Side Effect JS

Easy to setup and use - 3 lines of configuration and you can test your single page application like it's really connected to an API / Writes to files etc.

Version:

• V2.1.0

How to use:

• Install side-effect-js from npm : npm install side-effect-js --save

• Create side effects file - the name is not important and export an array of side effects:

  	//side-effects-my-app.js
  	import SideEffectJS from 'side-effect-js';
  	var newConsoleEffect = SideEffectJS.CreateEffect('console-effect',
  		() => { console.log("this is the real x"); },
  		() => { console.log("x simulate"); }
  	);
  
  	var fetchEffect = SideEffectJS.CreateEffect('fetch-effect',
  		() => { return fetch('http://www.google.com'); },
  		() => {
  			return new Promise(resolve => setTimeout(() => resolve("google test"), 2000));
  		}
  	);
  
  	export default [newConsoleEffect, fetchEffect];

  Note: side effect is an object that must contain: id: string, run: func, simulate: func - id is unique, defining duplicate ids will throw an error SideffectsJS load failed, Found duplicate id in effects:$id on load.

• On the root of your app - load all side effects to SideEffectJS:

  	//App.js or index.js
  	import sideEffects from './side-effects-my-app.js'
  	import SideEffectJS from 'side-effect-js';
  	SideEffectJS.Load(sideEffects);

• When you want to use the simulate effect - just add after SideEffectJS.Load this line: SideEffectJS.UseSimulator();

  Don't use SideEffectJS.UseSimulator() on production.

Better way to run in simulator mode without Changing the code (v2.0.0+)

Old way (changing code): In versions previous to 2.0.0 when you wanted to use the mock side effects you should call SideEffectJS.UseSimulator(); in your code. You can still do it - but then you need to change this line all the time. New way (use process.env): All you need to do is to initiate SIMULATE_SIDE_EFFECTS=1 and you will get the mock effects. For example instead of using: node index.js run: SIMULATE_SIDE_EFFECTS=1 node index.js. For react applications using CRA (create react app) - you can use REACT_APP_SIMULATE_SIDE_EFFECTS=1 in the .env files.

Read more on Create React App .env: https://create-react-app.dev/docs/adding-custom-environment-variables/

• Consume effects from SideEffectJS:

  	import SideEffectJS from 'side-effect-js';
  	var effect = SideEffectJS.Get('fetch-effect');
  	effect().then(x => console.log(x));

What do you need it for?

If you are developing single page application (Frontend) or Node.js apps (Backend) - most of the times you will have a server that returns responses to your client.

Almost any application has side effects:

• Using AJAX to call remote servers (Your REST API / Firebase or other serverless).
• Reading / Writing to files (on the server side)

In order to mock those I/O operations - you need to change your code frequently - means you should replace those I/O with some "stub" code.

So - what can we do better?

Use the side-effect method

Write all side effects in some aggregated place and load real / stub behaviour on run time.

Example:

Let's assume we are using redux-thunk or redux-saga and we need to fetch some data. What will we do with this data? save it to the state - and baiscally it will cause the UI to re-render and display something.

So instead of using this example thunk we will use the side-effects approach:

//some-redux-thunk-example

const myThunk = (username, password) => {
	dispatch('LOGIN_STARTED');
	// I.O operation
	fetch('myapi.com/login',
		{
			username:username,
			password:password
		})
		.then(token => dispatch('LOGIN_SUCCESS', token)
		.catch(err => dispatch('LOGIN_FAILED', err);
}

This is how it will look when using Side Effect JS:

//some-redux-thunk-example

import SideEffectJS from 'side-effect-js';

const myThunk = (username, password) => {
	dispatch('LOGIN_STARTED');
	// I.O operation
	SideEffectJS.Get('login')(username, password)
		.then(token => dispatch('LOGIN_SUCCESS', token)
		.catch(err => dispatch('LOGIN_FAILED', err);
}

What are the benefits of this approach?

• You are aware of IO - because you define side effect for each operation.
• You can mock and stub any i/o operation - and work on real app like it's consuming data from an API.
• You don't need to handle mock api and configure it all the time.
• You have one place that holds all the side effects of your app (in DDD terms it enfources infrastructure layer)
• You app will be easier to test
• By creating the side effects file you are creating documentations for you app's side effects.

What's next?

In the next versions there will be full support for:

1. Display all the side effects that your application actually uses in real time.
2. Better documentations & Examples.
3. Support for middlewares - so you will be able implement anything you want.

Release notes:

V2.1.0: - 04/27/2020 Deprecated (ts only): For TypeScript only - CreateEffect is deprecated (backward compatible), instead use: CreateEffectTyped. Typescript usage example:

const effect SideEffectJS.CreateEffectTyped<T,R>(`some-id`, realFunction, mockFunction)
//T - The model that mock and real function gets
//R - The result type of the mock and real function

//Example.ts
import SideEffectJS from '../side-effect-js';
type GetValue = {
    username: string,
    passowrd: string
}

const exampleFunctionReal = (loginDetails: GetValue): string => {
    return "real";
}

const exampleFunctionMock = (loginDetails: GetValue): string => {
    return "mock";
}

//Old way of creating effect
const oldEffect = SideEffectJS.CreateEffect('id-old', exampleFunctionReal, exampleFunctionMock);

//New way of creating effect(V2.1.0+) - ensures real effect and mock effect has the same contract
const effect = SideEffectJS.CreateEffectTyped<GetValue, string>('xxx', exampleFunctionReal, exampleFunctionMock);

V2.0.0: - 04/26/2020 Instead of using SideEffectJS.UseSimulator() - which forces you to change code -> use SIMULATE_SIDE_EFFECTS=1 or REACT_APP_SIMULATE_SIDE_EFFECTS=1 for CRA apps. SideEffectJS.UseSimulator() is still an option but it forces you to change code before merging. Read more on Create React App .env: https://create-react-app.dev/docs/adding-custom-environment-variables/