, 'json']),
    'should get json from response',
  );

  const last = gen.next({ data: 'value' });
  t.ok(last.done, 'generator should finish');
  t.deepEqual(
    last.value,
    { data: 'value', extra: 'stuff' },
    'should return data',
  );
});

Using a little helper library called gen-tester we can make this even easier.

const { genTester, yields } = require('gen-tester');

test('test fetchBin', (t) => {
  t.plan(1);

  const respValue = { resp: 'value', json: 'hi' };
  const returnValue = { data: 'value', extra: 'stuff' };

  const tester = genTester(genCall);
  const { actual, expect } = tester(
    yields(
      call(fetch, 'http://httpbin.org/get'),
      respValue, // the result value of `resp` in the generator
    ),
    yields(
      call([respValue, 'json']),
      { data: 'value' }, // the result value of `data` in the generator
    ),
    returnValue,
  );

  t.deepEqual(actual, expected);
});

Take a close look here. When the generator function does not get called by task all it does is return JSON at every yield. This is the brilliance of describing side-effects as data: we can test our generator function synchronously, without needing any HTTP interceptors or mocking functions! So even though at every yield this library will make asynchronous calls, for testing, we can step through the generator one step after another and make sure the yield makes the correct call.

API

task

Manages async flow for a generator. This is an alias to the co function.

call

const { task, call } = require('cosed');
const fetch = require('node-fetch');

function* example() {
  yield call(fetch, 'http://google.com')
  return 'hi';
}

task(example);

all

Uses Promise.all to execute effects in parallel. Could be an array of effects or an object of effects.

const { task, call, all } = require('cosed');
const fetch = require('node-fetch');

function* example() {
  const resp = yield all([
    call(fetch, 'http://google.com'),
    call(fetch, 'http://something-else.com'),
  ]);
  const data = yield all(resp.map((r) => call([r, 'json'])));
  return data;
}

task(example);

spawn

Spawns an effect without the generator waiting for that effect to finish.

const { task, spawn } = require('cosed');
const fetch = require('node-fetch');

function effect() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve();
      console.log('ACTIVATE');
    }, 5000);
  });
}

function* example() {
  yield spawn(effect);
  console.log('COOL');
}

task(example);
// COOL
// ... five seconds later
// ACTIVATE

delay

This will sleep the generator for the designated amount of time.

const { task, delay } = require('cosed');

function* example() {
  console.log('INIT')
  yield delay(1000);
  console.log('END');
}

task(example);
// INIT
// ... one second later
// END

factory

This is what creates task. This allows end-developers to build their own effect middleware. When using middleware it must return a promise, something that co understands how to handle, and to allow other middleware to handle the effect as well, you must return next(effect);

const { factory } = require('cosed');

const ERROR = 'ERROR';
const error = (msg) => ({ type: ERROR, msg });
const middleware = (next) => (effect) => {
  if (effect.type === ERROR) {
    return Promise.reject(effect.msg);
  }

  return next(effect);
};

function* example() {
  yield error('SOMETHING HAPPENED');
}

const customTask = factory(middleware);
customTask(example).catch((err) => {
  console.log(`ERROR: ${err}`);
});

// ERROR: SOMETHING HAPPENED