else if construct in JavaScript

The else if construct allows you to set conditions in the else block. Let's look at an example:

let num = 1;

if (num == 1) {
	console.log('value1');
} else if (num == 2) {
	console.log('value2');
} else if (num == 3) {
	console.log('value3');
}

The advantage of using else if instead of several ifs is the ability to catch the situation when the value of the variable num does not match any of the conditions:

let num = 1;

if (num == 1) {
	console.log('value1');
} else if (num == 2) {
	console.log('value2');
} else if (num == 3) {
	console.log('value3');
} else {
	console.log('incorrect value of the variable num');
}

The variable day contains some number in the range from 1 to 31. Determine in which decade of the month this number falls (in the first, second or third).

Modify the previous task so that if the variable day is not a number from 1 to 31, an error message would be issued.