Components – React

Components are the building blocks of React. They help you divide the functionality of the UI into several pieces which can be reused throughout the application. You can inject value into props as given below:

// 1. Define our Welcome component function Welcome(props) { return <h1>Hello, {}</h1>; } // 2. Create our "app" const element = <Welcome name="Faisal Arkan" />; // 3. Render the app in the browser window ReactDOM.render( element, document.getElementById('root') );

First, we define our Component (this is a functional, stateless component). It takes a single argument, props, and from this object, only the value name is ever used. We could replace this with a firstName and lastName if we wanted. But we don’t right now.

Second, the Welcome component is called with the value name="Faisal Arkan". This will be assigned to {} in our component. The Welcome component called with this name returns the element <h1>Hello, Faisal Arkan</h1>. We save this into the const variable element.

The value name="Faisal Arkan" will be assigned to {} from function Welcome(props) and returns a component <h1>Hello, Faisal Arkan</h1> which is saved into the const variable element. The component can then be rendered via ReactDOM.render(element, document.getElementById('root'));. document.getElementById('root') in this case is the target location you would like the element component to be rendered.

Important note:

User defined components (stateful or stateless) must always start with a capital letter as React considers components starting with lowercase letters as DOM tags. For example, <Foo /> will be considered a user defined component but <foo /> will be considered DOM tags similar to <div />, <span /> or <p /> tags. This is explaned in detail here

Other ways to declare components

There are many ways to declare components when using React.js, but there are two kinds of components, stateless components and stateful components.

Briefly, stateful components are clever. They can hold their own information (in the state object) and inherit props (as seen above) from parents. Stateless components can only inherit props, so if they need to update they have to be told by their parent.

See the below examples of af Cat component. The stateful Cat component is aware of its feeling, the stateless one is not.

Stateful Components

Class Type Components

class Cat extends React.Component { constructor(props) { super(props); this.state = { humor: 'happy' } } render() { return( <div> <h1>Hi, my name is {}</h1> <p> I'm a {this.props.color} cat </p> <p> I am a {this.state.humor} cat. </p> </div> ); } }

Stateless Components

Functional Components (Arrow Function from ES6)

const Cat = props => { return ( <div> <h1>Hi, my name is {}</h1> <p>I am a {props.color} cat</p> <p>Sadly, I can't tell you how I feel, because I have no state.</p> </div>; ); };

Implicit Return Components

If all your component does is render something (i.e. no computing), you can omit the ‘return’.

const Cat = props => <div> <h1>{}</h1> <p>{props.color}</p> <p>Sadly, I can't tell you how I feel, because I have no state.</p> </div>;

Pure Components

This type of component was added in React 16 and can be used to declare stateless non-functional components.
These components work like normal stateful components (class-based component) but with shouldComponentUpdate() pre-defined.
They are the fastest components and make the render cycle much cleaner and leaner.

class Cat extends React.PureComponent { render() { return( <div> <h1>{}</h1> <p>{props.color}</p> </div> ); } }

This component will only render if there is a change in its props; not when the parent re-renders.

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