Passing and copying arrays by value. How-to?

[nachoexr]

I have this function

function test(n)
{
   alert(n[0]);
   var a = n;
   a.shift();
   alert(n[0]);
}

then I run this:

test(Array(1,2));

The first alert says:

1,2

but the second says:

2

How can I pass by value, or copy an array by value? I can't understand memory management in javascript.

[rnd me]

array are always passed by ref.

one way around it:

replace:

var a = n;

with

var a = [].concat(n);

EDIT:

your test function uses the same array.
shift both returns the bottom element and removes it from the array.

thus, index 1 becomes index 0 after the shift, and you see 2 instead of 1 and 2 because 1 is gone.

[nachoexr]

Oh, I see. Thank you for the tip.

With you answer I've created this simple function:

function copyArray(a,b)
{
   var n=a.length;
   for(var i=0;i<n;i++)
   {
      b[i]=a[i];
   }
}

As the only way to "copy" an array, don't you?

EDIT:
I've found this better function:

http://my.opera.com/GreyWyvern/blog/show.dml/1725165

I hope it helps.

[rnd me]

must faster to do:

 function copyArray(a,b) {
 b= [].concat(a);
}

[FLEB]

Yeah, it's a bit of an old thread, but just in case anyone else bumps into it... a smarter way to implement this would be as an Array method, not a simple function:

Array.prototype.copy = function () {
   return [].concat(this);
}

Then, it becomes a method of every Array. Easy to read, simple to remember:

var myArray = [1,2,3,4,5];
var myArrayCopy = myArray.copy();

Now, I've got to go change my own Array.copy() helper-method. I'd been using a for/in loop. I don't know why I didn't think of concat() or similar.

[rnd me]

Yeah, it's a bit of an old thread, but just in case anyone else bumps into it... a smarter way to implement this would be as an Array method, not a simple function:

Array.prototype.copy = function () {
   return [].concat(this);
}

Then, it becomes a method of every Array. Easy to read, simple to remember:

var myArray = [1,2,3,4,5];
var myArrayCopy = myArray.copy();

Now, I've got to go change my own Array.copy() helper-method. I'd been using a for/in loop. I don't know why I didn't think of concat() or similar.

i think that if we are to pollute a global namespace like Array, we should have a very good reason for doing so, more than just renaming concat copy...

you don't need an extra copy method, or a blank array, just use concat or slice:

var myArray = [1,2,3,4,5];
var myArrayCopy = myArray.concat();