JavaScript array reduction

Question

function init(){
var panel = document.getElementById("panel");
var i;

var week = ["monday","tuesday","wednesday","thursday","friday"];
var weekend = ["saturday","sunday"];

panel.innerHTML+="weekdays: "+ week;
panel.innerHTML+="<hr>weekend days: "+ weekend;

for(i=0; i<weekend.length; i++)
{
    week[week.length]=weekend[i];
}

panel.innerHTML+="<hr>increased with weekend days: "+week;

week-=2;

panel.innerHTML+="<hr>reduced back to weekdays: "+week;
}
window.onload=init;

this is the main js code it is loaded from another html file. everything is working fine until the end. the last output should be "reduced back to weekdays: monday,tuesday,wednesday,thursday,friday" but what im getting is this "reduced back to weekdays: NaN", what does this error mean and how can i fix it?

Esailija · Accepted Answer · 2012-07-05 09:24:31Z

11

You should reduce the array length by 2, not the array itself:

week.length -= 2;

Example:

var week = [1,2,3,4,5,6,7];
week.length -= 2;

console.log(week);
//[1, 2, 3, 4, 5]

answered Jul 5, 2012 at 9:24

Esailija

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7 Comments

Platinum Azure Over a year ago

I feel like it's better practice to use the well-defined array methods rather than messing directly with its length, but maybe you can explain if I'm wrong?

Esailija Over a year ago

@PlatinumAzure reducing length is well defined. developer.mozilla.org/en/JavaScript/Reference/Global_Objects/…

Platinum Azure Over a year ago

Too used to data hiding, I guess. Link to the ECMAScript standard documenting this and I'll +1 you.

Lee Kowalkowski Over a year ago

15.4 array objects : whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted. ecma-international.org/ecma-262/5.1/#sec-15.4 - But I prefer to use functions to change things like this.

Lee Kowalkowski Over a year ago

Ha ha, yes, I'm talking about programming not the internals of a language specification. I don't mind using = to assign values to an object (instead of setAttribute()), but when it does something like delete items, or reload documents (location.href= vs location.assign) I think the functions are more readable.

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aravind_reddy · Accepted Answer · 2021-04-28 06:15:07Z

5

maybe you have to change:

week-=2;

to:

week.splice(-2);

Edit (thanks @Esailija):

splice return the removed elements, so you have just to call the splice method, without assign the result back to the array:

week.splice(-2);

edited Apr 28, 2021 at 6:15

aravind_reddy

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answered Jul 5, 2012 at 9:29

MaJac89

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2 Comments

Esailija Over a year ago

.splice returns an array of the removed elements, he wants it to have weekdays again not the weekend days

Lee Kowalkowski Over a year ago

This answer is the most appropriate for the desired behaviour. Assigning a value to length feels dirty (the length of other types is read-only), and pop() is not scalable (if you wanted to discard a lot of items from the end of an array) because it's a stack metaphor .

Platinum Azure · Accepted Answer · 2012-07-05 09:24:33Z

2

You can't subtract a number from an array and expect meaningful results. That is mixing types and of course the behavior will be weird.

To get the weekend days out, use the pop() method of the array object.

answered Jul 5, 2012 at 9:24

Platinum Azure

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Comments

Someth Victory · Accepted Answer · 2012-07-05 09:51:57Z

0

function init(){
var panel = document.getElementById("panel");
var i;

var week = ["monday","tuesday","wednesday","thursday","friday"];
var weekend = ["saturday","sunday"];

panel.innerHTML+="weekdays: "+ week;
panel.innerHTML+="<hr>weekend days: "+ weekend;

for(i=0; i<weekend.length; i++)
{
    week[week.length]=weekend[i];
}

panel.innerHTML+="<hr>increased with weekend days: "+week;

week.length -= 2;

panel.innerHTML+="<hr>reduced back to weekdays: "+week;
}
window.onload=init;

but it's not a good way to use .length for setter!

answered Jul 5, 2012 at 9:51

Someth Victory

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