# Components

# Creating Components

This is an extension of Basic Example:

# Basic Structure

import React, { Component } from 'react';
import { render } from 'react-dom';

class FirstComponent extends Component {
    render() {
        return (
            <div>
                Hello, {this.props.name}! I am a FirstComponent.
            </div>
        );
    }
}

render(
    <FirstComponent name={ 'User' } />,
    document.getElementById('content')
);

The above example is called a stateless component as it does not contain state (in the React sense of the word).

In such a case, some people find it preferable to use Stateless Functional Components, which are based on ES6 arrow functions.

# Stateless Functional Components

In many applications there are smart components that hold state but render dumb components that simply receive props and return HTML as JSX. Stateless functional components are much more reusable and have a positive performance impact on your application.

They have 2 main characteristics:

  1. When rendered they receive an object with all the props that were passed down
  2. They must return the JSX to be rendered
// When using JSX inside a module you must import React
import React from 'react';
import PropTypes from 'prop-types';

const FirstComponent = props => (
    <div>
        Hello, {props.name}! I am a FirstComponent.
    </div>
);

//arrow components also may have props validation
FirstComponent.propTypes = {
    name: PropTypes.string.isRequired,
}

// To use FirstComponent in another file it must be exposed through an export call:
export default FirstComponent;

# Stateful Components

In contrast to the 'stateless' components shown above, 'stateful' components have a state object that can be updated with the setState method. The state must be initialized in the constructor before it can be set:

import React, { Component } from 'react';

class SecondComponent extends Component {
    constructor(props) {
        super(props);

        this.state = {
            toggle: true
        };

        // This is to bind context when passing onClick as a callback
        this.onClick = this.onClick.bind(this);
    }

    onClick() {
        this.setState((prevState, props) => ({
            toggle: !prevState.toggle
        }));
    }
    
    render() {
        return (
            <div onClick={this.onClick}>
                Hello, {this.props.name}! I am a SecondComponent.
                <br />
                Toggle is: {this.state.toggle}
            </div>
        );
    }
}

Extending a component with PureComponent instead of Component will automatically implement the shouldComponentUpdate() lifecycle method with shallow prop and state comparison. This keeps your application more performant by reducing the amount of un-necessary renders that occur. This assumes your components are 'Pure' and always render the same output with the same state and props input.

# Higher Order Components

Higher order components (HOC) allow to share component functionality.

import React, { Component } from 'react';

const PrintHello = ComposedComponent => class extends Component {
    onClick() {
        console.log('hello');
    }
    
    /* The higher order component takes another component as a parameter 
    and then renders it with additional props */
    render() {
        return <ComposedComponent {...this.props } onClick={this.onClick} />
    }
}

const FirstComponent = props => (
    <div onClick={ props.onClick }>
        Hello, {props.name}! I am a FirstComponent.
    </div>
);

const ExtendedComponent = PrintHello(FirstComponent);

Higher order components are used when you want to share logic across several components regardless of how different they render.

# Basic Component

Given the following HTML file:

index.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8" />
    <title>React Tutorial</title>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.2.1/react.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.2.1/react-dom.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.34/browser.min.js"></script>
  </head>
  <body>
    <div id="content"></div>
    <script type="text/babel" src="scripts/example.js"></script>
  </body>
</html>

You can create a basic component using the following code in a separate file:

scripts/example.js

import React, { Component } from 'react';
import ReactDOM from 'react-dom';

class FirstComponent extends Component {
  render() {
    return (
      <div className="firstComponent">
        Hello, world! I am a FirstComponent.
      </div>
    );
  }
}
ReactDOM.render(
  <FirstComponent />, // Note that this is the same as the variable you stored above
  document.getElementById('content')
);

You will get the following result (note what is inside of div#content):

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8" />
    <title>React Tutorial</title>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.2.1/react.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.2.1/react-dom.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.34/browser.min.js"></script>
  </head>
  <body>
    <div id="content">
        <div className="firstComponent">
            Hello, world! I am a FirstComponent.
        </div>
    </div>
    <script type="text/babel" src="scripts/example.js"></script>
  </body>
</html>

# Nesting Components

A lot of the power of ReactJS is its ability to allow nesting of components. Take the following two components:

var React = require('react');
var createReactClass = require('create-react-class');

var CommentList = reactCreateClass({
  render: function() {
    return (
      <div className="commentList">
        Hello, world! I am a CommentList.
      </div>
    );
  }
});

var CommentForm = reactCreateClass({
  render: function() {
    return (
      <div className="commentForm">
        Hello, world! I am a CommentForm.
      </div>
    );
  }
});

You can nest and refer to those components in the definition of a different component:

var React = require('react');
var createReactClass = require('create-react-class');

var CommentBox = reactCreateClass({
  render: function() {
    return (
      <div className="commentBox">
        <h1>Comments</h1>
        <CommentList /> // Which was defined above and can be reused
        <CommentForm /> // Same here
      </div>
    );
  }
});

Further nesting can be done in three ways, which all have their own places to be used.

# 1. Nesting without using children

(continued from above)

var CommentList = reactCreateClass({
  render: function() {
    return (
      <div className="commentList">
        <ListTitle/>
        Hello, world! I am a CommentList.
      </div>
    );
  }
});

This is the style where A composes B and B composes C.

# Pros

  • Easy and fast to separate UI elements
  • Easy to inject props down to children based on the parent component's state

# Cons

  • Less visibility into the composition architecture
  • Less reusability

# Good if

  • B and C are just presentational components
  • B should be responsible for C's lifecycle

# 2. Nesting using children

(continued from above)

var CommentBox = reactCreateClass({
  render: function() {
    return (
      <div className="commentBox">
        <h1>Comments</h1>
        <CommentList>
            <ListTitle/> // child
        </CommentList>
        <CommentForm />
      </div>
    );
  }
});

This is the style where A composes B and A tells B to compose C. More power to parent components.

# Pros

  • Better components lifecycle management
  • Better visibility into the composition architecture
  • Better reusuability

# Cons

  • Injecting props can become a little expensive
  • Less flexibility and power in child components

# Good if

  • B should accept to compose something different than C in the future or somewhere else
  • A should control the lifecycle of C

B would render C using this.props.children, and there isn't a structured way for B to know what those children are for. So, B may enrich the child components by giving additional props down, but if B needs to know exactly what they are, #3 might be a better option.

# 3. Nesting using props

(continued from above)

var CommentBox = reactCreateClass({
  render: function() {
    return (
      <div className="commentBox">
        <h1>Comments</h1>
        <CommentList title={ListTitle}/> //prop
        <CommentForm />
      </div>
    );
  }
});

This is the style where A composes B and B provides an option for A to pass something to compose for a specific purpose. More structured composition.

# Pros

  • Composition as a feature
  • Easy validation
  • Better composaiblility

# Cons

  • Injecting props can become a little expensive
  • Less flexibility and power in child components

# Good if

  • B has specific features defined to compose something
  • B should only know how to render not what to render

#3 is usually a must for making a public library of components but also a good practice in general to make composable components and clearly define the composition features. #1 is the easiest and fastest to make something that works, but #2 and #3 should provide certain benefits in various use cases.

# Props

Props are a way to pass information into a React component, they can have any type including functions - sometimes referred to as callbacks.

In JSX props are passed with the attribute syntax

<MyComponent userID={123} />

Inside the definition for MyComponent userID will now be accessible from the props object

// The render function inside MyComponent
render() {
    return (
        <span>The user's ID is {this.props.userID}</span>
    )
}

It's important to define all props, their types, and where applicable, their default value:

// defined at the bottom of MyComponent
MyComponent.propTypes = {
    someObject: React.PropTypes.object,
    userID: React.PropTypes.number.isRequired,
    title: React.PropTypes.string
};

MyComponent.defaultProps = {
    someObject: {},
    title: 'My Default Title'
}

In this example the prop someObject is optional, but the prop userID is required. If you fail to provide userID to MyComponent, at runtime the React engine will show a console warning you that the required prop was not provided. Beware though, this warning is only shown in the development version of the React library, the production version will not log any warnings.

Using defaultProps allows you to simplify

const { title = 'My Default Title' } = this.props;
console.log(title);

to

console.log(this.props.title);

It's also a safeguard for use of object array and functions. If you do not provide a default prop for an object, the following will throw an error if the prop is not passed:

if (this.props.someObject.someKey)

In example above, this.props.someObject is undefined and therefore the check of someKey will throw an error and the code will break. With the use of defaultProps you can safely use the above check.

# Component states - Dynamic user-interface

Suppose we want to have the following behaviour - We have a heading (say h3 element) and on clicking it, we want it to become an input box so that we can modify heading name. React makes this highly simple and intuitive using component states and if else statements. (Code explanation below)

// I have used ReactBootstrap elements. But the code works with regular html elements also
var Button = ReactBootstrap.Button;
var Form = ReactBootstrap.Form;
var FormGroup = ReactBootstrap.FormGroup;
var FormControl = ReactBootstrap.FormControl;

var Comment = reactCreateClass({
  getInitialState: function() {
    return {show: false, newTitle: ''};
  },

  handleTitleSubmit: function() {
     //code to handle input box submit - for example, issue an ajax request to change name in database
  },

  handleTitleChange: function(e) {
    //code to change the name in form input box. newTitle is initialized as empty string. We need to update it with the string currently entered by user in the form
    this.setState({newTitle: e.target.value});
  },

  changeComponent: function() {
    // this toggles the show variable which is used  for dynamic UI
    this.setState({show: !this.state.show)};
  },

  render: function() {

    var clickableTitle;

    if(this.state.show) {
        clickableTitle = <Form inline onSubmit={this.handleTitleSubmit}>
                             <FormGroup controlId="formInlineTitle">
                                 <FormControl type="text" onChange={this.handleTitleChange}>
                             </FormGroup>
                         </Form>;
    } else {
        clickabletitle = <div>
                            <Button bsStyle="link" onClick={this.changeComponent}>
                                <h3> Default Text </h3>
                            </Button>
                         </div>;
    }

    return (
        <div className="comment">
            {clickableTitle}
        </div>
    );
  }
});

ReactDOM.render(
    <Comment />, document.getElementById('content')
);

The main part of the code is the clickableTitle variable. Based on the state variable show, it can be either be a Form element or a Button element. React allows nesting of components.

So we can add a {clickableTitle} element in the render function. It looks for the clickableTitle variable. Based on the value 'this.state.show', it displays the corresponding element.

# Variations of Stateless Functional Components

const languages = [
  'JavaScript',
  'Python',
  'Java',
  'Elm',
  'TypeScript',
  'C#',
  'F#'
]

// one liner
const Language = ({language}) => <li>{language}</li>

Language.propTypes = {
  message: React.PropTypes.string.isRequired
}

/**
* If there are more than one line.
* Please notice that round brackets are optional here,
* However it's better to use them for readability
*/
const LanguagesList = ({languages}) => {
  <ul>
    {languages.map(language => <Language language={language} />)}
  </ul>
}

LanguagesList.PropTypes = {
  languages: React.PropTypes.array.isRequired
}

/**
 * This syntax is used if there are more work beside just JSX presentation
 * For instance some data manipulations needs to be done.
 * Please notice that round brackets after return are required,
 * Otherwise return will return nothing (undefined)
 */
const LanguageSection = ({header, languages}) => {
  // do some work
  const formattedLanguages = languages.map(language => language.toUpperCase())
  return (
    <fieldset>
      <legend>{header}</legend>
      <LanguagesList languages={formattedLanguages} />
    </fieldset>
  )
}

LanguageSection.PropTypes = {
  header: React.PropTypes.string.isRequired,
  languages: React.PropTypes.array.isRequired
}

ReactDOM.render(
  <LanguageSection 
    header="Languages" 
    languages={languages} />,
  document.getElementById('app')
)

Here you can find working example of it.

# setState pitfalls

You should use caution when using setState in an asynchronous context. For example, you might try to call setState in the callback of a get request:

class MyClass extends React.Component {
    constructor() {
        super();

        this.state = {
            user: {}
        };
    }

    componentDidMount() {
        this.fetchUser();
    }

    fetchUser() {
        $.get('/api/users/self')
            .then((user) => {
                this.setState({user: user});
            });
    }

    render() {
        return <h1>{this.state.user}</h1>;
    }
}

This could call problems - if the callback is called after the Component is dismounted, then this.setState won't be a function. Whenever this is the case, you should be careful to ensure your usage of setState is cancellable.

In this example, you might wish to cancel the XHR request when the component dismounts:

class MyClass extends React.Component {
    constructor() {
        super();

        this.state = {
            user: {},
            xhr: null
        };
    }

    componentWillUnmount() {
        let xhr = this.state.xhr;

        // Cancel the xhr request, so the callback is never called
        if (xhr && xhr.readyState != 4) {
            xhr.abort();
        }
    }

    componentDidMount() {
        this.fetchUser();
    }

    fetchUser() {
        let xhr = $.get('/api/users/self')
            .then((user) => {
                this.setState({user: user});
            });

        this.setState({xhr: xhr});
    }
}

The async method is saved as a state. In the componentWillUnmount you perform all your cleanup - including canceling the XHR request.

You could also do something more complex. In this example, I'm creating a 'stateSetter' function that accepts the this object as an argument and prevents this.setState when the function cancel has been called:

function stateSetter(context) {
    var cancelled = false;
    return {
        cancel: function () {
            cancelled = true;
        },
        setState(newState) {
            if (!cancelled) {
                context.setState(newState);
            }
        }
    }
}

class Component extends React.Component {
    constructor(props) {
        super(props);
        this.setter = stateSetter(this);
        this.state = {
            user: 'loading'
        };
    }
    componentWillUnmount() {
        this.setter.cancel();
    }
    componentDidMount() {
        this.fetchUser();
    }
    fetchUser() {
        $.get('/api/users/self')
            .then((user) => {
                this.setter.setState({user: user});
            });
    }
    render() {
        return <h1>{this.state.user}</h1>
    }
}

This works because the cancelled variable is visible in the setState closure we created.

# Remarks

React.createClass was deprecated in v15.5 and expected to be removed in v16. There is a drop-in replacement package for those that still require it. Examples using it should be updated.