为什么此javascript日期数组不起作用？

Question

I'm trying to populate an array with a list of dates.

When I go back to retrieve a date in the array, they are all the same date.

var paydates = new Array();
var i;
var firstDate = new Date();
firstDate.setFullYear(2010, 11, 26); //set the first day in our array as 12/26/10

for (i=0; i < 200; i++) //put 200 dates in our array
{
    paydates[i] = firstDate;
    firstDate.setDate(firstDate.getDate()+14); //add 14 days
    document.write("1st: " + i + ":" + paydates[i] + "<br />");
    //this lists the dates correctly
}

//when I go back to retrieve them:
for (i=0; i < 200; i++)
{
    document.write("2nd: " + i + ":" + paydates[i] + "<br />");
    //this lists 200 instances of the same date
}

It's probably something stupid, but I'm at a loss.

Thanks

Answer 1

In your loop, you assign paydates[i] a reference to firstDate . At the end of 200 iterations, all 200 locations in the paydates array are pointing to the last firstDate .

You should create a new Date instance in each iteration and then assign it to an index in the paydates array.

Also, you'll notice that the first date listed in your example is not 12/26/2010, but 1/9/2011. I'm not sure if that's a mistake or intentional, but as your code is, the first firstDate isn't the date you used to seed your array of dates.

JSFiddle of a working example that also simplifies your code a little bit. Here is the barebones code from the fiddle:

var paydates = []; // new array
var firstDate = new Date(2010, 11, 26); // seed date

for (var i = 0; i < 200; i++) {
    paydates.push(new Date(firstDate.getTime()));
    firstDate.setDate(firstDate.getDate() + 14); // add 14 days
}

Answer 2

Replace the first loop with:

var temp;

for (i=0; i < 200; i++) //put 200 dates in our array
{
    temp = new Date(firstDate.getTime());
    paydates[i] = temp;
    firstDate.setDate(firstDate.getDate()+14); //add 14 days
    document.write("1st: " + i + ":" + paydates[i] + "<br />");
}

The problem was that you were storing a reference to your one and only firstDate object to each index in your array.

Answer 3

You must create a new Date based on firstDate to get individual date to each element in array

Instead:

paydates[i] = firstDate;
firstDate.setDate(firstDate.getDate()+14); //add 14 days

write:

paydates[i] = new Date( firstDate.setDate(firstDate.getDate()+14));