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];

Use Array.includes for multiple conditionals

See also