Making a Calendar in Vanilla Javascript

Arguably, time is the best and worst built-in function in Javascript. For my most recent project, I had the idea to make a bastardized Google calendars. I naively figured that making a general, functional calendar would be straightforward and allow me plenty of time to build out my site. I so was wrong.

Where to Start?

Pretty early on, with guidance from my instructor, I decided to hard code a month into the HTML. This allowed me to build out the general structure and style I wanted for my calendar. There was conflicting literature on whether using <div> or <table> would be better practice. I ultimately decided to go with a table.

If you have ever taken the time to really study a month calendar, the structure is pretty simple. There is a header at the top, usually, that indicates the month and year. Below that are 7 columns outlining the individual days of the week. Finally, there is a maximum of 6 rows to populate the number of days for a given month.

Below is a snapshot of my hardcoded HTML.

Again, this allowed me to mess with the CSS style sheet to render the overall look and feel I wanted for my calendar.

Harnessing the Power of Time

Now came the hard part. I needed to use the built-in time function to populate the accurate number of days in a given month and year. After doing some research, I learned that Javascript will return a number that corresponds to a months relative index in a year: i.e. January is 0, February is 1, etc. This can be called using the getMonth() function built-in to Javascript.

Javascript also does the same thing with days of the week and starts with Sunday. That makes Sunday - 0, Monday - 1, etc. This is called using the getDay() function from Javascript.

So, the question became how do I use this two crucial pieces of information: 1. How do I figure out what day of the week is the start of the month? 2. How do I calculate how many days are in a month?

Answering the first questions was simple enough. Javascript literature helped outline the various options for built-in functions. Miraculously, new Date(year, month) is incredibly powerful and will automatically render the first day of a given year and month argument. Using the .getDay() function on this allowed me to get the day index for the start of the month.

Next, I needed to get the number of days in a given month. Again, I used the all-powerful new Date() to get me started, and using some high school algebra, I reverse engineered a way to calculate the total days. The new Date() function can accept a large number of arguments, down to the millisecond. I did not need to get granular with my calendar, but thought I could use the month, year and day arguments. Given that the maximum number of days in any month is 31, putting in 32 days into new Date() would give me the relative date that would actually be in the next month. For example, July(index 6 of a year) has 31 days in a month. If I entered new Date(2019, 6, 32) into my console, I would expect the console to return August 1, 2019.

The new Date() function also has a getDate() function that will return the day number from a given date. Using this returned number, I subtracted this from 32 to get the accurate number of days in a given month.

To further prove my concept, I used February 2019 in my console. Passing in 2019 for year, 1 for month, and 32 for days, I should expect the above equation to return 28, as there were 28 days in February 2019.

One hurdle down, now I had to use this data to populate and render my table.

Populating the Beast

After removing the hardcoded HTML data from my table, I needed to populate the month span and the year span with the currently rendered month and year. This was fairly straightforward as I left a majority of my HTML code in the header. After finding the individual span elements using document.findElementById(), I changed the textContent to the given month and year.

After grabbing the table element, again using document.getElementById(), I then needed to add some for loops to render my rows and columns. I also knew that I would need to print the actual day number on the calendar. For obvious reasons, I started the count at 1 and assigned this 1 to a usable variable called renderNum. (Note: This variable will be incremented up by one with each column loop. But I’ll get to that in a bit)

Next, I generated the row for loop, as this was going to hold all the individual date tiles. (To be more proper, I will append my row <tr> with my table data <td> elements.) Since I knew that I would need at maximum 6 rows, I started my for loop at the traditional 0 and had it stop when i reached 6. Seeing as there was no text content for the row, all I needed was to create the <tr> element and assign this to a usable variable.

As stated before, I wanted to append each row with 7 table data elements. This meant that I needed a nested for loop. This 2nd for loop also started at 0 and ended when the count (c in this case) reached 7. This was simple enough, but the next step took some logical reasoning. Remembering that the new Date(month, year).getDay() gave me the index of day of the week, I needed to setup empty <td>s so that the count and calendar rendering started on the correct day. To do this, I added an if statement in my column for loop that would check if: _1. The loop was on the first row (aka i===0), and _2. The value of count loop was less than the new Date(month, year).getDay() value (aka c < startOfMonth).

If both those conditions were true, I wanted the loop to create a <td> element, add an empty class to the newly rendered <td> and append this to the current row element.

Next, I wanted to make sure that I was populating the correct day number on each one of the new table data elements. Modifying my empty td element method, I added a <td> element for each new day, added the textContent, and appended the row. The textContent in this case would be the renderNum variable defined earlier. To make sure that the renderNum printed the correct date, I incremented the variable by 1.

Using the power of truthiness, I used an else if statement to stop the calendar from incrementing and adding data past the last day of the month.

Once I reached the break and was out of the for loops, I needed to make sure that to append the table body with the newly formed and populated row. (I did rearrange and group the code according to variables, and my personal preference. Obviously, this is not necessary. It was just a personal preference.)

Finally, I added EventListeners on the click of the arrows to toggle to the month before and after a calendar snapshot. Given the length of this post already, I will spare you. But there is plenty of literature on EventListeners that I recommend reading if you have trouble with that step.

Below is a snapshot of the rendered calendar. I will be sure to update this post with the github link, once the project is complete.

Next up, appointments, time zones and scheduling. Wish me luck!