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Creating Calendar with JavaScript Date Object


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.


  • 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


  • 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">
<meta charset="UTF-8" />
content="width=device-width, initial-scale=1.0"
<title>JavaScript Calendar</title>
<div id="navigator"></div>
<div id="calendar"></div>
// code for rendering

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.js`
const html = (s, ...args) =>, i) => `${ss}${args[i] || ''}`).join('')

to be added – screenshot of highlighting


Then we connect calendar.js and index.html.

<!-- `index.html` -->
<script src="calendar.js"></script>

Defining constants will help before writing renderCalendar().

// `calendar.js`
const NAME_OF_DAYS = [
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.js`
const 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.js`
const 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),
💬Work in Progress

Work in Progress. Check back later.