array.slice() to copy a new array

Question

var arr  = [ ['hello', 'and', 'hi'], [2,3,4] ],
    arr2 = arr.slice();
arr2[1].push(44);
arr[0] = "new value";

console.log(arr, arr2);

//["new value", [2, 3, 4, 44]]
//[["hello", "and", "hi"], [2, 3, 4, 44]]

Isn't arr2 = arr.slice() supposed to create a new copy of arr? Therefore, arr2[1].push(44) won't effect the original arr

Can anyone tell me why the console logged arr has number 44 in its second element?

Because you copy only parent array, you also need copy all child arrays too, like this https://jsfiddle.net/o96hL8L9/2/ — Oleksandr T., Oct 21 '15 at 08:38
[Source](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice) : The slice() method returns a shallow copy of a portion of an array into a new array object. — Harsh Gupta, Oct 21 '15 at 08:39

score 1 · Answer 1 · answered Oct 21 '15 at 08:38

From documentation:

For object references (and not the actual object), slice copies object references into the new array. Both the original and new array refer to the same object. If a referenced object changes, the changes are visible to both the new and original arrays.

reference

So it's a desired behaviour - you have copied a reference to the second array so it has been changed in both arr and arr2 variables.

array.slice() to copy a new array

1 Answers1