React/Redux Style Guide

React Guidelines

Our UI is built on React. React makes it easy to build modular, declarative, components.

Useful Resources:

Thinking in React - Facebook React Docs
You're missing the point of React - by Dan Abramov
Best Practices for building large React apps - by Alex Lopatin
Atomic Design - by Brad Frost (not React specific, but applicable)

Organization

Useful Resources:

How to Scale React Applications - by Max Stoiber
Smart & Dumb Components - by Dan Abramov
Organizing Large React Applications - by Nathanael Beisiegel
Feature First Organization - by Ryan C. Collins

§ 1.1 Component Types: Separate components into Container (smart) and Presentational (dumb) component types.

	Container Components	Presentational Components
Location	top level, route handlers	middle + leaf components
Aware of Redux	Yes	No
Reading Data	subscribe to Redux State	From props
Changing Data	dispatch Redux actions	invoke callbacks from props

"Don’t take the presentational and container component separation as a dogma. Sometimes it doesn’t matter or it’s hard to draw the line. If you feel unsure about whether a specific component should be presentational or a container, it might be too early to decide. Don’t sweat it!" -- Dan Abramov

§ 1.2 Container Components: Try to organize container components around the following principles:

Are concerned with how things work.
May contain both presentational and container components inside but usually don’t have any DOM markup of their own except for some wrapping divs, and never have any styles.
Provide the data and behavior to presentational or other container components.
Call Redux actions and provide these as callbacks to the presentational components.

§ 1.3 Presentational Components: Try to organize presentational components around the following principles:

Are concerned with how things look.
May contain both presentational and container components inside, and usually have some DOM markup and styles of their own.
Often allow containment/composition via this.props.children.
Have no dependencies on the rest of the app, such as Redux actions or state.
Don’t specify how the data is loaded or mutated.
Receive data and callbacks exclusively via props.
Rarely have their own state (when they do, it’s UI state rather than data).
Should be written as functional components unless they need state, lifecycle hooks, or performance optimizations.

§ 1.3 Component Folder Organization: Take advantage of feature-first organization and separate presentational and container components into separate, top-level folders in your source tree; with each component getting its own folder.

Have two top level folders: src/containers and src/components (optionally, src/pages for calling out a distinction for container components used as router entry points)
Using feature-first organization we build modular, reusable components by co-locating all the files related to that component in a single folder:

src/
  containers/
    PendingRequestList/
      index.js
	  PendingRequestList.jsx
	  test.spec.js
    ...
    
  components/
    RequestSummaryItem/
	  index.js
	  RequestSummaryItem.jsx
	  styles.scss
	  test.spec.js
	...

Each component folder contains all the behavior, styles, tests and/or other assets related to that component. The index.js is simply a rollup file to allow easier autoloading of the component module, while keeping the component in a self-named file to ease fuzzy finding in various editors/IDEs.
These component folders could even potentially contain sub-component folders organized the same way if those sub-components are directly related to or composed by the main component.

Why? React encourages a functional, declarative style that lends to better reuse. Taking a feature-first approach, and isolating all the files related to a given component in a single folder helps ensure better reusability of that component. With no outside dependencies, the component can be dragged/dropped as a folder into any other project; or even pulled out into it's own npm module/package.

You also get the added benefit of easing the cognitive load on developers, keeping them from having to search around the file system looking for the styles, tests or other assets related to the component they're working on.

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Components

Useful Resources:

React Patterns - by Michael Chan
A Conceptual Intro to React Components - by Mark Thomas
Higher Order Components: React Pattern - by Jack Franklin

§ 2.1 Keep Components Small: Keeping components small maximizes their potential for reuse, reduces the chance for bugs, allows them to focus on a single reponsibility and improves readability and testing.

Why? Rule of thumb is that if your render method has more than 10-20 lines it's probably way too big. The whole idea behind React is code re-usability (separation of concerns, single responsibility principle, declarative) so if you just throw everything in one file you are losing clarity and simplicity.

§ 2.2 One Component per file: You should try to put each component in its own file to ensure they are easier to read, maintain and test.

Why? This follows from 2.1 to help in keeping components focused while being easier to read and test. If you're unsure where a larger component might be better broken out into smaller components, it's ok to break out smaller components together in the same file; you can come back and refactor as the requirements/design become more clear and pull them out into separate files then.

§ 2.3 Use Composition to extend functionality: Use composition and Higher Order Components (HoC's) to extend other components or add functionality.

Composition of components is handled via this.props.children, allowing one component to render the output of one or more components by containing them as nested components.

{/* Composition via children */}
<Modal show={true}>
  <ModalTitle>My Modal</ModalTitle>
  <ModalBody>
	  Welcome to composition!
  </ModalBody>
</Modal>

A higher-order component (HoC) is just a function that takes an existing component and returns another component that wraps it. The HoC could do any of the following:
- Do things before and/or after it calls the wrapped component
- Avoid rendering the wrapped component if certain criteria is not met
- Update the props passed to the wrapped component, or add new props
- Transform the output of rendering the wrapped component (e.g. wrap with extra DOM elements, etc.)
Example - React Timer using HoC (live demo)

// on-interval.jsx
// HoC to allow us to refresh a component 
// based on a timing interval 
const onInterval = (refresh) => (WrappedComponent) => {
  return class WithInterval extends Component {
	constructor(props) {
      super(props);
	  this.state = { ticks: 0 };
	  this.interval = setInterval(this.tick.bind(this), refresh);
	}
    
    tick() {
      this.setState({ ticks: this.state.ticks + 1 })
    }
    
    componentWillUnmount() {
	  clearInterval(this.interval);
	}
    
    render() {
      return <WrappedComponent {...this.props} />;
	}
  };
};
export default onInterval;

// timer.js
// Component to display current time HH:MM:SS A/PM
import onInterval from './on-interval';

const Timer = ({ label }) => {
  const now = new Date();
  const hours = (now.getHours() % 12) || 12;
  const mins = now.getMinutes();
  const secs = now.getSeconds();
  return (<p>
    <b>{label && `${label}: `}</b> 
    {(hours < 10 ? '0' : '') + hours}:
    {(mins < 10 ? '0' : '') + mins}:
    {(secs < 10 ? '0' : '') + secs}
    {hours < 12 ? ' PM' : ' AM'}
  </p>);
};

export default onInterval(1000)(Timer);

// app.js
import Timer from './timer';
import { render } from 'react-dom';

render(<Timer label="Current Time"/>, document.querySelector('#app'));

// Output, updating every second
// => Current Time: 12:35:28 AM
// => Current Time: 12:35:29 AM
// => ...

§ 2.4 Always declare propTypes, defaultProps and displayName: Always declare prop types and display name for all components (container or presentational); and declare default props for any non-required props.

Why? propTypes are a form of documentation, and providing defaultProps means the reader of your code doesn’t have to assume as much. In addition, it can mean that your code can omit certain type checks.

// bad
function Bookend({ left, right, children }) {
  return <div>{foo}{children}{bar}</div>;
}
Bookend.propTypes = {
  left: PropTypes.number.isRequired,
  right: PropTypes.string
};

// good
function Bookend({ left, right, children }) {
  return <div>{left}{children}{right}</div>;
}
Bookend.displayName = 'Bookend';
Bookend.propTypes = {
  left: PropTypes.string.isRequired,
  right: PropTypes.string,
  children: PropTypes.node
};
Bookend.defaultProps = {
  right: '',
  children: null,
};

default props on functional components can also be handled using ES2015 default argument values.

function Bookend({ left, right = '', children = null }) { ... }

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Container Components

Useful Resources:

Presentational & Container Components - by Dan Abramov
Container Components by Brad Westfall
Understanding the React Component Lifecycle - by A. Sharif

§ 3.1 Containers understand behavior and state: As it relates to the application, container components understand the behavior and state needed to operate. Regardless of where that state comes from, the container's responsibility is to manage that state and provide the appropriate application level behaviors, via functions; and pass those down to presentational components via props.

// container component
class NumberListContainer extends Component {
  constructor(props) {
    super(props);
    this.state = { numbers: [1,2,3,4,5] };
  }
  render() {
    return <NumberList {...this.state}/>;
  }
}

// presentational components
const NumberList = ({ numbers }) => (
  <ul>{ numbers.map(n => <Number value={n}/>) }</ul>
);

const Number = ({ value }) => <li>{value}</li>;

§ 3.2 Connect Containers to state using connect(): Containers, likely, will need access to the global Redux state. Use react-redux's connect() higher order component to connect to the Redux store and receive state as props when Redux state changes.

// src/containers/MyContainer/my-container.jsx
import { connect } from 'react-redux';

export class MyContainer extends Component { ... }

const mapStateToProps = (state) => ({
  // map state to component props...
});
const mapDispatchToProps = (dispatch) => ({
  // map dispatch to action creators to props...
});

export default connect(mapStateToProps, mapDispatchToProps)(MyContainer);

mapStateToProps() is used to assign slices of your redux state to properties that should be passed to your component every time state changes
mapDispatchToProps() allows you to pass action creators as props to your component and bind them in a call to the redux store's dispatch() method.

§ 3.3 Containers should not perform layout: Containers components should perform data fetching, pull necessary data from state or compute dynamic state, provide behavior as callback methods and then pass those items as props to presentational components for rendering.

// bad
@connect(
  (state) => ({ comments: state.comments }),
  { fetchComments }
)
class CommentList extends React.Component {
  componentDidMount() {
    this.props.fetchComments();
  }
  render() {
    return (
      <ul>
      { this.props.comments.map(renderComment) }
      </ul>
    );
  }
  renderComment({body, author}) {
    return <li>{body}—{author}</li>;
  }
}

// good
@connect(
  (state) => ({ comments: state.comments }),
  { fetchComments }
)
class CommentListContainer extends React.Component {
  componentDidMount() {
    this.props.fetchComments();
  }
  render() {
    return <CommentList comments={this.props.comments} />;
  }
}

// Presentational Components
const Comment = ({ body, author }) => (
  <li>{body}—{author}</li>
);

const CommentList = ({ comments }) => (
    <ul>
     {this.props.comments.map(comment => <Comment {...comment}/>)}
    </ul>
);

Why? This separates concerns, allow our Container to focus on just gather data while layout concerns are handled by presentational components.

§ 3.4 Export both the connected and non-connected component: Container components should export the undecorated component (for unit testing) as a named export and the connected component as the default export (for use within the app).

// export named component
export class MyContainer extends Component { ... }

// export connected component as default
export default connect(...)(MyContainer);

Why? When writing unit tests, we don't need to test the functionality of react-redux's <Provider/> or the connect() function itself. You can safely assume those have been tested and do their job. What you want to test is the actual container component itself. By exporting the un-connected container, you can easily mock plain objects as props to test the container across different scenarios.

This also makes the unit tests simpler to design and setup, as you reduce the amount of boilerplate you need for the test to work.

§ 3.5 Fetch data within the correct lifecycle method: If you're not doing server-side rendering (SSR) then componentDidMount() or the constructor() is the best place to handle async requests for resources.

Why? The component's constructor() and componentWillMount() are called on both server-side and client-side; while componentDidMount() is called only on client-side.

Here's some relevant clarification from the React documentation (see lifecycle methods for more details)

constructor(props) The constructor for a React component is called before it is mounted. The constructor is the right place to initialize state. If you don't initialize state and you don't bind methods, you don't need to implement a constructor for your React component.

componentWillMount() is invoked immediately before mounting occurs. It is called before render(), therefore setting state in this method will not trigger a re-rendering. Avoid introducing any side-effects or subscriptions in this method.

This is the only lifecycle hook called on server rendering. Generally, we recommend using the constructor() instead.

componentDidMount() is invoked immediately after a component is mounted. Initialization that requires DOM nodes should go here. If you need to load data from a remote endpoint, this is a good place to instantiate the network request. Setting state in this method will trigger a re-rendering.

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Presentational Components

Useful Resources:

Make your React Components Pretty - by Mark Brouch
Get your App outta my Component - by Adam Terlson

§ 4.1 Presentational Components should be stateless functional components: Presentational components (should) rely on props only and don't manage or access state. Writing them as stateless functional components keeps them simple and allows various perf improvements within React's rendering of the virtual-dom.

rely on props only (leave state management to containers)
are pure (deterministic) functions, their output is solely determined by their input (props). And, they don't cause external side-effects.

// bad
class Author extends Component {
  render() { 
    const { author, email } = this.props;
    return <p><b>{author}</b> - {email}</p>;
  }
}

// good
const Author = ({ author, email }) => (
  <p><b>{author}</b> - {email}</p>
);

§ 4.2 (if used) Local state should be UI specific or transient; and minimal: Local state is not "bad", per se; and there are instances where it can be useful. If the state in consideration is useful to other components in the application or should be modified by other components, move it from local state to the Redux store.

"There is so much FUD about local state. “setState is bad”, “no-set-state”, “root of all evil”, “use functional components”. State is fine.

When people say setState() is an anti-pattern, they mean 500-line components with a complex setState() soup scattered across event handlers.

There is no “anti-pattern” in having a <DropDown> keep isOpen in local state. If it was an anti-pattern, React wouldn’t have this feature."

-- Dan Abramov via twitter

§ 4.2.1 Expose any local state via props: When your component does have any local state, you should expose it via props to allow sharing and persistence of that state at the application level, if needed.

// bad
class FlipCard extends Component {
  constructor(props) {
    super(props);
    this.state = { flipped: false };
  }

  toggleFlip() {
    this.setState({ flipped: !this.state.flipped });
  }

  render() {
    const { flipped } = this.state;
    return (
      <div className={flipped ? 'showBack' : 'showFront' }/>
        <span className="flip-control" onClick={() => this.toggleFlip()} />
        {/* ... */}
      </div>
    );
}

Why? The initial state of the component, flipped or not, can't be controlled by the application in the above code. You'll always get a card that's initially showing the front; and the application has no way of knowing if/when the card gets flipped. Although the component is reusable, it is less useful to applications that wish to incorporate it.

// good
class FlipCard extends Component {
  static propTypes = {
    // expose internal state via props
    flipped: PropTypes.bool,
    onFlip: PropTypes.func
    // ...
  };
  
  constructor(props) {
    super(props);
    this.state = { 
      flipped: props.flipped || false
    };
  }
  
  toggleFlip() {
    const { onFlip } = this.props;
    this.setState({ 
      flipped: !this.state.flipped 
    }, () => onFlip && onFlip(this.state.flipped));
  }
  
  render() {
    const { flipped } = this.state; 
    return (
      <div className={flipped ? 'showBack' : 'showFront' }/>
        <span className="flip-control" onClick={() => this.toggleFlip()} />
        {/* ... */}
      </div>
    );
  }
}

Why? With very little effort, we can keep our component reusable, and allow applications that manage state at the global level with Redux, Flux, etc. to persist that state and have more control over the behavior of the component (if desired).

§ 4.2.2 Reusable components should minimize assumptions and dependencies: Components that may be used across projects, or put in a shared npm package, should make no assumptions about the application architecture; and minimize any dependencies.

Why? When writing components we should always be thinking about applicability and reusability. If we keep components agnostic about the application environment they are running in, we gain a more solid, flexible component or library that will be useful for projects over a longer period.

Assumptions like:

our applications will always use Redux, or redux-thunk, ...

we'll always have bootstrap's css globally available, so I'll use those class names...

jQuery or fetch will always be there for AJAX calls...

will end up coupling the component or library to a very specific application stack. Application stacks change, new projects might choose to go a different direction; and for whatever reason, those components now no longer work and must be abandoned or re-written.

Don't make assumptions when writing reusable components; and keep dependencies to a minimum; and when there is a dependency, make it explicit.

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JSX

§ 5.1 Keep conditional rendering clean and simple: Handle conditional rendering in your component by using one of the following techniques to help keep code clean, readable and easy to extend:

Using if..else outside of JSX, or a ternary ?: inline w/ the JSX

// good
render() {
  if (this.props.username) {
    return <UserGreeting user={this.props.username}/>;
  }
  else {
    return <GuestGreeting />;
  }
}

// good 
render() {
  return this.props.username ?
    <UserGreeting user={this.props.username} /> :
    <GuestGreeting>;
}

if the condition is complex or it clutters the render() logic, pull it out into a separate method.

class Header extends Component {
  isValidUser({ user } = this.props) {
    return user.isLoggedIn && user.hasEntitlement('ADMIN');
  }
  render() {
    const { user } = this.props;
    return (
      <div>
        <h1>{title}</h1>
        <div className="pull-right">
          { this.isValidUser(user) ?
              <UserNavProfile user={user}/> :
              <GuestNavProfile />
           }
        </div>
      </div>
    );
  }
}

Simple conditional tests using &&

// good
render() {
  const { error } = this.props;
  return (
    <p>
      { error && <Notification message={error}/> }
    </p>
  );
}

Tip: When using conditional expressions in JSX, be sure your conditional expression evaluates to a one of: false, null, undefined, and true which are values ignored by React for rendering. Some falsy values, like the number 0 (zero), are still rendered by React, so the following would actually output the string 0 for the conditional expression if the records array had zero length:

// bad
const Rows = ({ records }) => ( 
  <div>
    { records.length && records.map(/*...*/) }
  </div>
);

Tip: If a condition becomes too complex or cumbersome, it might be time to pull that portion out into a new, smaller component to encapsulate that logic.

§ 5.2 Use map to output/render a list of items: JSX allows any valid javascript expression within {} curly braces, which allows you to use functions like Array#map to loop over lists and return an array of valid JSX elements.

Why? Keep in mind that React/JSX will attempt to render the result of any valid javascript expression that doesn't result in one of these values:

false
null
undefined
true

So, returning any other value, including JSX, or an array of any of the above, will get rendered. Since map(), and even related methods like filter() and reduce() return arrays, they can all be used to iterate over lists inline with JSX.

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Component Styles

Useful Resources:

Modular CSS with React - by Philippe Masset
CSS Modules: Welcome to the Future - by Glen Maddern
What are CSS Modules and Why do we need them - by Robin Rendle
npm: classnames - author Jed Watson

§ 6.1 Use CSS Modules for component styles: each React component gets its own CSS file, which is scoped to that file and component. The magic happens at build time, when local class names – which can be super simple without risking collisions – are mapped to automatically-generated ones and exported as a JS object literal to use within React

// thumbnail/styles.scss 
.image {
  border-radius: 3px;
}

// thumbnail/thumbnail.jsx 
import styles from './styles.scss';
export const Thumbnail = () => (
  <img className={styles.image}/>
);

/* Rendered DOM */
<img class="Thumbnail__image___1DA66"/>

/* Processed thumbnail/styles.scss */
.Thumbnail__image___1DA66 {
  border-radius: 3px;
}

§ 6.2 Following feature-first, co-locate styles with their component: Using feature-first principles, we keep to our modular organization by co-locating the component's style in the same folder as the component code. Depending on your webpack setup, you can use plain css, or scss (SaSS) or less (Less) for writing styles using CSS modules.

MyComponent/
  index.js
  MyComponent.jsx
  styles.scss

§ 6.3 Use classnames for handling component classes: The classnames module is a useful utility for building up dynamic, conditional class names for use within React's className prop, which takes a string.

// bad
class Button extends Component {
  // ... 
  render () {
    var btnClass = 'btn';
    if (this.state.isPressed) btnClass += ' btn-pressed';
    else if (this.state.isHovered) btnClass += ' btn-over';
    return <button className={btnClass}>{this.props.label}</button>;
  }
}

// good
import cx from 'classnames';
 
class Button extends Component {
  // ... 
  render () {
    var btnClass = classNames('btn', {
      'btn-pressed': this.state.isPressed,
      'btn-over': !this.state.isPressed && this.state.isHovered
    });
    return <button className={btnClass}>{this.props.label}</button>;
  }
}

Why? Using a utility likes classnames can help pull out CSS class name logic into one place, simplify building the full class string and makes it easier to both read and extend if styling changes

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Naming Things

§ 7.1 Use consistent naming for events and event handlers: Name the handler methods after their triggering event.

Event handlers should:

begin with handle
end with the name of the event they handle (eg, Click, Change)
be present-tense

// bad
punchABadger () { /*...*/ },

render () {
  return <div onClick={this.punchABadger} />;
}

// good
handleClick () { /*...*/ },

render () {
  return <div onClick={this.handleClick} />;
}

Event names:

in props should start with on
should not clash with builtin/native event names, eg. use onSelect instead of onFocus or onClick

handleClick() { 
  const { onFlip } = this.props;
  onFlip(this.state.flipped);
}

render() {
  return (
    <div onClick={this.handleClick}>
      { this.props.children }
    </div>
  );
}

Tip: If you need to disambiguate handlers or event names, add additional information between handle or on and the event name. For example, you can distinguish between onChange handlers: handleNameChange and handleAgeChange and use props onNameChange and onAgeChange, respectively.

When you do this, ask yourself if you should be creating a new component.

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Redux

Currently, Velmat Inventory uses Redux for application state management.

Useful Resources:

Redux Docs
Getting Started with Redux - (video series) by Dan Abramov
Idiomatic Redux - (video series) by Dan Abramov
A maintainable project struture for React/Redux - by Matteo Mazzarolo

Organization

§ 8.1 Co-locate reducers, actions, action-types and reducers (similar to ducks standard): Organizing your redux related code around the reducer (the slice of store state it manages) by bundling your actions, action types w/ the reducer helps organize your redux code into reusable modules, is clean, reduces unnecessary boilerplate and eases developer effort by co-locating.

Tip: Our suggested format is a slight modification of the DUCKS standard which is widely adopted. Instead of including the reducer, actions, types in a single file, we separate them to ease usage/importing and keep them in a shared folder for the module.

A redux module (bundled folder) can be organized as follows:

src/redux-modules/
  authorization/
    index.js     # default exports reducer
    actions.js   # named exports of action creators
    types.js     # exports action type constants
    selectors.js # exports selectors

Example:

// src/redux-modules/auth/index.js
import * as types from './types';

// initial state for reducer
const initialState = { /* ... */ };

// reducer
export default (state = initialState, action) => {
  switch (action.type) {
    case types.ACTION_TYPE: return {
        ...state,
        /* modify state in immutable fashion */
      };
    /* other ACTION_TYPEs... */
    default: 
      return state;
  }
};

// src/redux-modules/auth/types.js

// named action type constants
export const ACTION_TYPE_ONE = '@@auth/action-type-one';
export const ACTION_TYPE_TWO = '@@auth/action-type-two';
/* ... */

// src/redux-modules/auth/actions.js
import * as types from './types';

export const actionCreator = (/* params... */) => ({
  type: types.ACTION_TYPE_ONE,
  /* params... */
});

/* other action creators... */

Why? This helps us build modules that:

focused on the managing a slice of application state, making for better reuse across pages/components

keeps us from coupling redux state w/ container components

gives us an easy way to import reducers, as well as action creators across components, and action types across other redux modules if needed.

Grouping by file type, ie. src/actions, src/reducers, src/action-types, src/selectors, etc. doesn't scale well for large applications, is less reusable because files related to a slice of state are spread out over the filesystem; and making changes means developers end up having to edit multiple files all over the filesystem as well.

For example... Importing reducer(s) is easy:

import authReducer from 'redux-modules/authorization';
import otherReducer from 'redux-modules/other';

const reducer = combineReducers({
  authReducer,
  otherReducer
});

Reusing actions across components:

// some-component.js
import * as authActions from 'redux-modules/authorization/actions';
import * as otherActions from 'redux-modules/other/actions';

// ...
const mapDispatchToProps = (dispatch) => ({
  ...bindActionCreators({
    ...authActions,
    ...otherActions
  }, dispatch)
});
// ...

Listening to actions across reducers:

// redux-modules/some-other/index.js
import * as authTypes from 'redux-modules/auth/types';
import * as types from './types';

// reducer...
export default (state = {}, action) => {
  case authTypes.OUTSIDE_ACTION:
    /* respond to OUTSIDE_ACTION somehow ... */
  case types.OWN_ACTION:
    /* handle our own actions ... */
  // ...
};

§ 8.2 Related to 8.1 colocate your unit tests for your reducer and actions with the redux-module: Since we're bundling our redux code to be reusable as modules, keeping the unit tests for each module colocated with the bundle makes sense.

Why? Similar to the feature-first organization for components and the reasons for our previous rule, with the unit tests bundled, this redux-module folder could easily be dropped into another project "as is" or easily pulled out into an npm module for wider reuse.

For clear ideas on testing redux reducers and actions, see Writing Tests in the Redux docs and Testing Redux Applications by Randy Coulman

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Action Creators

§ 9.1 Use redux-thunk and async/await for asynchronous action creators As a first resort, using the redux-thunk middleware, along with taking advantage of async/await is a clean approach to organizing asynchronous action creators.

Why? Using the redux-thunk middleware allows you to create asynchronous actions in a manageable way. You can use other potential redux middleware for this as well, like redux-promise or redux-sagas; but using thunks will likely handle most of your needs.

Taking advantage of ES2016's async/await syntax when doing async fetch request will also make your code cleaner and easier to follow vs using Promises and chained/nested .then() handlers. The more complex the response to the various sequence of actions needs to be, the more cumbersome, nested and unreadable the Promise chain tends to be. Using async and await allows you to write asynchronous code the reads like it was synchronous.

// bad
const complexAction = (/*...*/) => (dispatch) => {
  dispatch(doRequestAction(/*...*/));
  fetch(/*...*/)
  .then(handleResponse)
  .then(
    json => dispatch(doSuccessAction(json)),
    error => dispatch(doErrorAction(json))
  );
};

// good
const complextAction = (/*...*/) => async (dispatch) => {
  dispatch(doRequestAction(/*...*/);
  try {
    const json = await fetch(/*...*/);
    dispatch(doSuccessAction(/*...*/);
  }
  catch (err) {
    dispatch(doErrorAction(/*...*/);
  }
};

§ 9.2 Don't attempt to access store state from async redux-thunk action creators When using redux-thunk, the returned function is passed two parameters, 1) the dispatch method from the store, and 2) the getState method of the store. To keep action creators, and their redux modules decoupled from the application's store structure it's best to pass needed state as props to the action creator rather than use getState().

It's tempting to move conditional logic into the action creators when using redux-thunk because of the availability of the store's getState method as the second param:

// brittle
const loadTodo = (id) => (dispatch, getState) => {
  // only fetch the todo if it isn't already loaded
  if (!getState().todos.includes(id)) {
    const todo = await fetch(`/todos/${id}`);
    dispatch(addTodo(todo));
  }
}

Why? This kind of state-dependent logic in the action creator is brittle because the action creator is now tied to the global state shape decided by the application. If the application decided to change the state shape (ie., nested reducers in combineReducers or changed the state key the todos reducer is assigned to) this action creator would break. This problem is compounded if you have multiple action creators all relying on a specific state shape and key names.

A better approach is to do one of the following:

move the determination to call an action based on state to a container component, making the check in componentDidMount(), componentWillReceiveProps(), or appropriate event handler on container:

// good
// redux-module/actions
const loadTodo = (id) => (dispatch) => {
  const todo = await fetch(`/todos/${id}`);
  dispatch(addTodo(todo));
};

// src/containers/Container
@connect(
  (state) => ({ todos: selectTodos(state) }),
  (dispatch) => { loadTodo }
)
class Container extends Component {
	handleClick(id) {
	  const { todos, loadTodo } = this.props;
	  if (!todos.includes(id)) {
	    loadTodo(id);
	  }
	}
	//...
}

pass the required state or props needed for the conditional logic to the action creator as params:

// good
const loadTodo = (id, todos) => (dispatch) => {
  // only fetch the todo if it isn't already loaded
  if (!todos.includes(id)) {
    const todo = await fetch(`/todos/${id}`);
    dispatch(addTodo(todo));
  }
}

or, if the logic in question is being used by multiple components, you can dynamically access the correct state slice by passing the state key to access rather than the actual state (as in the last example), allowing us to keep the logic in one place and not duplicate across components:

// use lodash's `_.get` to create a generic selector that
// can be used to access dynamic paths within a state object
// https://lodash.com/docs/4.17.2#get
const getStateIn = (state, path) => _.get(state, path);

const loadTodo = (id, todosPath) => (dispatch, getState) => {
  // only fetch the todo if it isn't already loaded
  if (!getStateIn(getState(), todosPath).includes(id)) {
    const todo = await fetch(`/todos/${id}`);
    dispatch(addTodo(todo));
  }
}

// given state shape of
// { 
//    collections: {
//      todos: [],
//      contacts: [],
//    }
// }
loadTodos(4, 'collections.todos');

In the above implementation, the application can change the shape of the state and as long as it passes the correct key path, the action creators stay decoupled and don't have to be modified.

Tip: Note that #1 or #3 above can run into problems when relying on getState() if another async action is dispatched prior to accessing the state. dispatch calls are synchronous; but when dispatching another async action, the state might get modified before or after the use of getState(), which still makes state access brittle and prone to hard to debug errors.

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Reducers

§ 10.1 Reducers can't be reused without implementing Higher Order Reducers In order to reuse reducer logic in your store (or have multiple instances of a connected component) you'll need to use a higher order reducer to create instance specific reducers for each slice of state.

it's tempting to think that just reusing a reducer is simple. Consider a simple set of counters:

 // reducer
 const counter = (state = 0, action) => {
   switch(action.type) {
     case INC: return state + 1;
     case DEC: return state - 1;
     default: return state;
   }
 }

 const rootReducer = combineReducers({
 	counterA: counter,
 	counterB: counter,
 	counterC: counter
 });

However, after dispatching an INC action, all three counters would be equal to 1. This is because combineReducers calls each slice-reducer with the same action.

To get this to work correctly, you'll need to create a higher order reducer (a reducer factory, essentially) that takes a reducer as an argument and returns a new reducer that modifies the behavior of the passed reducer.

We can do this in a generic fashion by creating a higher order reducer that takes a reducer and a predicate function for checking the action. If the predicate function returns true, it will pass the action on to the original reducer; otherwise, it just returns the state "as is."

// higher-order reducer, to limit a reducer to work 
// on actions matching a predicate only
const limited = (reducer, predicate) => (state, action) => {
  if (predicate(action)) {
    return reducer(state, action);
  }
  return state;
};

const rootReducer = combineReducers({
	counterA: limited(counter, action => action.name == 'A'),
	counterB: limited(counter, action => action.name == 'B'),
	counterC: limited(counter, action => action.name == 'C')
});

// Now, just dispatch actions with an added 
// `name` property to identify the proper counter.
dispatch({ type: INC, name: 'A' });
dispatch({ type: DEC, name: 'C' });
// => new state
//  {
//    counterA: 1,
//    counterB: 0,
//    counterC: -1
//  }

§ 10.2 Use string constants instead of inline strings for action types There are a number of benefits to defining action types as string constants, rather than as inline strings in the reducers and action creators.

It helps keep the naming consistent, and allows you to gather all action types in one place. (could be in same file as reducer, actions or in separate file)
It's helpful to see all the existing actions before working on a new feature. Maybe someone added the action you need already.
Easy to see changes and scope of features on pull requests by visually diffing new/removed/changed action types.
You'll get undefined if you mistype a constant. Redux will immediately throw when dispatching such an action, and you'll find the mistake sooner.

// bad
const reducer(state = {}, action) {
  switch(action.type) {
    case 'DO_STUFF': //...
    case 'GET_STUFF': //...
    //...
  }
}
const doStuff = () => ({ type: 'DO_STUFF', /*...*/ });
const getStuff = () => ({ type: 'GET_STUFF', /*...*/ });

// good
const DO_STUFF = '@@stuff/do-it';
const GET_STUFF = '@@stuff/get-it';

const reducer(state = {}, action) {
  switch(action.type) {
    case DO_STUFF: //...
    case GET_STUFF: //...
    //...
  }
}
const doStuff = () => ({ type: DO_STUFF, /*...*/ });
const getStuff = () => ({ type: GET_STUFF, /*...*/ });

Tip: Using a prefix for the action type based on the reducer/actions they work (the @@stuff/* above) with is a good way to namespace your action types and ensure you don't get any collisions across reducers when using simple names like ADD_RECORD or GET_TODO and end up with an unstable state.

§ 10.2 Use selectors liberally, and co-locate selectors with their reducer Reducers represent a slice of your data store, including it's "shape", and selectors are the query language for accessing that state at the component level. Keep them together and use them to decouple your state model from your view.

If components only use selectors to access state, typically via a call to mapStateToProps() when using react-redux, you've effectively decoupled your view from your state model and logic.

// bad
const initialState = { 
  step: 1,
  counter: 0 
};

const reducer(state = initialState, action) { /*...*/ }

// View Component
@connect((state) => ({ 
	count: state.counter
	nextCount: state.counter + state.step
})
//...

Here, if we change our reducer or state shape, any component accessing that state directly will break. Using state directly, as above, tightly couples your components to your redux store.

// good
const initialState = { 
  step: 1,
  counter: 0 
};

const reducer(state = initialState, action) { /*...*/ }

// simple selectors
const selectCount = (state) => state.counter;
const selectNextCount = (state) => state.counter + state.step;

// View Component
@connect((state) => ({ 
	count: selectCount(state),
	nextCount: selectNextCount(state)
})
//...

In this example, if we rename our state key from counter to count, any component using our selectors to access state still works without changes. We are decoupled and can make changes to just the reducer/selectors.

§ 10.3 Keep reducers pure, with no side-effects Reducers should be pure functions, ie. deterministic. Redux works on the assumption that your state is immutable; and a reducer is intended to accept a state₁, along with an action and return a completely new state₂ (or the exact same state if nothing has changed).

**reducer** `=>` ( **state**_cur, **action** ) `=>` **state**_next
A pure function is a function where the return value is only determined by its input values, without observable side effects.

Side effects like ajax calls, logging, dom updates, etc, should never be done within a reducer.

§ 10.4 Try to keep your state shape flat; and normalize data where possible [TODO]...

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Connecting Components

[TODO]

Always use a state selector function that returns the minimal state subset you need.
Expose the connected component as default export.
Expose the unconnected component as named export for unit testing.

Organizing Redux State

[TODO]

Naming Things

action types - use constants, [NOUN]_[VERB], eg. USER_FETCH or MATERIAL_REQUEST_UPDATE
action creators - [verb][Noun]() eg. fetchCurrentUser(), acceptMaterialRequest()
selectors - get[NOUN]() or select[NOUN]() eg. getSelectedCards(), selectActiveRows()

Utility/Helper Functions

You'll likely need or want to build utility functions or libraries of them that can be reused by components and redux actions. Organize them under a src/utils/ folder and follow some basic guidelines.

Group related util functions under a common name. ie, async helper functions might be in src/utils/async.js
Expose each function as named export.
Don't use a default export.
Util functions must be pure.
Util functions should be reusable, but have a single purpose.

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luaysuarna/react-redux-style-guide.md

React/Redux Style Guide

Table of Contents:

React Guidelines

Organization

Components

Container Components

Presentational Components

JSX

Component Styles

Naming Things

Redux

Organization

Action Creators

Reducers

Connecting Components

Organizing Redux State

Naming Things

Utility/Helper Functions