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2.1.0 • Public • Published

Side Effect JS side-effect-js CI

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.


  • 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:

        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(''); },
            () => {
                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';
  • 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:

  • 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.


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:

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

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

import SideEffectJS from 'side-effect-js';
const myThunk = (username, password) => {
    // 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
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:

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  • eladbash