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JavaScript

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HTML 

              
                body
	div#content
!

CSS 

              
                // Variables
// =========
$dark:				#000411
$black:				#353535
$white:				#FCFCFC
$gray:				#F1FFFA
$green:				#3DDC97
$red:					#FF495C

$off-state: 	$red
$on-state: 		$green

$react-blue:	#61dafb

// Mixins
// ======

@mixin flex($dir: row, $wrap: nowrap, $justify: center, $align: center)
	display: flex
	flex-flow: $dir $wrap
	justify-content: $justify
	align-items: $align


// Base
// ====

\:root
	font-size: 20px

body
	width: 100%
	height: 100vh
	user-select: none

// Modules

.component-wrapper
	width: 100%
	height: 100vh
	@include flex(column, wrap, space-around)
	background-color: $dark
	transition: background-color 200ms ease-out

.header
	font-size: 3rem
	font-family: "Helvetica Neue", Helvetica, sans-serif
	color: $dark

.switch
	position: relative
	width: 8rem
	height: 5rem
	border-radius: 20%
	transition: background-color 100ms ease-out
	z-index: 1
	&:before, &:after
		content: ""
		position: absolute
		top: 0
		background-color: inherit
		border-radius: 50%
		width: 5rem
		height: 5rem
		z-index: 2
	&:before
		left: -1rem
	&:after
		right: -1rem
	
.toggle-button
	position: absolute
	width: 5rem
	height: 5rem
	background-color: $black
	border-radius: 50%
	transition: transform 100ms ease-in-out
	z-index: 3
	border: 0.05rem solid $black
	top: -0.05rem

// State

.component-wrapper_is-light
	background-color: $white
	
component-wrapper_is-dark
	background-color: $dark

.header_is-lit
	color: $react-blue
	text-shadow: 0

.header_is-dark
	color: $dark
	text-shadow: 0 0 3rem $react-blue

.switch_is-off
	background-color: $off-state

.switch_is-on
	background-color: $on-state

.toggle-button_position-left
	transform: translateX(-1.05rem)

.toggle-button_position-right
	transform: translateX(4.05rem)
!

JS 

              
                class ComponentWrapper extends React.Component {
	constructor(props) {
		super(props);
		this.state = {isOn: false};
	}
	
	handleToggle(e) {
		this.setState({isOn: !this.state.isOn});
	}
	
	render() {
		let classNames = ["component-wrapper", (this.state.isOn) ? "component-wrapper_is-light" : "component-wrapper_is-dark"].join(" ");
		return (
			<div className={classNames}>
				<Header 
					isOn={this.state.isOn}	
				/>
				<Switch 
					isOn={this.state.isOn}	
					handleToggle={this.handleToggle.bind(this)}
				/>
			</div>		
		);
	}
}

const Header = function(props) {
	let classNames = ["header", (props.isOn) ? "header_is-lit" : "header_is-dark"].join(" ");
	return (
		<h1 className={classNames}>React</h1>
	);
}

const Switch = function(props) {
	let classNames = ["switch", (props.isOn) ? "switch_is-on" : "switch_is-off"].join(" ");
	return (
		<div className={classNames} onClick={props.handleToggle}>
			<ToggleButton 
				isOn={props.isOn}	
			/>
		</div>
	);
}

const ToggleButton = function(props) {
		let classNames = ["toggle-button", (props.isOn) ? "toggle-button_position-right" : "toggle-button_position-left"].join(" ");
		return (<div className={classNames}></div>);
};

ReactDOM.render(<ComponentWrapper />, document.getElementById("content"));
!
999px

Console