Use Array.includes for multiple conditionals

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