Welcome to ReactAddressMock

React Address Mock


Mock Local Storage

We are using localstorage in our browser. This is not going to be available in our tests since we are not executing our code inside a browser. Instead, we need to mock localstorage. We need to create a fake localstorage object that emulates what the browser local storage object.

In the browser, the global object is called window. localStorage is a method of this global objects: window.localStorage. In NodeJs the global object is called global. So we create global.localStorage. In both cases, one can access localStorage directly by name. We don't have to write global.localStorage or window.localStorage. It is not an error to do so, but it is not necessary. Therefore, in both our tests and in a browser, we can just call localStorage even though in one case it is called global.localStorage and in the other it is called window.localStorage.

At the risk of belaboring the point too long, I'll just sum up by saying that we are creating a mock localStorage object that acts just like the real localStorage object but does not require the presence of the browser and the window object.

Below is the code which implements the parts of localStorage that we use in our app. This code will be executed during our tests. Place the code near the top of the files that will need access to our data. For instance, if you checking for the value of firstName you will probably need the mock. A simple way to determine where it belongs is to simply run the tests in a module and see if they throw an exception saying that localStorage is not defined.

NOTE: Remember that we can press the letter 'p' when running tests, then type in all or part of the name of the file for a test. Then only that file will be run. For instance, typing Address will run only Address.test.js. This works best if you can uniquely identify each file. For instance, if you have tests suites called FooBar.test.js and Bar.test.js then trying to filter on Bar.test.js will get both test suites because both names contain Bar.test.js. Typing FooBar.test.js will run only the tests in FooBar.test.js.

// http://stackoverflow.com/a/32911774/253576
beforeEach(function() {
    const localStorageMock = (function() {
        let storage = {};
        return {
            getItem: function(key) {
                return storage[key];
            setItem: function(key, value) {
                storage[key] = value.toString();
            clear: function() {
                storage = {};
    Object.defineProperty(global, 'localStorage', {value: localStorageMock});


Create Mocks Folder

So how do we perform this miracle? To make a long story short: we use the mock library built into Jest. Here is how to proceed:

Mock Data

First lets create a simple module that contains the data we will use in our mock and call it __mocks__/mock-data.js:

 * Created by charlie on 4/18/17.

const getData = (url) => {
    switch (url) {
        case './address-list.json':
            return [{
                "firstName": "Lamar",
                "lastName": "Alexander",
                "street": "455 Dirksen Senate Office Building",
                "city": "Washington DC",
                "state": "TN",
                "zip": " 20510",
                "phone": "202-224-4944",
                "website": "https://www.alexander.senate.gov/public",
                "email": "",
                "contact": "http://www.alexander.senate.gov/public/index.cfm?p=Email"
                "firstName": "Roger",
                "lastName": "Wicker",
                "street": "555 Dirksen Senate Office Building",
                "city": "Washington DC",
                "state": "MS",
                "zip": " 20510",
                "phone": "202-224-6253",
                "website": "https://www.wicker.senate.gov",
                "email": "",
                "contact": "https://www.wicker.senate.gov/public/index.cfm/contact"
                "firstName": "Timothy",
                "lastName": "Kaine",
                "street": "231 Russell Senate Office Building",
                "city": "Washington DC",
                "state": "VA",
                "zip": " 20510",
                "phone": "202-224-4024",
                "website": "https://www.kaine.senate.gov",
                "email": "",
                "contact": "https://www.kaine.senate.gov/contact"

            return [];

export default getData;

This code simply creates sets of data that mimic what our server would return given a call to a specific url.

Mock fetch

Below is the source code for our new mock for fetch called whatwg-fetch.js. Save it and mock-data.js in the __mocks__ folder. Note in particular the call to jest.genMockFromModule. That call asks Jest to generate a mock object for the module we want to replace with a mock:

 * Created by charlie on 4/17/17.

import getData from './mock-data';

'use strict';

const whatwgFetch = jest.genMockFromModule('whatwg-fetch');

const fetch = function(url) {

    const objectState = getData(url);

    const response = {};
    response.json = function() {
        return objectState;

    //console.log("FETCH STATER", objectState);
    return {
        then: function(func) {
            //console.log('FETCH TEST ONE', func(response));
            return {
                then: function(func) {
                    return {
                        catch: function() {


whatwgFetch.fetch = fetch;
window.fetch = fetch;

module.exports = whatwgFetch;

Over time, you can comment out the calls to console.log. But they might be helpful at first when you are trying to understand what is going on. Note in particular that we are now putting calls to the callbacks (func) passed into our labyrinthine series of return statements. The most important is the second call to then where we pass back the stater object. Recall that this is used as follows in our call to fetch:

.then(function (json) {
    console.log('parsed json', json);
    that.setState(foo => (json));

Using Mocks in Tests

At the top of your address.test.js file, do this:

Is this still necessary? (Was it ever necessary?) I think you can just do this at the top of your test:

import 'whatwg-fetch';


In the constructor for Address we have the following code. Its responsibility is to be sure that localStorage contains at least 100 records representing the addresses we placed in address-list.json.

import DataLoader from './DataLoader';
const dataLoader = new DataLoader();

// Code ommitted here.
// Then, in the constructor:
constructor(props) {
    logger.log('Constructor called');

    this.addressIndex = 0;

    const that = this;
    dataLoader.loadAddresses(function(addressCount) {
        if (!addressCount) {
            throw new Error('Cannot get address count in address.js');
        that.addressCount = addressCount;
    // AND SO ON

This code performs a little trick you have seen before:

The loadAddress function has one responsibility but two parts:

Here is some of DataLoader:

 * Created by bcuser on 5/10/17.

import Logger from '../assets/elf-logger';
const logger = new Logger('data-loader', 'yellow', 'green', '18px');
import {saveByIndex} from '../assets/elf-local-storage';
import 'whatwg-fetch';

export default class DataLoader {

    constructor() {
        this.STORE_SET = ['elven-store', 'set', 'elven-count'];
        this.loadAddresses = this.loadAddresses.bind(this);

    dataLoaded() {
        const elfStore = localStorage.getItem(this.STORE_SET[0]);
        return (elfStore === this.STORE_SET[1]);

    setLocalStorage(addresses) {
        logger.log('SET LOCAL', addresses);
        //localStorage.setItem('elven-store', 'set');
        localStorage.setItem(this.STORE_SET[0], this.STORE_SET[1]);
        //localStorage.setItem('elven-count', addresses.length);
        localStorage.setItem(this.STORE_SET[2], addresses.length);
        addresses.forEach(function(address, index) {
            saveByIndex(address, index);
        return addresses;

    loadAddresses(callback) {
        const that = this;
        if (this.dataLoaded()) {
            logger.log('Using data from localstore');
        } else {
            logger.log('Loading data');
            fetch('./address-list.json').then(function(data) {
                const addresses = data.json();
                return addresses;
            }).then(function(data) {
                logger.log(JSON.stringify(data, null, 4));
            }).catch(function (err) {

Elf Local Storage

The DataLoader code is specific to our current project. Here is a helper object with localStorage calls that could be reused in multiple programs. Save it as elf-local-storage.js in your assets directory.

 * Created by Charlie on 5/8/17.

const ELF_TAG = 'elf';

const padNumber = function(numberToPad, width, padValue) {
    padValue = padValue || '0';
    numberToPad += '';
    if (numberToPad.length >= width) {
        return numberToPad;
    } else {
        return new Array(width - numberToPad.length + 1).join(padValue) + numberToPad;

function saveByIndex(item, index) {
    if (typeof item === 'object') {
        item = JSON.stringify(item, null, 4);
    const key = ELF_TAG + padNumber(index, 4, 0);
    localStorage.setItem(key, item);

function getByIndex(index) {
    const key = ELF_TAG + padNumber(index, 4, 0);
    return JSON.parse(localStorage.getItem(key));

function removeElfKeys() {
    for (var key in localStorage) {
        if (key.startsWith(ELF_TAG)) {

function clearLocalStorage() {

export {saveByIndex, getByIndex, removeElfKeys, clearLocalStorage};

Load Local Data

In onAddressChange load data from the localStore:

onAddressChange(event) {
    detailLogger.log('onAddressChange called with', event.target.id);
    if (event.target.id.startsWith('dec')) {
        if (this.addressIndex > 0) {
            this.addressIndex -= 1;
    } else {
        if (this.addressIndex < this.addressCount) {
            this.addressIndex += 1;
    detailLogger.log('addressIndex', this.addressIndex);
    const address = getByIndex(this.addressIndex);

        address: address

Turn it in

I'll be grading React Address Mock and React Address DataMaven assignments at the same time from the same codebase. You will get two grades, but I will be looking at one copy of CongressAddress when I grade them. I don't want to have to get two versions of CongressAddress going. Therefore, I will start a single version of the program, run the tests, and expect to be able to grade both assignments based on the code from the same commit. Two assignments, one version of CongressAddress:

Once you have a version of CongressAddress that contains code fulfilling the requirements for both assignments, then you should push, branch and tag:

git add .
git commit -m "Code for React Address Mock and React Address DataMaven"
git push
git branch week07-DataMavenMock
git tag -a v7.X.X -m "Code for React Address Mock and React Address DataMaven"
git push origin v7.X.X

Of course, the X.X bit would contain your idea of the appropriate numbering scheme. For instance: v7.0.0.

Hint Tagging