All array elements have same value

Question

Following is my JavaScript code:

var enddate, finaldate, startyear, endyear, i;
startyear = 2010;
endyear = 2011;
enddate = new Date(endyear, 11, 31)
finaldate = new Date(enddate)
var a = [];
j = 0;
enddate.setDate(enddate.getDate() + 6)
for (i = new Date(startyear, 0, 1); i <= enddate; i.setDate(i.getDate() + 6)) {
    if (i > finaldate) {
        console.log(finaldate);
    } else {
        console.log("Value of i: " + i);
        a.push(i);

        console.log(a[j++]);
    }
}

for (var k = 0; k <= a.length; k++) {
    console.log(a[k])
}

The statement console.log("Value of i: " +i); prints the correct values in console but when I try printing the values of an array a at the end, it gives me ALL the values as Mon Jan 09 2012 00:00:00 GMT-0500 (EST), cannot understand why would this happen.

It's happening because you have a for loop that's iterating over date objects, and i is a global reference to an object that you keep changing. To be clearer, you're not pushing unique values to the array, but the same reference. — adeneo
– adeneo, Commented Oct 14, 2015 at 21:09

davcs86 · Accepted Answer · 2015-10-14 21:12:21Z

1

You need to create a new date object inside the loop. When using .push() on an array in javascript, the objects and arrays are pushed by reference. Built-in types like numbers are pushed as a copy.

var enddate, finaldate, startyear, endyear, i;
startyear = 2010;
endyear = 2011;
enddate = new Date(endyear, 11, 31)
finaldate = new Date(enddate)
var a = [];
j = 0;
enddate.setDate(enddate.getDate() + 6)
for (i = new Date(startyear, 0, 1); i <= enddate; i.setDate(i.getDate() + 6)) {
    if (i > finaldate) {
        console.log(finaldate);
    } else {
        console.log("Value of i: " + i);
        var m = new Date(i);
        a.push(m);

        console.log(a[j++]);
    }

}


for (var k = 0; k <= a.length; k++) {

    console.log(a[k])
}

answered Oct 14, 2015 at 21:12

davcs86

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2 Comments

davcs86 Over a year ago

console.log(a[j++]); give the "correct " date, because as all the items of a are references to i, it displays the last value of i

davcs86 Over a year ago

and for the same reason all the values are "jan 9 2012", which it is the last value assigned to i before exit the for loop.

Quentin · Accepted Answer · 2015-10-14 21:15:08Z

1

You create a date object at the start of the loop and store a reference to it in i.

Each time you go around the loop, you copy that reference into the array and modify the value of the date object.

You end up with an array containing a bunch of references to the same date object which is set to the final value you set it to.

You need to create a new date object each time you go around the loop.

e.g.

a.push(new Date(i));

answered Oct 14, 2015 at 21:15

Quentin

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4 Comments

Ketcomp Over a year ago

On line 16, why does "console.log(a[j++]);" give the correct dates?

pku Over a year ago

okay, that given solution worked but I am still unclear as to why the date being stored in array "a" is "jan 9 2012" my end date according to code should be maximum "jan 06 2012"

davcs86 Over a year ago

Because "jan 9 2012" is the last value assigned to i before exit the for loop

Quentin Over a year ago

@Ketcomp — console.log(a[j++]); works because you are testing inside the original loop, so you are testing before you assign the new value.

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