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HTML

              
                <main>
	<h1>📚 Reading Planner</h1>
	
	<form id="form" class="form form--pages">
		<div class="form__field">
			<label for="input-total-pages">How many pages are in your book?</label>
			<input id="input-total-pages" name="total_pages" type="number" value="100" min="0" step="1">
		</div>
		
		<div class="form__field">
			<label for="input-current-page">What page are you on?</label>
			<input id="input-current-page" name="current_page" type="number" value="1" min="0" step="1">
		</div>
		
		<div class="form__field">
			<p>Would you rather read some number of pages per day, or finish by some date?</p>
			<input id="input-method-pages-per-day" type="radio" name="method" value="pages" checked>
			<label for="input-method-pages-per-day">Pages per day</label>
			<input id="input-method-finish-by-date" type="radio" name="method" value="date">
			<label for="input-method-finish-by-date">Finish by date</label>
		</div>
		
		<div class="form__field form__field--pages">
			<label for="input-pages-per-day">How many pages do you want to read each day?</label>
			<input id="input-pages-per-day" name="pages_per_day" type="number" value="10" min="0" step="1">
		</div>

		<div class="form__field form__field--date">
			<label for="input-finish-date">When do you want to finish by?</label>
			<input id="input-finish-date" name="finish_date" type="date">
		</div>
		
		<div class="form__field">
			<button type="submit">Update</button>
		</div>
	</form>
	
	<h2>Result</h2>
	<div id="result">If you read 10 pages per day, you’ll finish your book in <strong>10 days</strong>.</div>
</main>

              
            
!

CSS

              
                /* ------------------------------ */
/* Base                           */
/* ------------------------------ */

html {
	box-sizing: border-box;
}

*,
::after,
::before {
	box-sizing: inherit;
}

body {
	margin: 1rem auto 3rem;
	line-height: 1.4;
	tab-size: 4;
}

main {
	display: block; /* Fix IE rendering main as inline */
	margin: 0 auto;
	max-width: 42rem;
	padding-left: 1rem;
	padding-right: 1rem;
}

pre {
	border: 1px solid #ccc;
	padding: 1rem;
	overflow: auto;
	-webkit-overflow-scroll: touch;
}

button,
input,
select {
	font-size: 1rem;
}



/* ------------------------------ */
/* Form                           */
/* ------------------------------ */

.form__field {
	margin-bottom: 1rem;
}

/* 💡 Remove Child Margin Utility */
/* https://codepen.io/tannerhodges/pen/gObaGvw */
.form__field > :first-of-type { margin-top: 0; }
.form__field > :last-of-type  { margin-bottom: 0; }

/* Dim unused fields */
.form--pages .form__field--date,
.form--date .form__field--pages {
	filter: blur(2px);
	opacity: 0.5;
	cursor: not-allowed;
	pointer-events: none;
	user-select: none;
}

              
            
!

JS

              
                /**
 * Update results based on form data.
 * @param   Event  event
 * @return  void
 */
function update(event) {	
	var output = '😵 Whoops! I screwed up. Would you leave a comment to let me know? Thanks!';
	
	// Get form values
	var currentPage = parseInt(form.elements.current_page.value, 10);
	var totalPages = parseInt(form.elements.total_pages.value, 10);
	var method = form.elements.method.value;
	var pagesPerDay = parseInt(form.elements.pages_per_day.value, 10);
	var finishDate = form.elements.finish_date.value;
	
	// Common variables
	var pagesLeft = totalPages - currentPage;

	// Calculate - Pages per day
	// NOTE: Round up so we always return full days
	var daysToRead = Math.ceil(pagesLeft / pagesPerDay);
		
	// Calculate - Finish by date
	// NOTE: Round up so we always return full days
	var today = new Date();
	var daysLeft = Math.ceil((Date.parse(finishDate) - today.getTime()) / (1000 * 3600 * 24));
	// NOTE: Round up so we always return full pages
	var newPagesPerDay = Math.ceil(pagesLeft / daysLeft);

	// Update CSS classes
	form.className = form.className.replace(/form--\w+/, 'form--' + method);
	
	// Output - Finish by date
	if (method === 'date') {
		output = 'You’ll need to read <strong>' + newPagesPerDay + ' pages per day</strong> to finish by ' + finishDate + '.';
	// Output - Pages per day
	} else {
		output = 'If you read ' + pagesPerDay + ' pages per day, you’ll finish your book in <strong>' + daysToRead + ' days</strong>, on ' + getDateFromNow(daysToRead) + '.';
	}
	
	// Save data to localStorage
	localStorage.setItem('readingPlanner', JSON.stringify({
		current_page: currentPage,
		total_pages: totalPages,
		method: method,
		pages_per_day: pagesPerDay,
		finish_date: finishDate,
	}));
	
	result.innerHTML = output;
}

/**
 * Load form data from localStorage.
 * @return void
 */
function loadData() {
	var data = JSON.parse(localStorage.getItem('readingPlanner') || 'false');
	
	if (!data) {
		return;
	}
	
	// Set data for each form input
	Object.entries(data).forEach(function ([key, value]) {
		form.elements[key].value = value;
	});
}

/**
 * Get date N days from now. Defaults to today.
 * @param   Number  Number of days
 * @return  String  Date in mm/dd/yyyy
 */
function getDateFromNow(days) {
	var date = new Date();
	
	// Increment days
	date.setDate(date.getDate() + (days || 0));
	
	// Return formatted string
	return date
		.toISOString()
		.replace(/(\d+-\d+-\d+).*/, '$1');
}

// Init default values
window['input-finish-date'].value = getDateFromNow(30);
loadData();
update();

// Init event listeners
form.addEventListener('input', update);
form.addEventListener('submit', function (event) {
	event.preventDefault();
	update();
});

              
            
!
999px

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