Copy an arbitrary n-dimensional array in JavaScript?

This works for arrays, it won't work for nested objects (that aren't Arrays):

function copy(arr){
    var new_arr = arr.slice(0);
    for(var i = new_arr.length; i--;)
        if(new_arr[i] instanceof Array)
            new_arr[i] = copy(new_arr[i]);
    return new_arr;
}

Use it like this:

var arr = [
   [[1,2,3],[75]], 100,
   [[[1]]], [],
   [1,[2,[3],4],5], 6, 7, 8
];

var new_arr = copy(arr);

Using jQuery (works for arrays and "plain" objects):

var a = [[1,[2,3]],[4]];
var b = $.extend(true, [], a);
a[0][0] = 42;             // modify object in a
alert(JSON.stringify(b)); // [[1,[2,3]],[4]]

Or JSON direct (if all objects are JSON-izable):

var a = [[1,[2,3]],[4]];
var b = JSON.parse(JSON.stringify(a))
a[0][0] = 42;             // modify object in a
alert(JSON.stringify(b)); // [[1,[2,3]],[4]]

Older versions of IE (8? 7?) will need a shim for the JSON object.

I just wanted to add to Paulpro's answer. Note that this is identical to his answer except that I changed copy(new_arr[i]) to copy(arr[i]), and new_arr.length to arr.length.

function copy(arr){
var new_arr = arr.slice(0);
for(var i = arr.length; i--;)
    if(new_arr[i] instanceof Array)
        new_arr[i] = copy(arr[i]);
return new_arr;
}

The reason copy(new_arr[i]) worked is because the .slice copied over what arr[i] was pointing at, making them equal.

Also, while Paulpro's answer works for all cases, if by chance each member of each dimension of the multi-dimensional array is either an array or a non-array you can make it more efficient by only slicing non-array dimensions. I mean what is the point of copying over an array of pointers that will simply be overwritten by the following recursion?

function copy(arr){
if(arr[0] instanceof Array){
    var new_arr = new Array(arr.length);
    for(var i = arr.length; i--;)
        new_arr[i] = copy(arr[i]);
}
else{var new_arr = arr.slice(0);}
return new_arr;
}

This is my solution to clone a multidimensional array; Actually i had to invent Array.prototype.clone() in order to invent a function to generate N dimension array and initialize it with a value.

Array.prototype.clone = function(){
  return this.reduce((p,c,i) => (p[i] = Array.isArray(c) ? c.clone() : c, p),[])
}

function arrayND(...n){
  return n.reduceRight((p,c) => c = (new Array(c)).fill(true).map(e => Array.isArray(p) ? p.clone() : p ));
}

var arr = arrayND(...[4,4,4],8); //last argument is the initializing value
arr[0][1][3] = "eight";
console.log(JSON.stringify(arr));