The “Get Up and Running with React and TypeScript” series is made up of posts from chapters in my book “Hello React and TypeScript“. If you have questions, comments or corrections, please reach out to me in the comments or on Twitter @charleslbryant.

You can view the posts in this series on the Hello React and TypeScript category page.

Component Composition

https://charleslbryant.gitbooks.io/hello-react-and-typescript/content/Samples/ComponentComposition.html

This sample gives a basic example of refactoring the Hello World application by composing the UI with modular components.

Source Code

https://github.com/charleslbryant/hello-react-and-typescript/releases/tag/0.0.7

src/helloworld.tsx

/// <reference path="../typings/tsd.d.ts" />

import * as React from 'react';
import HelloForm from 'helloform';
import HelloContent from 'hellocontent';

export default class HelloWorld extends React.Component<any, any> {
    constructor(props: any){
        super(props);
        this.state = { name: this.props.defaultName };
        this.handleChange = this.handleChange.bind(this)
    }

    public handleChange(event: any) : void {

        this.setState({ name: event.target.value });
    }

    public render() {
        return (
            <div>
                <HelloForm 
                    name = { this.state.name }
                    handleChange = { this.handleChange } 
                />
                <HelloContent 
                    name = { this.state.name }
                />
            </div>
        );
    }
}

The HelloWorld component is updated to compose the same UI as the previous example using two modular components, HelloForm and HelloContent.

import * as React from 'react';
import HelloForm from 'helloform';
import HelloContent from 'hellocontent';

To compose with components the components you want to compose have to be in scope. To do this we import the components. HelloForm and HelloContent are imported by using the import statement with the from value being the name of the component.

public render() {
    return (
        <div>
            <HelloForm 
                name = { this.state.name }
                handleChange = { this.handleChange } 
            />
            <HelloContent 
                name = { this.state.name }
            />
        </div>
    );
}

The actual composition occurs in the render method. We add the HelloForm and HelloContentcomponents. Notice that the name of the components start with uppercase to differentiate them from HTML elements. Each of the components accept some properties and we pass them with a syntax that is similar to defining HTML attributes.

By composing UIs in this manner we move from an imperative style of building UIs to a declarative one. Instead of defining every element and attribute we want to use in the UI we delegate the definition to a modular reusable component.

Since the modular components are reusable we can compose multiple UIs with them. We can compose with components developed by other teams. We get to focus on the unique aspects of our domain and delegate other lower level concerns to modular components.

src/helloform.tsx

/// <reference path="../typings/tsd.d.ts" />

import * as React from 'react';

export default class HelloForm extends React.Component<any, any> {
    constructor(props: any){
        super(props);
    }

    public render() {
        return (
            <div>
                <input 
                    value={ this.props.name }
                    onChange={ e => this.props.handleChange(e) }
                />
            </div>
        );
    }
}

Here we have a new component responsible for the collecting user input for our tiny application. By now this code should be familiar. We just moved the input element from the HelloWorld component and encapsulated it in this new component.

For this simple example we are storing all of our components in one folder. As your application grows you may want to use a folder structure that makes the number of component files more manageable. When you are at the point of doing this you will have to address the reference path for the TypeScript typings. It will become a maintenance issue having to keep the path in sync as you move components and build new folder structures, but we won’t go into that just yet.

src/hellocontent.tsx

/// <reference path="../typings/tsd.d.ts" />

import * as React from 'react';

export default class HelloContent extends React.Component<any, any> {
    constructor(props: any){
        super(props);
    }

    public render() {
        return (
            <div>
                Hello { this.props.name }!
            </div>
        );
    }
}

This is a component responsible for displaying the hello message. Again, this component is a result of encapsulating this section of the DOM out of the HelloWorld component.

Stateless Components

One interesting observation about these components is that they are stateless. They don’t hold any state and they rely on a parent component to pass props to it in order to do its work. This includes event handlers and data as you can see in the sample code.

When possible using stateless components are preferred in React. They help to improve performance because they lessen the amount of processing that React has to do with the component.

The “Get Up and Running with React and TypeScript” series is made up of posts from chapters in my book “Hello React and TypeScript“. If you have questions, comments or corrections, please reach out to me in the comments or on Twitter @charleslbryant.

You can view the posts in this series on the Hello React and TypeScript category page.

Accept User Input

This sample gives an basic example of accepting user input.

Source Code

https://github.com/charleslbryant/hello-react-and-typescript/releases/tag/0.0.6

src/helloworld.tsx

/// <reference path="../typings/tsd.d.ts" />

import * as React from 'react';

export default class HelloWorld extends React.Component<any, any> {
    constructor(props: any){
        super(props);
        this.state = { name: this.props.defaultName };
    }

    public handleOnChange(event: any) : void {
        this.setState({ name: event.target.value });
    }

    public render() {
        return (
            <div>
                <div>
                    <input 
                        onChange={ e => this.handleOnChange(e) }
                    />
                </div>
                <div>
                    Hello { this.state.name }!
                </div>
            </div>
        );
    }
}

The significant changes here are we removed the button and handleOnClick method. We also added a text box and a handleOnChange method to handle changes as a user adds input to the text box.

public handleOnChange(event: any) : void {
    this.setState({ name: event.target.value });
}

This is a new method to handle the onChange event. In the method we are calling this.setState and updating the value of name in this.state to the value of the target element passed in the event.

public render() {
    return (
        <div>
            <div>
                <input 
                    onChange={ e => this.handleOnChange(e) }
                />
            </div>
            <div>
                Hello { this.state.name }!
            </div>
        </div>
    );
}

Here we added an input element and set its default value to this.props.name. We also bound the text box’s onChange event to handleOnChange method.

With these changes we have implemented a unidirectional data flow.

1. When the user enters something in the text box it triggers the onChange event.
2. The onChange event is handled by the handleOnChange method.
3. The handleOnChange method updates the value of name in this.state and triggers a re-render of the component with this.setState.
4. this.setState ends in a call to the render method that updates the name in our “Hello” message.

State is only changes as the result of an event. The Hello message is no bound to an external model state and can only be updated as a result of an event being triggered by user input. This is different than two way binding or bi-directional data flow where changes in a model can also update the state of a view.

Unidirectional Data Flow (UDF)

event > event handler > state > render

Components are representations of the state of a view over time. As events are triggered over time they update state and re-render the component with the new state. The flow can be seen as a stream of events that flow in one direction that eventually update component state causing a component to re-render. If you know about CQRS, event streaming, or stream processing, there are similar concepts in each. UDF is a redundant theme in learning React, hence a redundant theme in this book.

The sample is a simple naive example because we aren’t dealing with external or persisted data. The scope of the example makes it a little hard to understand UDF. In the example we don’t have to worry about updating an external store.

If you are having trouble understanding UDF, when you learn about Flux it will makes more sense. The Flux architecture helps you visualize data flow in a circular one way round trip. Even though it may be hard to see UDF within the context of a single component the same event flow is used to accomplish UDFin React whether within a single component, a Flux architecture, or using Relay (another Facebook library). When you get into Flux or other data flow patterns or libraries, UDF will be expanded to add additional concepts into the data flow.

UDF vs Bi-directional Data Flow

The important thing to notice about UDF is that we aren’t attempting to create a complex bi-directional view binding with some external model. We are binding to the state modeled within a component with no dependency or complex mapping to some external data model. The component is responsible for expressing its own state, updating its state, passing properties to its child components, and re-rendering itself and children when state changes as the result of some event.

If we were to use a bi-directional binding to an external model, we would not know why state is being updated. Any number of views could have the same binding to the same model. With bi-directional binding a change to a view or model could cause updates to multiple models or views and it becomes hard to understand the data flow, especially when you are trying to solve a Sev1 incident.

If you’d like to know more about UDF, there is a lot about it online that can be found with a simple search. Actually, the original MVC pattern is an example of UDF. It was distorted when it moved to web clients. If you’d like to dig into the theory behind React and UDF, you can research Functional Reactive Programming – https://en.wikipedia.org/wiki/Reactive_programming.

The “Get Up and Running with React and TypeScript” series is made up of posts from chapters in my book “Hello React and TypeScript“. If you have questions, comments or corrections, please reach out to me in the comments or on Twitter @charleslbryant.

You can view the posts in this series on the Hello React and TypeScript category page.

Component Interactivity

This sample gives an basic example of using adding interactivity to a component.

Source Code

https://github.com/charleslbryant/hello-react-and-typescript/releases/tag/0.0.5

src/helloworld.tsx

/// <reference path="../typings/tsd.d.ts" />

import * as React from 'react';

export default class HelloWorld extends React.Component<any, any> {
    constructor(props: any){
        super(props);
        this.state = { name: this.props.defaultName };
    }

    public handleChange(event: any) : void {
        this.setState({ name: "Charles" });
    }

    public render() {
        return (
            <div>
                Hello { this.state.name }!
                <button 
                    name = "Update"
                    onClick = { e => this.handleOnClic(e) }
                >Update</button>
            </div>
        );
    }
}

To add interactivity we:

• added an element for a user to interact with
• created a method to handle an event triggered by user interaction
• bound the new element’s onClick event to the new event handler method

public handleOnClick(event: any) : void {
    this.setState({ name: "Charles" });
}

This is a new method to handle the click event. We expect callers of this method to send the event as an argument so we defined the argument as type any so we can accept any event. We could define a specific or generic event type and restrict the argument to that type. We also define the method as returning void or nothing.

In the method we are calling this.setState and updating the value of name. When we callthis.setState it causes the component to re-render with the new state.

return (
    <div>
        Hello { this.state.name }!
        <button 
            name = "Update"
            onClick = { e => this.handleOnClick(e) }
        >Update</button>
    </div>
);

We are adding a new HTML element, button (remember that HTML elements start with lowercase letter). The interesting bit is that we are binding the element’s onClick event to the handleOnClick method. To bind the event handler with the event we are using the new ES6 arrow function expression to simplify function context binding. Notice that the event handler is camelCased which is consistent with JavaScript style.

Function Context Binding

In the end, React is just plain JavaScript. The React Team has resisted the urge to add a lot of magic and have made strides to remove magic to reduce coupling. Magic is the encapsulated work that is hidden behind abstractions. When a library does work behind the scenes without you knowing it, its magic. Reducing this hidden work helps to losen coupling and that is a good thing because it enables reuse. The React team believes our code shouldn’t have a lot of coupling to React.

One example of this, related to our use of the arrow function above, is function context binding. There was a time when React automatically bound functions to this. If you aren’t a JavaScript developer there is a chance that you don’t know that this in JavaScript is different from this in C# or Java or self in Ruby.

In some strongly typed languages, this is bound to an object instance. In JavaScript this is bound to the function it is called in. This causes a lot of confusion, but this decoupling of this from their parent function allows us to reuse functions outside of the parent, powerful and dangerous at the same time.

To get this function reuse we can define the context for a function and have this represent what we want it to. In our example the arrow function is assigning this from the class HelloWorld to thehandleOnClick method. If we wanted, we could use this same method in say a GoodByeWorld class and the arrow function could set the context of the handleOnClick method.

Without the arrow function we would use JavaScripts bind method to achieve the same effect. The arrow function is just some syntactic sugar for

onClick = { handleOnClick().bind(this) }

You can get more on arrow functions here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions.