← Back toFront-end tips

Use Array.includes for multiple conditionals

Written byPhuoc Nguyen
23 Mar, 2021
Let's consider a function that determines the total number of days in a given month.
const getDays = (month, year) => {
Since months in JavaScripts start from 0, we assume that the `month` parameter is zero-based index. We can make the function quickly return if the month is April, June, September or November:
if (month === 3 || month === 5 || month === 8 || month === 10) {
return 30;
These conditionals can be replaced with a single check by using the `Array.includes` function:
if ([3, 5, 8, 10].includes(month)) {
return 30;
For this specific function, we also can use a lookup table:
const isLeapYear = (year) => year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);

const getDays = (month, year) => [31, isLeapYear(year) ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];

See also

If you found this post helpful, please consider giving the repository a star on GitHub or sharing the post on your favorite social networks 😍. Your support would mean a lot to me!

Questions? 🙋

Do you have any questions about front-end development? If so, feel free to create a new issue on GitHub using the button below. I'm happy to help with any topic you'd like to learn more about, even beyond what's covered in this post.
While I have a long list of upcoming topics, I'm always eager to prioritize your questions and ideas for future content. Let's learn and grow together! Sharing knowledge is the best way to elevate ourselves 🥷.
Ask me questions

Recent posts ⚡

Newsletter 🔔

If you're into front-end technologies and you want to see more of the content I'm creating, then you might want to consider subscribing to my newsletter.
By subscribing, you'll be the first to know about new articles, products, and exclusive promotions.
Don't worry, I won't spam you. And if you ever change your mind, you can unsubscribe at any time.
Phước Nguyễn