jdanyow/app.html

## app.html
<template>
  <h1>${message}</h1>
</template>

## app.js
export class App {
  message = 'Hello World!';
}

## au-class.js
import {customAttribute} from 'aurelia-framework';
/**
 * The au-class custom attribute is intended to simplify aurelia binding code in the standard html class attribute.
 *
 * This is an unofficial community proposal to add to Aurelia (aurelia.io)
 *
 * Before:
 <div class="base-class ${foo} ${someVMProp < 10 ? 'red' : ''} ${!otherVMProp ? 'blue' : ''}"></div>
 * After:
 <div au-class.bind="['base-class', foo, {red: someVMProp < 10, blue: !otherVMProp}]"></div>
 */
@customAttribute('au-class')
export class AuClass {

  classes = new Set();

  static inject = [Element];
  constructor(element: Element) {
    this.element = element;
  }

  valueChanged(newVal = {}) {
    let newClasses = new Set(AuClass._extractClasses(newVal));
    let removedClasses = [...this.classes].filter(x => !newClasses.has(x));
    let addedClasses = [...newClasses].filter(x => !this.classes.has(x));

    this.removeClasses(removedClasses);
    this.addClasses(addedClasses);

    this.classes = newClasses;
  }

  addClasses(classes) {
    classes.forEach(c => this.element.classList.add(c))
  }

  removeClasses(classes) {
    classes.forEach(c => this.element.classList.remove(c))
  }

  static _extractClasses(args) {
    let classesFound;

    if (args && typeof args === 'string') {
      classesFound = [args];
    }
    if (Array.isArray(args)) {
      classesFound = args.reduce((acc, arg) => acc.concat(AuClass._extractClasses(arg)), []);
    }
    else if (typeof args === 'object') {
      classesFound = Object.keys(args).filter(p => args[p]);
    }

    return classesFound;
  }
}

## index.html
<!doctype html>
<html>
  <head>
    <title>Aurelia</title>
    <link rel="stylesheet" href="styles.css">
    <meta name="viewport" content="width=device-width, initial-scale=1">
  </head>
  <body aurelia-app="main">
    <h1>Loading...</h1>

    <script src="https://cdn.rawgit.com/jdanyow/aurelia-bundle/v1.0.3/jspm_packages/system.js"></script>
    <script src="https://cdn.rawgit.com/jdanyow/aurelia-bundle/v1.0.3/config.js"></script>
    <script>
      System.import('aurelia-bootstrapper');
    </script>
  </body>
</html>

## main.js
export function configure(aurelia) {
  aurelia.use
    .standardConfiguration()
    .developmentLogging();

  aurelia.start().then(a => a.setRoot());
}

## styles.css
/* styles */
	import {customAttribute} from 'aurelia-framework';
	/**
	* The au-class custom attribute is intended to simplify aurelia binding code in the standard html class attribute.
	*
	* This is an unofficial community proposal to add to Aurelia (aurelia.io)
	*
	* Before:
	<div class="base-class ${foo} ${someVMProp < 10 ? 'red' : ''} ${!otherVMProp ? 'blue' : ''}"></div>
	* After:
	<div au-class.bind="['base-class', foo, {red: someVMProp < 10, blue: !otherVMProp}]"></div>
	*/
	@customAttribute('au-class')
	export class AuClass {

	classes = new Set();

	static inject = [Element];
	constructor(element: Element) {
	this.element = element;
	}

	valueChanged(newVal = {}) {
	let newClasses = new Set(AuClass._extractClasses(newVal));
	let removedClasses = [...this.classes].filter(x => !newClasses.has(x));
	let addedClasses = [...newClasses].filter(x => !this.classes.has(x));

	this.removeClasses(removedClasses);
	this.addClasses(addedClasses);

	this.classes = newClasses;
	}

	addClasses(classes) {
	classes.forEach(c => this.element.classList.add(c))
	}

	removeClasses(classes) {
	classes.forEach(c => this.element.classList.remove(c))
	}

	static _extractClasses(args) {
	let classesFound;

	if (args && typeof args === 'string') {
	classesFound = [args];
	}
	if (Array.isArray(args)) {
	classesFound = args.reduce((acc, arg) => acc.concat(AuClass._extractClasses(arg)), []);
	}
	else if (typeof args === 'object') {
	classesFound = Object.keys(args).filter(p => args[p]);
	}

	return classesFound;
	}
	}
	<!doctype html>
	<html>
	<head>
	<title>Aurelia</title>
	<link rel="stylesheet" href="styles.css">
	<meta name="viewport" content="width=device-width, initial-scale=1">
	</head>
	<body aurelia-app="main">
	<h1>Loading...</h1>

	<script src="https://cdn.rawgit.com/jdanyow/aurelia-bundle/v1.0.3/jspm_packages/system.js"></script>
	<script src="https://cdn.rawgit.com/jdanyow/aurelia-bundle/v1.0.3/config.js"></script>
	<script>
	System.import('aurelia-bootstrapper');
	</script>
	</body>
	</html>
	export function configure(aurelia) {
	aurelia.use
	.standardConfiguration()
	.developmentLogging();

	aurelia.start().then(a => a.setRoot());
	}