• I wrote this post more than 2 years ago.
• That's enough time for things to change.
• Possibly, I may not endorse the content anymore.

# Creating Calendar in JavaScript 🗓self.__wrap_b=(e,t,r)=>{let _=(r=r||document.querySelector([data-br="${e}"])).parentElement,a=e=>r.style.maxWidth=e+"px";r.style.maxWidth="";let l,i=_.clientWidth,s=_.clientHeight,b=i/2-.25,n=i+.5;if(i){for(;b+1<n;)l=Math.round((b+n)/2),a(l),_.clientHeight===s?n=l:b=l;a(n*t+i*(1-t))}r.__wrap_o||(r.__wrap_o=new ResizeObserver((()=>{self.__wrap_b(0,+r.dataset.brr,r)}))).observe(_)},self.__wrap_b(":Rmjbas:",1) Counting... Let's create a calendar with JavaScript but without any external library. This project is based on my previous internship at Woowa Bros, a unicorn food-delivery startup in Seoul. ## Show me the code first.​ GitHub - anaclumos/calendar.js: Vanilla JS Calendar ## Show me the demo first.​ ## Goals​ • Use functional programming* instead of Object-oriented programming. • No DOM manipulation after initializing. This philosophy is based on the React framework (or any other Single Page Application libraries.) DOM manipulation can be highly confusing if 30 different codes are trying to edit the same thing. So instead, we will rerender the components if we need to edit something. 💡 Don't fix it. Buy a new one. — Rerendering in Front-end ## Stack​ • JavaScript Date Object • CSS display: grid will be useful. ## Basic Idea​ • There will be a global displayDate object that represents the displaying month. • navigator.js will change this displayDate object, and trigger renderCalendar() function with displayDate as an argument. • renderCalendar() will rerender with the calendar. ## Before anything, prettier!​ Prettier helps write clean and neat codes with automatic formatting. // .prettierrc{ "semi": false, "singleQuote": true, "arrowParens": "always", "tabWidth": 2, "useTabs": false, "printWidth": 60, "trailingComma": "es5", "endOfLine": "lf", "bracketSpacing": true} ## Now throw in some HTML.​ <!-- index.html --><!DOCTYPE html><html lang="en"> <head> <meta charset="UTF-8" /> <meta name="viewport" content="width=device-width, initial-scale=1.0" /> <title>JavaScript Calendar</title> </head> <body> <div id="navigator"></div> <div id="calendar"></div> </body> <script> // code for rendering </script></html> I generated this boilerplate with VS Code. ## Then trick VS Code to read JS String as HTML Tags.​ Since we use Vanilla JavaScript, we don't have access to fancy JSX-style highlighting. Instead, our generated HTML codes will live inside JavaScript String, which doesn't have syntax highlighting or Intellisense. Therefore, let's create a function that tricks VS Code to recognize JavaScript String as HTML Tags. // util.jsconst html = (s, ...args) => s.map((ss, i) => ${ss}${args[i] || ''}).join('') to be added - screenshot of highlighting ## calendar.js​ Then we connect calendar.js and index.html. <!-- index.html --><script src="calendar.js"></script> Defining constants will help before writing renderCalendar(). // calendar.jsconst NUMBER_OF_DAYS_IN_WEEK = 7const NAME_OF_DAYS = ['sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat']const LONG_NAME_OF_DAYS = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']const ACTUAL_TODAY = new Date() Note that we use NUMBER_OF_DAYS_IN_WEEK to remove magic numbers inside our code. It can be tough to decipher if we meet a random 7 during a code. Instead, using such constant increases the maintainability of the code. for (let d = 0; d < NUMBER_OF_DAYS_IN_WEEK; d++) { // do something} If there was a random 7, who knows if we are iterating through the number of Harry Potter Books? This code block will be the baseline for our calendar generation. We will pass in the HTML target and day object. today represents the month being displayed. Thetoday object will come from navigator,js. Navigator will return the actual date for the current month and return on the first day of the month for other months. // calendar.jsconst renderCalendar = ($target, today) => {  let html = getCalendarHTML(today)  // minify html  html = html.replace(/\n/g, '')  // replace multiple spaces with single space  html = html.replace(/\s{2,}/g, ' ')  $target.innerHTML = html} Now, we need four different Date objects for displaying the calendar. We could've used fewer objects, but it is up to the implementation. I think reducing date objects here would cause a minimal performance increase but spike the understandability of the code, so using four objects seems like a fair middle ground. ### Four Date objects we need​ • The last day of last month: needed to highlight last month's weekend and display the correct date for last month's row. • The first day of this month: needed to highlight this month's weekend and figure out how many days of last month we need to render. • The last day of this month: needed for rendering this month with iteration. • The first day of next month: needed to highlight the weekend of next month. I made a function that would process these four dates when inputted a specific Date. // calendar.jsconst processDate = (day) => { const month = day.getMonth() const year = day.getFullYear() return { lastMonthLastDate: new Date(year, month, 0), thisMonthFirstDate: new Date(year, month, 1), thisMonthLastDate: new Date(year, month + 1, 0), nextMonthFirstDate: new Date(year, month + 1, 1), }} I created a function that binds these 4 dates into an object and returns them. It receives a Date object as argument, and in this calendar, a Date object corresponding to "today" will be inserted. const processDate = (day) => { const date = day.getDate() const month = day.getMonth() const year = day.getFullYear() return { lastMonthLastDate: new Date(year, month, 0), thisMonthFirstDate: new Date(year, month, 1), thisMonthLastDate: new Date(year, month + 1, 0), nextMonthFirstDate: new Date(year, month + 1, 1), }} ### 2-2. Create getCalendarHTML​ Now let's draw a calendar in earnest. I created a getCalendarHTML function that returns the contents of the calendar as HTML. The getCalendarHTML function is a bit bulky, so I framed it first. const getCalendarHTML = () => { let today = new Date() let { lastMonthLastDate, thisMonthFirstDate, thisMonthLastDate, nextMonthFirstDate } = processDate(today) let calendarContents = [] // ... return calendarContents.join('')} Add a line at the top to display the day of the week. Use the const we added at the beginning to remove the magic number. for (let d = 0; d < NUMBER_OF_DAYS_IN_WEEK; d++) { calendarContents.push(html<div class="${NAME_OF_DAYS[d]} calendar-cell">${NAME_OF_DAYS[d]}</div>)} Then let's draw the last month. For example, if the first day of this month is Wednesday, the role of drawing the last month corresponding to Sunday, Monday, and Tuesday. For days corresponding to Sunday, sun HTML Class is added. for (let d = 0; d < thisMonthFirstDate.getDay(); d++) { calendarContents.push( html<div class="${d % 7 === 0 ? 'sun' : ''}          calendar-cell          past-month        "    >      ${lastMonthLastDate.getMonth() + 1}/${lastMonthLastDate.getDate() - thisMonthFirstDate.getDay() + d}    </div>  )}

Let's draw this month on a similar principle. For today's day, today HTML Class and "today" String are added. Similarly, sat and sun HTML Class are added for Saturday and Sunday respectively.

for (let d = 0; d < thisMonthLastDate.getDate(); d++) {  calendarContents.push(    html<div      class="          ${today.getDate() === d + 1 ? 'today' : ''}${(thisMonthFirstDate.getDay() + d) % 7 === 0 ? 'sun' : ''}          ${(thisMonthFirstDate.getDay() + d) % 7 === 6 ? 'sat' : ''} calendar-cell this-month " >${d + 1} ${today.getDate() === d + 1 ? ' today' : ''} </div> )} Finally, draw the days of the next month in the remaining cells. let nextMonthDaysToRender = 7 - (calendarContents.length % 7)for (let d = 0; d < nextMonthDaysToRender; d++) { calendarContents.push( html<div class="${(nextMonthFirstDate.getDay() + d) % 7 === 6 ? 'sat' : ''}          calendar-cell          next-month        "    >      ${nextMonthFirstDate.getMonth() + 1}/${d + 1}    </div>  )}

## 3. Writing CSS​

### 3-1. Using display: grid​

If you use display: grid on an element, you can neatly put its child elements into a grid (table).

• grid-template-columns: Information on how to arrange columns. 1fr means 1 fraction, and since it is written 7 times in total, 7 columns with the same width are created.
• grid-template-rows: You can define the size of rows. Here, there is only one 3rem, so the first row is defined as 3rem.
• grid-auto-rows: You can define the size of the next row. Here, it says 6rem, so all subsequent rows have a row size of 6rem.

#App {  /* grid */  display: grid;  grid-template-columns: 1fr 1fr 1fr 1fr 1fr 1fr 1fr;  grid-template-rows: 3rem;  grid-auto-rows: 6rem;  /* style */  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;  border: 1px solid black;  max-width: 720px;  margin-left: auto;  margin-right: auto;}
• When drawing a table, you want to wrap all cells with a uniform border, just like Excel, but there are cases where only the outermost cells have thin lines. In terms of HTML, borders are applied only to th and td.
• I prefer to apply this "n px to all cell borders, n px to table borders" border. This will give you a uniform border of 2n px overall.
.calendar-cell {  border: 1px solid black;  padding: 0.5rem;}

### 3-2. 토요일과 일요일, 오늘 하이라이팅​

.past-month,.next-month {  color: gray;}.sun {  color: red;}.sat {  color: blue;}.past-month.sun {  color: pink;}.next-month.sat {  color: lightblue;}.today {  color: #e5732f;}

## I felt that​

• At first, I got a little lost when connecting with JS to "initialize" the calendar. This is because you connected renderCalendar to the top of body. Since the DOM is executed sequentially, if you connect it to the top, if the #App div does not appear, renderCalendar will be executed and the DOM element will not be found.
• Also, I couldn't remember how to render codes that can be expressed in JS associations on the screen. It was simply to querySelect the app in js, which plays the role of index.js, and then insert it into innerHTML.
• In the Woowa Tech Camp project, magic numbers were used. This time, the magic number was removed to improve readability.
• The Woowa Techcamp project was written in Object Oriented JavaScript (more precisely, Singleton pattern), but this time it was written in small functions.
• Tried to use ES6+ syntax. For example, I used it by putting variables in backticks or destructuring the return data of processDate. Also let and const were mainly used.
• I regret that getCalendarHTML could not have been written a little shorter.