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react-fe-interview (wip)

  • This is a version of a Front End Technical Interview app which generates random test questions (for interviewer or candidate preparing for interviews)

  • This version is using

    • react and react-router4
    • localforage for browser-based storage
    • react-bootstrap

  • This project's text content was sourced from the h5bp repo, Front-end Developer Interview Questions with MIT license

Dependencies

Using node v6.9.2 (via nvm)

Resources

localforage - this is a really handy library with a much easier api to use than the original indexedDB api

surge module - I'm using an npm module that allows easier hosting of react-router applications on a 3rd party host

Running the Application

demo version

  • Known issue: currently filtering on the surge host is not working; it does work when the application is run on a local server... looking into this, sorry!

Running the application locally

1 From the terminal: npm install

2 npm run build

3 npm start

===

This project was bootstrapped with Create React App.

Below you will find some information on how to perform common tasks.
You can find the most recent version of this guide here.

Table of Contents

Updating to New Releases

Create React App is divided into two packages:

  • create-react-app is a global command-line utility that you use to create new projects.
  • react-scripts is a development dependency in the generated projects (including this one).

You almost never need to update create-react-app itself: it delegates all the setup to react-scripts.

When you run create-react-app, it always creates the project with the latest version of react-scripts so you’ll get all the new features and improvements in newly created apps automatically.

To update an existing project to a new version of react-scripts, open the changelog, find the version you’re currently on (check package.json in this folder if you’re not sure), and apply the migration instructions for the newer versions.

In most cases bumping the react-scripts version in package.json and running npm install in this folder should be enough, but it’s good to consult the changelog for potential breaking changes.

We commit to keeping the breaking changes minimal so you can upgrade react-scripts painlessly.

Sending Feedback

We are always open to your feedback.

Folder Structure

After creation, your project should look like this:

my-app/
  README.md
  node_modules/
  package.json
  public/
    index.html
    favicon.ico
  src/
    App.css
    App.js
    App.test.js
    index.css
    index.js
    logo.svg

For the project to build, these files must exist with exact filenames:

  • public/index.html is the page template;
  • src/index.js is the JavaScript entry point.

You can delete or rename the other files.

You may create subdirectories inside src. For faster rebuilds, only files inside src are processed by Webpack.
You need to put any JS and CSS files inside src, or Webpack won’t see them.

Only files inside public can be used from public/index.html.
Read instructions below for using assets from JavaScript and HTML.

You can, however, create more top-level directories.
They will not be included in the production build so you can use them for things like documentation.

Available Scripts

In the project directory, you can run:

npm start

Runs the app in the development mode.
Open http://localhost:3000 to view it in the browser.

The page will reload if you make edits.
You will also see any lint errors in the console.

npm test

Launches the test runner in the interactive watch mode.
See the section about running tests for more information.

npm run build

Builds the app for production to the build folder.
It correctly bundles React in production mode and optimizes the build for the best performance.

The build is minified and the filenames include the hashes.
Your app is ready to be deployed!

See the section about deployment for more information.

npm run eject

Note: this is a one-way operation. Once you eject, you can’t go back!

If you aren’t satisfied with the build tool and configuration choices, you can eject at any time. This command will remove the single build dependency from your project.

Instead, it will copy all the configuration files and the transitive dependencies (Webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except eject will still work, but they will point to the copied scripts so you can tweak them. At this point you’re on your own.

You don’t have to ever use eject. The curated feature set is suitable for small and middle deployments, and you shouldn’t feel obligated to use this feature. However we understand that this tool wouldn’t be useful if you couldn’t customize it when you are ready for it.

Syntax Highlighting in the Editor

To configure the syntax highlighting in your favorite text editor, head to the relevant Babel documentation page and follow the instructions. Some of the most popular editors are covered.

Displaying Lint Output in the Editor

Note: this feature is available with [email protected] and higher.

Some editors, including Sublime Text, Atom, and Visual Studio Code, provide plugins for ESLint.

They are not required for linting. You should see the linter output right in your terminal as well as the browser console. However, if you prefer the lint results to appear right in your editor, there are some extra steps you can do.

You would need to install an ESLint plugin for your editor first.

A note for Atom linter-eslint users

If you are using the Atom linter-eslint plugin, make sure that Use global ESLint installation option is checked:

For Visual Studio Code users

VS Code ESLint plugin automatically detects Create React App's configuration file. So you do not need to create eslintrc.json at the root directory, except when you want to add your own rules. In that case, you should include CRA's config by adding this line:





{
// ...
"extends": "react-app"
}



Then add this block to the `package.json` file of your project:

```js
{
  // ...
  "eslintConfig": {
    "extends": "react-app"
  }
}



Finally, you will need to install some packages globally:


npm install -g [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected]


We recognize that this is suboptimal, but it is currently required due to the way we hide the ESLint dependency. The ESLint team is already working on a solution to this so this may become unnecessary in a couple of months.


Changing the Page <title>


You can find the source HTML file in the public folder of the generated project. You may edit the <title> tag in it to change the title from “React App” to anything else.


Note that normally you wouldn't edit files in the public folder very often. For example, adding a stylesheet is done without touching the HTML.


If you need to dynamically update the page title based on the content, you can use the browser document.title API. For more complex scenarios when you want to change the title from React components, you can use React Helmet, a third party library.


Finally, if you use a custom server for your app in production and want to modify the title before it gets sent to the browser, you can follow advice in this section.


Installing a Dependency


The generated project includes React and ReactDOM as dependencies. It also includes a set of scripts used by Create React App as a development dependency. You may install other dependencies (for example, React Router) with npm:


npm install --save <library-name>



Importing a Component


This project setup supports ES6 modules thanks to Babel.

While you can still use require() and module.exports, we encourage you to use import and export instead.


For example:


Button.js


import React, { Component } from 'react';

class Button extends Component {
  render() {
    // ...
  }
}

export default Button; // Don’t forget to use export default!


DangerButton.js


import React, { Component } from 'react';
import Button from './Button'; // Import a component from another file

class DangerButton extends Component {
  render() {
    return <Button color="red" />;
  }
}

export default DangerButton;


Be aware of the difference between default and named exports. It is a common source of mistakes.


We suggest that you stick to using default imports and exports when a module only exports a single thing (for example, a component). That’s what you get when you use export default Button and import Button from './Button'.


Named exports are useful for utility modules that export several functions. A module may have at most one default export and as many named exports as you like.


Learn more about ES6 modules:



Adding a Stylesheet


This project setup uses Webpack for handling all assets. Webpack offers a custom way of “extending” the concept of import beyond JavaScript. To express that a JavaScript file depends on a CSS file, you need to import the CSS from the JavaScript file:


Button.css


.Button {
  padding: 20px;
}


Button.js


import React, { Component } from 'react';
import './Button.css'; // Tell Webpack that Button.js uses these styles

class Button extends Component {
  render() {
    // You can use them as regular CSS styles
    return <div className="Button" />;
  }
}


This is not required for React but many people find this feature convenient. You can read about the benefits of this approach here. However you should be aware that this makes your code less portable to other build tools and environments than Webpack.


In development, expressing dependencies this way allows your styles to be reloaded on the fly as you edit them. In production, all CSS files will be concatenated into a single minified .css file in the build output.


If you are concerned about using Webpack-specific semantics, you can put all your CSS right into src/index.css. It would still be imported from src/index.js, but you could always remove that import if you later migrate to a different build tool.


Post-Processing CSS


This project setup minifies your CSS and adds vendor prefixes to it automatically through Autoprefixer so you don’t need to worry about it.


For example, this:


.App {
  display: flex;
  flex-direction: row;
  align-items: center;
}


becomes this:


.App {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: horizontal;
  -webkit-box-direction: normal;
      -ms-flex-direction: row;
          flex-direction: row;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
}


There is currently no support for preprocessors such as Less, or for sharing variables across CSS files.


Adding Images and Fonts


With Webpack, using static assets like images and fonts works similarly to CSS.


You can import an image right in a JavaScript module. This tells Webpack to include that image in the bundle. Unlike CSS imports, importing an image or a font gives you a string value. This value is the final image path you can reference in your code.


Here is an example:


import React from 'react';
import logo from './logo.png'; // Tell Webpack this JS file uses this image

console.log(logo); // /logo.84287d09.png

function Header() {
  // Import result is the URL of your image
  return <img src={logo} alt="Logo" />;
}

export default Header;


This ensures that when the project is built, Webpack will correctly move the images into the build folder, and provide us with correct paths.


This works in CSS too:


.Logo {
  background-image: url(./logo.png);
}


Webpack finds all relative module references in CSS (they start with ./) and replaces them with the final paths from the compiled bundle. If you make a typo or accidentally delete an important file, you will see a compilation error, just like when you import a non-existent JavaScript module. The final filenames in the compiled bundle are generated by Webpack from content hashes. If the file content changes in the future, Webpack will give it a different name in production so you don’t need to worry about long-term caching of assets.


Please be advised that this is also a custom feature of Webpack.


It is not required for React but many people enjoy it (and React Native uses a similar mechanism for images).

An alternative way of handling static assets is described in the next section.


Using the public Folder




Note: this feature is available with [email protected] and higher.




Changing the HTML


The public folder contains the HTML file so you can tweak it, for example, to set the page title.
The <script> tag with the compiled code will be added to it automatically during the build process.


Adding Assets Outside of the Module System


You can also add other assets to the public folder.


Note that we normally encourage you to import assets in JavaScript files instead.
For example, see the sections on adding a stylesheet and adding images and fonts.
This mechanism provides a number of benefits:


    

  • Scripts and stylesheets get minified and bundled together to avoid extra network requests.
    • 

  • Missing files cause compilation errors instead of 404 errors for your users.
    • 

  • Result filenames include content hashes so you don’t need to worry about browsers caching their old versions.
    • 



However there is an escape hatch that you can use to add an asset outside of the module system.


If you put a file into the public folder, it will not be processed by Webpack. Instead it will be copied into the build folder untouched.   To reference assets in the public folder, you need to use a special variable called PUBLIC_URL.


Inside index.html, you can use it like this:


<link rel="shortcut icon" href="%PUBLIC_URL%/favicon.ico">


Only files inside the public folder will be accessible by %PUBLIC_URL% prefix. If you need to use a file from src or node_modules, you’ll have to copy it there to explicitly specify your intention to make this file a part of the build.


When you run npm run build, Create React App will substitute %PUBLIC_URL% with a correct absolute path so your project works even if you use client-side routing or host it at a non-root URL.


In JavaScript code, you can use process.env.PUBLIC_URL for similar purposes:


render() {
  // Note: this is an escape hatch and should be used sparingly!
  // Normally we recommend using `import` for getting asset URLs
  // as described in “Adding Images and Fonts” above this section.
  return <img src={process.env.PUBLIC_URL + '/img/logo.png'} />;
}


Keep in mind the downsides of this approach:


    

  • None of the files in public folder get post-processed or minified.
    • 

  • Missing files will not be called at compilation time, and will cause 404 errors for your users.
    • 

  • Result filenames won’t include content hashes so you’ll need to add query arguments or rename them every time they change.
    • 



When to Use the public Folder


Normally we recommend importing stylesheets, images, and fonts from JavaScript.
The public folder is useful as a workaround for a number of less common cases:


    

  • You need a file with a specific name in the build output, such as manifest.webmanifest.
    • 

  • You have thousands of images and need to dynamically reference their paths.
    • 

  • You want to include a small script like pace.js outside of the bundled code.
    • 

  • Some library may be incompatible with Webpack and you have no other option but to include it as a <script> tag.
    • 



Note that if you add a <script> that declares global variables, you also need to read the next section on using them.


Using Global Variables


When you include a script in the HTML file that defines global variables and try to use one of these variables in the code, the linter will complain because it cannot see the definition of the variable.


You can avoid this by reading the global variable explicitly from the window object, for example:


const $ = window.$;


This makes it obvious you are using a global variable intentionally rather than because of a typo.


Alternatively, you can force the linter to ignore any line by adding // eslint-disable-line after it.


Adding Bootstrap


You don’t have to use React Bootstrap together with React but it is a popular library for integrating Bootstrap with React apps. If you need it, you can integrate it with Create React App by following these steps:


Install React Bootstrap and Bootstrap from NPM. React Bootstrap does not include Bootstrap CSS so this needs to be installed as well:


npm install react-bootstrap --save
npm install bootstrap@3 --save



Import Bootstrap CSS and optionally Bootstrap theme CSS in the src/index.js file:


import 'bootstrap/dist/css/bootstrap.css';
import 'bootstrap/dist/css/bootstrap-theme.css';


Import required React Bootstrap components within src/App.js file or your custom component files:


import { Navbar, Jumbotron, Button } from 'react-bootstrap';


Now you are ready to use the imported React Bootstrap components within your component hierarchy defined in the render method. Here is an example App.js redone using React Bootstrap.


Adding Flow


Flow is a static type checker that helps you write code with fewer bugs. Check out this introduction to using static types in JavaScript if you are new to this concept.


Recent versions of Flow work with Create React App projects out of the box.


To add Flow to a Create React App project, follow these steps:


    

  1. Run npm install --save-dev flow-bin.
    2. 

  2. Add "flow": "flow" to the scripts section of your package.json.
    3. 

  3. Add // @flow to any files you want to type check (for example, to src/App.js).
    4. 



Now you can run npm run flow to check the files for type errors.
You can optionally use an IDE like Nuclide for a better integrated experience.
In the future we plan to integrate it into Create React App even more closely.


To learn more about Flow, check out its documentation.


Adding Custom Environment Variables




Note: this feature is available with [email protected] and higher.




Your project can consume variables declared in your environment as if they were declared locally in your JS files. By
default you will have NODE_ENV defined for you, and any other environment variables starting with
REACT_APP_.




Note: You must create custom environment variables beginning with REACT_APP_. Any other variables except NODE_ENV will be ignored to avoid accidentally exposing a private key on the machine that could have the same name.




These environment variables will be defined for you on process.env. For example, having an environment
variable named REACT_APP_SECRET_CODE will be exposed in your JS as process.env.REACT_APP_SECRET_CODE, in addition
to process.env.NODE_ENV.




Note: Changing any environment variables will require you to restart the development server if it is running.




These environment variables can be useful for displaying information conditionally based on where the project is
deployed or consuming sensitive data that lives outside of version control.


First, you need to have environment variables defined. For example, let’s say you wanted to consume a secret defined
in the environment inside a <form>:


render() {
  return (
    <div>
      <small>You are running this application in <b>{process.env.NODE_ENV}</b> mode.</small>
      <form>
        <input type="hidden" defaultValue={process.env.REACT_APP_SECRET_CODE} />
      </form>
    </div>
  );
}


During the build, process.env.REACT_APP_SECRET_CODE will be replaced with the current value of the REACT_APP_SECRET_CODE environment variable. Remember that the NODE_ENV variable will be set for you automatically.


When you load the app in the browser and inspect the <input>, you will see its value set to abcdef, and the bold text will show the environment provided when using npm start:


<div>
  <small>You are running this application in <b>development</b> mode.</small>
  <form>
    <input type="hidden" value="abcdef" />
  </form>
</div>


Having access to the NODE_ENV is also useful for performing actions conditionally:


if (process.env.NODE_ENV !== 'production') {
  analytics.disable();
}


The above form is looking for a variable called REACT_APP_SECRET_CODE from the environment. In order to consume this
value, we need to have it defined in the environment. This can be done using two ways: either in your shell or in
a .env file.


Adding Temporary Environment Variables In Your Shell


Defining environment variables can vary between OSes. It's also important to know that this manner is temporary for the
life of the shell session.


Windows (cmd.exe)


set REACT_APP_SECRET_CODE=abcdef&&npm start


(Note: the lack of whitespace is intentional.)


Linux, macOS (Bash)


REACT_APP_SECRET_CODE=abcdef npm start


Adding Development Environment Variables In .env




Note: this feature is available with [email protected] and higher.




To define permanent environment variables, create a file called .env in the root of your project:


REACT_APP_SECRET_CODE=abcdef



These variables will act as the defaults if the machine does not explicitly set them.

Please refer to the dotenv documentation for more details.




Note: If you are defining environment variables for development, your CI and/or hosting platform will most likely need
these defined as well. Consult their documentation how to do this. For example, see the documentation for Travis CI or Heroku.




Can I Use Decorators?


Many popular libraries use decorators in their documentation.

Create React App doesn’t support decorator syntax at the moment because:


    

  • It is an experimental proposal and is subject to change.
    • 

  • The current specification version is not officially supported by Babel.
    • 

  • If the specification changes, we won’t be able to write a codemod because we don’t use them internally at Facebook.
    • 



However in many cases you can rewrite decorator-based code without decorators just as fine.

Please refer to these two threads for reference:



Create React App will add decorator support when the specification advances to a stable stage.


Integrating with an API Backend


These tutorials will help you to integrate your app with an API backend running on another port,
using fetch() to access it.


Node


Check out this tutorial.
You can find the companion GitHub repository here.


Ruby on Rails


Check out this tutorial.
You can find the companion GitHub repository here.


Proxying API Requests in Development




Note: this feature is available with [email protected] and higher.




People often serve the front-end React app from the same host and port as their backend implementation.

For example, a production setup might look like this after the app is deployed:


/             - static server returns index.html with React app
/todos        - static server returns index.html with React app
/api/todos    - server handles any /api/* requests using the backend implementation



Such setup is not required. However, if you do have a setup like this, it is convenient to write requests like fetch('/api/todos') without worrying about redirecting them to another host or port during development.


To tell the development server to proxy any unknown requests to your API server in development, add a proxy field to your package.json, for example:


  "proxy": "http://localhost:4000",


This way, when you fetch('/api/todos') in development, the development server will recognize that it’s not a static asset, and will proxy your request to http://localhost:4000/api/todos as a fallback. The development server will only attempt to send requests without a text/html accept header to the proxy.


Conveniently, this avoids CORS issues and error messages like this in development:


Fetch API cannot load http://localhost:4000/api/todos. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.



Keep in mind that proxy only has effect in development (with npm start), and it is up to you to ensure that URLs like /api/todos point to the right thing in production. You don’t have to use the /api prefix. Any unrecognized request without a text/html accept header will be redirected to the specified proxy.


The proxy option supports HTTP, HTTPS and WebSocket connections.

If the proxy option is not flexible enough for you, alternatively you can:



Using HTTPS in Development




Note: this feature is available with [email protected] and higher.




You may require the dev server to serve pages over HTTPS. One particular case where this could be useful is when using the "proxy" feature to proxy requests to an API server when that API server is itself serving HTTPS.


To do this, set the HTTPS environment variable to true, then start the dev server as usual with npm start:


Windows (cmd.exe)


set HTTPS=true&&npm start


(Note: the lack of whitespace is intentional.)


Linux, macOS (Bash)


HTTPS=true npm start


Note that the server will use a self-signed certificate, so your web browser will almost definitely display a warning upon accessing the page.


Generating Dynamic <meta> Tags on the Server


Since Create React App doesn’t support server rendering, you might be wondering how to make <meta> tags dynamic and reflect the current URL. To solve this, we recommend to add placeholders into the HTML, like this:


<!doctype html>
<html lang="en">
  <head>
    <meta property="og:title" content="%OG_TITLE%">
    <meta property="og:description" content="%OG_DESCRIPTION%">


Then, on the server, regardless of the backend you use, you can read index.html into memory and replace %OG_TITLE%, %OG_DESCRIPTION%, and any other placeholders with values depending on the current URL. Just make sure to sanitize and escape the interpolated values so that they are safe to embed into HTML!


If you use a Node server, you can even share the route matching logic between the client and the server. However duplicating it also works fine in simple cases.


Running Tests




Note: this feature is available with [email protected] and higher.

Read the migration guide to learn how to enable it in older projects!




Create React App uses Jest as its test runner. To prepare for this integration, we did a major revamp of Jest so if you heard bad things about it years ago, give it another try.


Jest is a Node-based runner. This means that the tests always run in a Node environment and not in a real browser. This lets us enable fast iteration speed and prevent flakiness.


While Jest provides browser globals such as window thanks to jsdom, they are only approximations of the real browser behavior. Jest is intended to be used for unit tests of your logic and your components rather than the DOM quirks.


We recommend that you use a separate tool for browser end-to-end tests if you need them. They are beyond the scope of Create React App.


Filename Conventions


Jest will look for test files with any of the following popular naming conventions:


    

  • Files with .js suffix in __tests__ folders.
    • 

  • Files with .test.js suffix.
    • 

  • Files with .spec.js suffix.
    • 



The .test.js / .spec.js files (or the __tests__ folders) can be located at any depth under the src top level folder.


We recommend to put the test files (or __tests__ folders) next to the code they are testing so that relative imports appear shorter. For example, if App.test.js and App.js are in the same folder, the test just needs to import App from './App' instead of a long relative path. Colocation also helps find tests more quickly in larger projects.


Command Line Interface


When you run npm test, Jest will launch in the watch mode. Every time you save a file, it will re-run the tests, just like npm start recompiles the code.


The watcher includes an interactive command-line interface with the ability to run all tests, or focus on a search pattern. It is designed this way so that you can keep it open and enjoy fast re-runs. You can learn the commands from the “Watch Usage” note that the watcher prints after every run:


Jest watch mode


Version Control Integration


By default, when you run npm test, Jest will only run the tests related to files changed since the last commit. This is an optimization designed to make your tests runs fast regardless of how many tests you have. However it assumes that you don’t often commit the code that doesn’t pass the tests.


Jest will always explicitly mention that it only ran tests related to the files changed since the last commit. You can also press a in the watch mode to force Jest to run all tests.


Jest will always run all tests on a continuous integration server or if the project is not inside a Git or Mercurial repository.


Writing Tests


To create tests, add it() (or test()) blocks with the name of the test and its code. You may optionally wrap them in describe() blocks for logical grouping but this is neither required nor recommended.


Jest provides a built-in expect() global function for making assertions. A basic test could look like this:


import sum from './sum';

it('sums numbers', () => {
  expect(sum(1, 2)).toEqual(3);
  expect(sum(2, 2)).toEqual(4);
});


All expect() matchers supported by Jest are extensively documented here.

You can also use jest.fn() and expect(fn).toBeCalled() to create “spies” or mock functions.


Testing Components


There is a broad spectrum of component testing techniques. They range from a “smoke test” verifying that a component renders without throwing, to shallow rendering and testing some of the output, to full rendering and testing component lifecycle and state changes.


Different projects choose different testing tradeoffs based on how often components change, and how much logic they contain. If you haven’t decided on a testing strategy yet, we recommend that you start with creating simple smoke tests for your components:


import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

it('renders without crashing', () => {
  const div = document.createElement('div');
  ReactDOM.render(<App />, div);
});


This test mounts a component and makes sure that it didn’t throw during rendering. Tests like this provide a lot value with very little effort so they are great as a starting point, and this is the test you will find in src/App.test.js.


When you encounter bugs caused by changing components, you will gain a deeper insight into which parts of them are worth testing in your application. This might be a good time to introduce more specific tests asserting specific expected output or behavior.


If you’d like to test components in isolation from the child components they render, we recommend using shallow() rendering API from Enzyme. You can write a smoke test with it too:


npm install --save-dev enzyme react-addons-test-utils


import React from 'react';
import { shallow } from 'enzyme';
import App from './App';

it('renders without crashing', () => {
  shallow(<App />);
});


Unlike the previous smoke test using ReactDOM.render(), this test only renders <App> and doesn’t go deeper. For example, even if <App> itself renders a <Button> that throws, this test will pass. Shallow rendering is great for isolated unit tests, but you may still want to create some full rendering tests to ensure the components integrate correctly. Enzyme supports full rendering with mount(), and you can also use it for testing state changes and component lifecycle.


You can read the Enzyme documentation for more testing techniques. Enzyme documentation uses Chai and Sinon for assertions but you don’t have to use them because Jest provides built-in expect() and jest.fn() for spies.


Here is an example from Enzyme documentation that asserts specific output, rewritten to use Jest matchers:


import React from 'react';
import { shallow } from 'enzyme';
import App from './App';

it('renders welcome message', () => {
  const wrapper = shallow(<App />);
  const welcome = <h2>Welcome to React</h2>;
  // expect(wrapper.contains(welcome)).to.equal(true);
  expect(wrapper.contains(welcome)).toEqual(true);
});


All Jest matchers are extensively documented here.

Nevertheless you can use a third-party assertion library like Chai if you want to, as described below.


Using Third Party Assertion Libraries


We recommend that you use expect() for assertions and jest.fn() for spies. If you are having issues with them please file those against Jest, and we’ll fix them. We intend to keep making them better for React, supporting, for example, pretty-printing React elements as JSX.


However, if you are used to other libraries, such as Chai and Sinon, or if you have existing code using them that you’d like to port over, you can import them normally like this:


import sinon from 'sinon';
import { expect } from 'chai';


and then use them in your tests like you normally do.


Initializing Test Environment




Note: this feature is available with [email protected] and higher.




If your app uses a browser API that you need to mock in your tests or if you just need a global setup before running your tests, add a src/setupTests.js to your project. It will be automatically executed before running your tests.


For example:


src/setupTests.js


const localStorageMock = {
  getItem: jest.fn(),
  setItem: jest.fn(),
  clear: jest.fn()
};
global.localStorage = localStorageMock


Focusing and Excluding Tests


You can replace it() with xit() to temporarily exclude a test from being executed.

Similarly, fit() lets you focus on a specific test without running any other tests.


Coverage Reporting


Jest has an integrated coverage reporter that works well with ES6 and requires no configuration.

Run npm test -- --coverage (note extra -- in the middle) to include a coverage report like this:


coverage report


Note that tests run much slower with coverage so it is recommended to run it separately from your normal workflow.


Continuous Integration


By default npm test runs the watcher with interactive CLI. However, you can force it to run tests once and finish the process by setting an environment variable called CI.


When creating a build of your application with npm run build linter warnings are not checked by default. Like npm test, you can force the build to perform a linter warning check by setting the environment variable CI. If any warnings are encountered then the build fails.


Popular CI servers already set the environment variable CI by default but you can do this yourself too:


On CI servers


Travis CI


    

  1. Following the Travis Getting started guide for syncing your GitHub repository with Travis.  You may need to initialize some settings manually in your profile page.
    2. 

  2. Add a .travis.yml file to your git repository.
    3. 



language: node_js
node_js:
  - 4
  - 6
cache:
  directories:
    - node_modules
script:
  - npm test
  - npm run build



    

  1. Trigger your first build with a git push.
    2. 

  2. Customize your Travis CI Build if needed.
    3. 



On your own environment


Windows (cmd.exe)


set CI=true&&npm test


set CI=true&&npm run build


(Note: the lack of whitespace is intentional.)


Linux, macOS (Bash)


CI=true npm test


CI=true npm run build


The test command will force Jest to run tests once instead of launching the watcher.




If you find yourself doing this often in development, please file an issue to tell us about your use case because we want to make watcher the best experience and are open to changing how it works to accommodate more workflows.




The build command will check for linter warnings and fail if any are found.


Disabling jsdom


By default, the package.json of the generated project looks like this:


  // ...
  "scripts": {
    // ...
    "test": "react-scripts test --env=jsdom"
  }


If you know that none of your tests depend on jsdom, you can safely remove --env=jsdom, and your tests will run faster.

To help you make up your mind, here is a list of APIs that need jsdom:



In contrast, jsdom is not needed for the following APIs:



Finally, jsdom is also not needed for snapshot testing.


Snapshot Testing


Snapshot testing is a feature of Jest that automatically generates text snapshots of your components and saves them on the disk so if the UI output changes, you get notified without manually writing any assertions on the component output. Read more about snapshot testing.


Editor Integration


If you use Visual Studio Code, there is a Jest extension which works with Create React App out of the box. This provides a lot of IDE-like features while using a text editor: showing the status of a test run with potential fail messages inline, starting and stopping the watcher automatically, and offering one-click snapshot updates.


VS Code Jest Preview


Developing Components in Isolation


Usually, in an app, you have a lot of UI components, and each of them has many different states.
For an example, a simple button component could have following states:


    

  • With a text label.
    • 

  • With an emoji.
    • 

  • In the disabled mode.
    • 



Usually, it’s hard to see these states without running a sample app or some examples.


Create React App doesn't include any tools for this by default, but you can easily add React Storybook to your project. It is a third-party tool that lets you develop components and see all their states in isolation from your app.


React Storybook Demo


You can also deploy your Storybook as a static app. This way, everyone in your team can view and review different states of UI components without starting a backend server or creating an account in your app.


Here’s how to setup your app with Storybook:


First, install the following npm package globally:


npm install -g getstorybook


Then, run the following command inside your app’s directory:


getstorybook


After that, follow the instructions on the screen.


Learn more about React Storybook:



Making a Progressive Web App


You can turn your React app into a Progressive Web App by following the steps in this repository.


Deployment


npm run build creates a build directory with a production build of your app. Set up your favourite HTTP server so that a visitor to your site is served index.html, and requests to static paths like /static/js/main.<hash>.js are served with the contents of the /static/js/main.<hash>.js file. For example, Python contains a built-in HTTP server that can serve static files:


cd build
python -m SimpleHTTPServer 9000


If you're using Node and Express as a server, it might look like this:


const express = require('express');
const path = require('path');
const app = express();

app.use(express.static('./build'));

app.get('/', function (req, res) {
  res.sendFile(path.join(__dirname, './build', 'index.html'));
});

app.listen(9000);


Create React App is not opinionated about your choice of web server. Any static file server will do. The build folder with static assets is the only output produced by Create React App.


However this is not quite enough if you use client-side routing. Read the next section if you want to support URLs like /todos/42 in your single-page app.


Serving Apps with Client-Side Routing


If you use routers that use the HTML5 pushState history API under the hood (for example, React Router with browserHistory), many static file servers will fail. For example, if you used React Router with a route for /todos/42, the development server will respond to localhost:3000/todos/42 properly, but an Express serving a production build as above will not.


This is because when there is a fresh page load for a /todos/42, the server looks for the file build/todos/42 and does not find it. The server needs to be configured to respond to a request to /todos/42 by serving index.html. For example, we can amend our Express example above to serve index.html for any unknown paths:


 app.use(express.static('./build'));

-app.get('/', function (req, res) {
+app.get('/*', function (req, res) {
   res.sendFile(path.join(__dirname, './build', 'index.html'));
 });


Now requests to /todos/42 will be handled correctly both in development and in production.


Building for Relative Paths


By default, Create React App produces a build assuming your app is hosted at the server root.

To override this, specify the homepage in your package.json, for example:


  "homepage": "http://mywebsite.com/relativepath",


This will let Create React App correctly infer the root path to use in the generated HTML file.


Serving the Same Build from Different Paths




Note: this feature is available with [email protected] and higher.




If you are not using the HTML5 pushState history API or not using client-side routing at all, it is unnecessary to specify the URL from which your app will be served. Instead, you can put this in your package.json:


  "homepage": ".",


This will make sure that all the asset paths are relative to index.html. You will then be able to move your app from http://mywebsite.com to http://mywebsite.com/relativepath or even http://mywebsite.com/relative/path without having to rebuild it.


Firebase


Install the Firebase CLI if you haven't already by running npm install -g firebase-tools. Sign up for a Firebase account and create a new project. Run firebase login and login with your previous created Firebase account.


Then run the firebase init command from your project's root. You need to choose the Hosting: Configure and deploy Firebase Hosting sites and choose the Firebase project you created in the previous step. You will need to agree with database.rules.json being created, choose build as the public directory, and also agree to Configure as a single-page app by replying with y.


    === Project Setup

    First, let's associate this project directory with a Firebase project.
    You can create multiple project aliases by running firebase use --add,
    but for now we'll just set up a default project.

    ? What Firebase project do you want to associate as default? Example app (example-app-fd690)

    === Database Setup

    Firebase Realtime Database Rules allow you to define how your data should be
    structured and when your data can be read from and written to.

    ? What file should be used for Database Rules? database.rules.json
    ✔  Database Rules for example-app-fd690 have been downloaded to database.rules.json.
    Future modifications to database.rules.json will update Database Rules when you run
    firebase deploy.

    === Hosting Setup

    Your public directory is the folder (relative to your project directory) that
    will contain Hosting assets to uploaded with firebase deploy. If you
    have a build process for your assets, use your build's output directory.

    ? What do you want to use as your public directory? build
    ? Configure as a single-page app (rewrite all urls to /index.html)? Yes
    ✔  Wrote build/index.html

    i  Writing configuration info to firebase.json...
    i  Writing project information to .firebaserc...

    ✔  Firebase initialization complete!


Now, after you create a production build with npm run build, you can deploy it by running firebase deploy.


    === Deploying to 'example-app-fd690'...

    i  deploying database, hosting
    ✔  database: rules ready to deploy.
    i  hosting: preparing build directory for upload...
    Uploading: [==============================          ] 75%✔  hosting: build folder uploaded successfully
    ✔  hosting: 8 files uploaded successfully
    i  starting release process (may take several minutes)...

    ✔  Deploy complete!

    Project Console: https://console.firebase.google.com/project/example-app-fd690/overview
    Hosting URL: https://example-app-fd690.firebaseapp.com


For more information see Add Firebase to your JavaScript Project.


GitHub Pages




Note: this feature is available with [email protected] and higher.




Step 1: Add homepage to package.json


The step below is important!

If you skip it, your app will not deploy correctly.


Open your package.json and add a homepage field:


  "homepage": "https://myusername.github.io/my-app",


Create React App uses the homepage field to determine the root URL in the built HTML file.


Step 2: Install gh-pages and add deploy to scripts in package.json


Now, whenever you run npm run build, you will see a cheat sheet with instructions on how to deploy to GitHub Pages.


To publish it at https://myusername.github.io/my-app, run:


npm install --save-dev gh-pages


Add the following scripts in your package.json:


  // ...
  "scripts": {
    // ...
    "predeploy": "npm run build",
    "deploy": "gh-pages -d build"
  }


The predeploy script will run automatically before deploy is run.


Step 3: Deploy the site by running npm run deploy


Then run:


npm run deploy


Step 4: Ensure your project's settings use gh-pages


Finally, make sure GitHub Pages option in your GitHub project settings is set to use the gh-pages branch:


gh-pages branch setting


Step 5: Optionally, configure the domain


You can configure a custom domain with GitHub Pages by adding a CNAME file to the public/ folder.


Notes on client-side routing


GitHub Pages doesn't support routers that use the HTML5 pushState history API under the hood (for example, React Router using browserHistory). This is because when there is a fresh page load for a url like http://user.github.io/todomvc/todos/42, where /todos/42 is a frontend route, the GitHub Pages server returns 404 because it knows nothing of /todos/42. If you want to add a router to a project hosted on GitHub Pages, here are a couple of solutions:


    

  • You could switch from using HTML5 history API to routing with hashes. If you use React Router, you can switch to hashHistory for this effect, but the URL will be longer and more verbose (for example, http://user.github.io/todomvc/#/todos/42?_k=yknaj). Read more about different history implementations in React Router.
    • 

  • Alternatively, you can use a trick to teach GitHub Pages to handle 404 by redirecting to your index.html page with a special redirect parameter. You would need to add a 404.html file with the redirection code to the build folder before deploying your project, and you’ll need to add code handling the redirect parameter to index.html. You can find a detailed explanation of this technique in this guide.
    • 



Heroku


Use the Heroku Buildpack for Create React App.

You can find instructions in Deploying React with Zero Configuration.


Resolving "Module not found: Error: Cannot resolve 'file' or 'directory'"


Sometimes npm run build works locally but fails during deploy via Heroku with an error like this:


remote: Failed to create a production build. Reason:
remote: Module not found: Error: Cannot resolve 'file' or 'directory'
MyDirectory in /tmp/build_1234/src  



This means you need to ensure that the lettercase of the file or directory you import matches the one you see on your filesystem or on GitHub.


This is important because Linux (the operating system used by Heroku) is case sensitive. So MyDirectory and mydirectory are two distinct directories and thus, even though the project builds locally, the difference in case breaks the import statements on Heroku remotes.


Modulus


See the Modulus blog post on how to deploy your react app to Modulus.


Netlify


To do a manual deploy to Netlify's CDN:


npm install netlify-cli
netlify deploy


Choose build as the path to deploy.


To setup continuous delivery:


With this setup Netlify will build and deploy when you push to git or open a pull request:


    

  1. Start a new netlify project
    2. 

  2. Pick your Git hosting service and select your repository
    3. 

  3. Click Build your site
    4. 



Support for client-side routing:


To support pushState, make sure to create a public/_redirects file with the following rewrite rules:


/*  /index.html  200



When you build the project, Create React App will place the public folder contents into the build output.


Now


See this example for a zero-configuration single-command deployment with now.


S3 and CloudFront


See this blog post on how to deploy your React app to Amazon Web Services S3 and CloudFront.


Surge


Install the Surge CLI if you haven't already by running npm install -g surge. Run the surge command and log in you or create a new account. You just need to specify the build folder and your custom domain, and you are done.


              email: [email protected]
           password: ********
       project path: /path/to/project/build
               size: 7 files, 1.8 MB
             domain: create-react-app.surge.sh
             upload: [====================] 100%, eta: 0.0s
   propagate on CDN: [====================] 100%
               plan: Free
              users: [email protected]
         IP Address: X.X.X.X

    Success! Project is published and running at create-react-app.surge.sh


Note that in order to support routers that use HTML5 pushState API, you may want to rename the index.html in your build folder to 200.html before deploying to Surge. This ensures that every URL falls back to that file.


Advanced Configuration


You can adjust various development and production settings by setting environment variables in your shell or with .env.






Variable
Development
Production
Usage






BROWSER
✅
❌
By default, Create React App will open the default system browser, favoring Chrome on macOS. Specify a browser to override this behavior, or set it to none to disable it completely.




HOST
✅
❌
By default, the development web server binds to localhost. You may use this variable to specify a different host.




PORT
✅
❌
By default, the development web server will attempt to listen on port 3000 or prompt you to attempt the next available port. You may use this variable to specify a different port.




HTTPS
✅
❌
When set to true, Create React App will run the development server in https mode.




PUBLIC_URL
❌
✅
Create React App assumes your application is hosted at the serving web server's root or a subpath as specified in package.json (homepage). Normally, Create React App ignores the hostname. You may use this variable to force assets to be referenced verbatim to the url you provide (hostname included). This may be particularly useful when using a CDN to host your application.




CI
🔶
✅
When set to true, Create React App treats warnings as failures in the build. It also makes the test runner non-watching. Most CIs set this flag by default.






Troubleshooting


npm start doesn’t detect changes


When you save a file while npm start is running, the browser should refresh with the updated code.

If this doesn’t happen, try one of the following workarounds:


    

  • If your project is in a Dropbox folder, try moving it out.
    • 

  • If the watcher doesn’t see a file called index.js and you’re referencing it by the folder name, you need to restart the watcher due to a Webpack bug.
    • 

  • Some editors like Vim and IntelliJ have a “safe write” feature that currently breaks the watcher. You will need to disable it. Follow the instructions in “Working with editors supporting safe write”.
    • 

  • If your project path contains parentheses, try moving the project to a path without them. This is caused by a Webpack watcher bug.
    • 

  • On Linux and macOS, you might need to tweak system settings to allow more watchers.
    • 



If none of these solutions help please leave a comment in this thread.


npm test hangs on macOS Sierra


If you run npm test and the console gets stuck after printing react-scripts test --env=jsdom to the console there might be a problem with your Watchman installation as described in facebookincubator/create-react-app#713.


We recommend deleting node_modules in your project and running npm install (or yarn if you use it) first. If it doesn't help, you can try one of the numerous workarounds mentioned in these issues:



It is reported that installing Watchman 4.7.0 or newer fixes the issue. If you use Homebrew, you can run these commands to update it:


watchman shutdown-server
brew update
brew reinstall watchman



You can find other installation methods on the Watchman documentation page.


If this still doesn't help, try running launchctl unload -F ~/Library/LaunchAgents/com.github.facebook.watchman.plist.


There are also reports that uninstalling Watchman fixes the issue. So if nothing else helps, remove it from your system and try again.


npm run build silently fails


It is reported that npm run build can fail on machines with no swap space, which is common in cloud environments. If the symptoms are matching, consider adding some swap space to the machine you’re building on, or build the project locally.


npm run build fails on Heroku


This may be a problem with case sensitive filenames.
Please refer to this section.


Something Missing?


If you have ideas for more “How To” recipes that should be on this page, let us know or contribute some!


   



      


  
      

      

        

          

            

  
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        Remake of Front End Interview test generator (react, JSON)
      

      

        
        
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