Reverse array confusion

[vikrac]

Hi All,

I have a very strange confusion which i am i am not able to understand. The issue is as below:
Output of following code is: col, up, dl, ny

<script type="text/javascript">

var city = ["ny", "dl", "up", "col"];
document.write(city .reverse());

</script>

Now, the output of the following code is the same: col, up, dl, ny

<script type="text/javascript">

var city = ["ny", "dl", "up", "col"];
var newCity = city;
newCity.reverse()
document.write(city);

</script>

Why is variable city reversed in this case?

Can anyone explain please? This is the first time i encountered this.

You can try it here: http://www.w3schools.com/jsref/tryit...yjsref_reverse

Thanks in advance,
Vikram

[Declan1991]

It's to do with the fact it's an object. newCity actually refers to the same object in memory as city does, so changing one is identical to changing the other.
A better explanation.

[rnd me]

declan explains half the effect: the part about all objects in js are passed byRef.

the other trick is that Array.reverse is mutative, which means that it actually alters the Array it's called on. splice(), sort(), and the stack operators like push/pop also mutate.

many array methods dont: map(), filter(), slice(), join(), etc all leave the orig intact.

if you want another copy to reverse: newCity = city.slice().reverse()

[JMRKER]

Would .concat() work just as well?

<script type="text/javascript">

var city = ["ny", "dl", "up", "col"];
var newCity = [].concat(city);
newCity.reverse()
document.write(city);
document.write('<p>');
document.write(newCity);

</script>

Leaves original untouched and a new array with reversed values.

[rnd me]

i've found slice to bench slightly higher than concat in enough browsers to make it worth using slice(). plus, slice is one fewer key to press than concat...