Javascript date setMonth does not set month correctly

Question

It looks like Javascript setMonth method does not change month correctly if it was used for 31st.

var event = new Date('July 31, 2018 10:48:30');

console.log(event.getMonth());
// output: 6

// Change to previous month
event.setMonth(event.getMonth() - 1);

console.log(event.getMonth());
// expected output: 5 actual output: 6

console.log(event);

Does anyone have a better option available to change month and set date of 30th if 31st is not in the month?

Answer 1

  let previous = event.getMonth();
  while(event.getMonth() === previous)
    event.setDate(event.getDate() - 1);

Just go back day by day.

Answer 2

Does anyone have a better option available to change month and set date of 30th if 31st is not in the month?

Just go a day before:

var event = new Date('July 31, 2018 10:48:30');

console.log(event.getMonth());

// Change to previous month
var current = event.getMonth();
event.setMonth(current - 1);
if (event.getMonth() === current) {
  event.setDate(event.getDate() - 1);
}

console.log(event.getMonth());
// actual output: 5

console.log(event);