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HTML

              
                <div id="app"></div>
              
            
!

CSS

              
                
              
            
!

JS

              
                // The data object (or state)
var data = {
	listName: 'Starting at Hogwarts',
	todos: []
};

// The state-based UI
var template = function (data) {

	// Create the list heading
	var html = '<h1>' + data.listName + '</h1>';

	// If there are no list items, show a message
	// Otherwise, display the list items
	if (data.todos.length < 1) {
		html += '<p><em>You do not have any items to complete yet. Please add some.</em></p>';
	} else {
		html += '<ul>' + data.todos.map(function (todo) {
			return '<li>' + todo + '</li>';
		}).join('') + '</ul>';
	}

	// Return the template
	return html;

};

/*!
 * Sanitize and encode all HTML in a user-submitted string
 * (c) 2018 Chris Ferdinandi, MIT License, https://gomakethings.com
 * @param  {String} str  The user-submitted string
 * @return {String} str  The sanitized string
 */
var sanitizeHTML = function (str) {
	var temp = document.createElement('div');
	temp.textContent = str;
	return temp.innerHTML;
};

/**
 * Create an immutable copy of an object and recursively encode all of its data
 * @param  {*} obj The object to clone
 * @return {*}     The immutable, encoded object
 */
var clone = function (obj) {

	// Get the object type
	var type = Object.prototype.toString.call(obj).slice(8, -1).toLowerCase();

	// If an object, loop through and recursively encode
	if (type === 'object') {
		var cloned = {};
		for (var key in obj) {
			if (obj.hasOwnProperty(key)) {
				cloned[key] = clone(obj[key]);
			}
		}
		return cloned;
	}

	// If an array, create a new array and recursively encode
	if (type === 'array') {
		return obj.map(function (item) {
			return clone(item);
		});
	}

	// If the data is a string, encode it
	if (type === 'string') {
		return sanitizeHTML(obj);
	}

	// Otherwise, return object as is
	return obj;

};

/**
 * Reactivity update the data object
 * @param {Object} obj The data to update
 */
var setData = function (obj) {
	for (var key in obj) {
		if (obj.hasOwnProperty(key)) {
			data[key] = obj[key];
		}
	}
  var sanitized = clone(data);
	app.innerHTML = template(sanitized);
};

// Render the initial UI
var app = document.querySelector('#app');
app.innerHTML = template(clone(data));

// Uncomment this to update the UI
// You can also open up the console feature and run it there
/*
setData({todos: [
	'Fix my wand',
	'Buy new robes',
	'Enroll in courses'
]});
*/
              
            
!
999px

Console