guillaumearm/handle-io

:sparkles: - Wrap side effects, combine them, and make this combination testable

handle-io ✨

Highly inspired by funkia/io and redux-saga, this library intends to wrap small pieces of impure code, orchestrates and tests them.

Purpose

Test side effects orchestration without pain

testHandler(logTwice('hello world'))
  .matchIo(log('hello world'))
  .matchIo(log('hello world'))
  .run();

This piece of code is an assertion, an error will be thrown if something goes wrong:

• wrong io function
• wrong io arguments
• too much io ran
• not enough io ran

Getting started

Install

npm install --save handle-io

---

IO

io is just a wrapper for functions and arguments. In some way, it transforms impure functions into pure functions.

Conceptually, an io function could just be defined in this way:

const log = (...args) => [console.log, args];

but in handle-io, it isn't.

Create IO functions

You can use io to create one:

const { io } = require('handle-io');
const log = io(console.log);

Run IO functions

Running log with arguments:

log('Hello', 'World').run(); // print Hello World

Running log without arguments:

log().run();
// or
log.run();

Keep in mind: pieces of code using .run() cannot be tested properly.

The idea of this library is to apply an IO function inside a structure called handler.

---

Handlers

A handler is a wrapped pure generator which just apply some IO function and/or handler.

e.g.

const { io, handler } = require('handle-io');

const log = io(console.log);

const logTwice = handler(function*(...args) {
  yield log(...args);
  yield log(...args);
});

Writing tests for handlers

Writing tests for handlers is very simple (please see the first example above).

What about testing a handler which applies an IO function and returns values ?

There is a very simple way:

• using the second argument of the .matchIo() method to mock returned values
• using .shouldReturn() to assert on the final value

e.g.

const { io, handler } = require('handle-io');

const getEnv = io((v) => process.env[v]);

const addValues = handler(function*() {
  const value1 = yield getEnv('VALUE1');
  const value2 = yield getEnv('VALUE2');
  return value1 + value2;
});

testHandler(addValues())
  .matchIo(getEnv('VALUE1'), 32)
  .matchIo(getEnv('VALUE2'), 10)
  .shouldReturn(42)
  .run();

Running handlers

Same as for IO functions, there is a .run() method:

addValues().run(); // => 42
// or
addValue.run();

Likewise, don't use handlers' .run() everywhere in your codebase.

handlers are combinable together: you can yield a handler.

---

Promise support

handle-io supports promises and allows you to create asynchronous IO.

e.g.

const { io, handler, testHandler } = require('handle-io');

// async io
const sleep = io((ms) => new Promise(resolve => setTimeout(resolve, ms)));

// create an async combination
const sleepSecond = handler(function*(s) {
  yield sleep(s * 1000);
  return s;
});

// test this combination synchronously
testHander(sleepSecond(42))
  .matchIo(sleep(42000))
  .shouldReturn(42)
  .run();

Please note that sleep(n) and sleepSecond(n) will expose .run() methods that return a promise.

e.g.

sleepSecond(1).run().then((n) => {
  console.log(`${n} second(s) waited`);
});

---

Dealing with errors

using Try/Catch

The simplest way to handle errors with handle-io is to use try/catch blocks.

As you can see in the example below, you can try/catch any errors:

• inside a handler:
  • thrown error
• inside an io function:
  • thrown error
  • unhandled promise rejection

e.g.

const { io, handler } = require('handle-io');

const handler1 = handler(function*() {
  throw new Error();
});

// Synchronous IO
const io1 = io(() => { throw new Error() });

// Asynchronous IO
const io2 = io(() => Promise.reject(new Error()));

// handler2 is safe, it can't throw because it handles errors
const handler2 = handler(function*() {
  try {
    yield io1();
    yield io2();
    yield handler1();
  } catch (e) {
    console.error(e);
  }
});

using catchError helper

A functional helper exits to avoid try/catchs block, it allows to easily ignore errors and/or results.

Under the hood, catchError uses a try/catch block and works similarly.

e.g.

const { io, handler, catchError } = require('handle-io');

const ioError = io((e) => { throw new Error(e) });

const myHandler = handler(function*() {
  const [res, err] = yield catchError(ioError('error'));
  if (err) {
    yield log(err);
  }
  return res;
});

How to test errors

By default, no mocked IO throws any error.

It's possible to simulate throws with testHandler using the simulateThrow test utility.

Writing tests for myHandler means two cases need to be handled:

• when ioError throws:

testHandler(myHandler())
  .matchIo(ioError('error'), simulateThrow('error'))
  .matchIo(log('error'))
  .shouldReturn(undefined)
  .run();

• when ioError doesn't throw:

testHandler(myHandler())
  .matchIo(ioError('error'), 42)
  .shouldReturn(42)
  .run();

---

Custom testHandler

A custom testHandler can be created using createTestHandler.

e.g.

import { io, createTestHandler } from 'handle-io';


const createCustomTestHandler = (h, mockedIOs = [], expectedRetValue, assertRet = false, constructor = createCustomTestHandler) => {
  return {
    ...createTestHandler(h, mockedIOs, expectedRetValue, assertRet, constructor),
    matchLog: (arg, ret) => constructor(
      h,
      [...mockedIOs, [io(console.log)(arg), ret]],
      expectedRetValue,
      assertRet,
      constructor,
    ),
  };
};

const customTestHandler = h => createCustomTestHandler(h);

const log = io(console.log);
const myHandler = handler(function*(value) {
  yield log(value);
  yield log(value);
  return 42;
});

customTestHandler(myHandler('hello world'))
  .shouldReturn(42)
  .matchLog('hello world')
  .matchLog('hello world')
  .run()

---

Use with Redux

There is a way to use handler as redux middleware.

Please take a look to redux-fun Handlers.

License

MIT