get number of days in the CURRENT month using javascript

Does this do what you want?

function daysInThisMonth() {
  var now = new Date();
  return new Date(now.getFullYear(), now.getMonth()+1, 0).getDate();
}

based on the answer from this post: What is the best way to determine the number of days in a month with javascript?

It should be easy to modify this to work for the current month Here's your code and the function from the other post:

function myFunction() {
    var today = new Date();
    var month = today.getMonth();
    console.log(daysInMonth(month + 1, today.getFullYear()))
}

function daysInMonth(month,year) {
  return new Date(year, month, 0).getDate();
}

myFunction();

Note that the function date.getMonth() returns a zero-based number, so just add 1 to normalize.