moment.js startOf问题

Question

I am trying to get the start and end of a day (which is a few days from today) using moment.js . This is the code I have:

var today = moment();
var day = today.add(-5, "days");
var startOfDay = day.startOf("day");
var endOfDay = day.endOf("day");

console.log("today " + today.format());
console.log("day " + day.format());
console.log("start " + startOfDay.format());
console.log("end " + endOfDay.format());

And these are the logs:

I2015-11-10T15:19:02.930Z]today 2015-11-10T15:19:02+00:00
I2015-11-10T15:19:02.931Z]day 2015-11-05T15:19:02+00:00
I2015-11-10T15:19:02.932Z]start 2015-11-05T23:59:59+00:00
I2015-11-10T15:19:02.933Z]end 2015-11-05T23:59:59+00:00

As you can see, the start and end dates are exactly the same. The end date is as expected, however, the startOf function appears to be doing exactly what the endOf function does.

Is there perhaps something I am missing?

Answer 1

Dates are mutable, and are altered by the method calls. Your two dates are both actually the same date object. That is, day.startOf("day") returns the value of day both times you call it. You can make copies however:

var startOfDay = moment(day).startOf("day");
var endOfDay = moment(day).endOf("day");

That constructs two new instances.