.slice() method in Javascript not copy by value

Question

Okay so I am trying to make an AI for a game called notakto. That much isn't relevant however, in order to make the search algorithm that I do I need to duplicate array. So I have a global array called board which looks like this [[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0]] where all of the 0s are different numbers. To copy this I have the following line. var newboard=board.slice(). Problem is when I run a line of code like newboard[0][0]=1 it changes also acts as if I have run the following board[0][0]=1.

.slice() makes a copy of the array it's given, but it doesn't work recursively. — Barmar
– Barmar, Commented Mar 28, 2016 at 21:31
If you're using jQuery, you can get a deep copy with $.extend. — Barmar
– Barmar, Commented Mar 28, 2016 at 21:32
There's always the old var copy = JSON.parse(JSON.stringify(board)); trick — adeneo
– adeneo, Commented Mar 28, 2016 at 21:35

Querenker · Accepted Answer · 2016-03-28 22:28:52Z

1

You copy your array, but the inner arrays are not copied. This is a litte bit hacky but it works:

var newboard = JSON.parse(JSON.stringify(board));

In this context it will work, but if your object has any functions these are lost through stringify. Moreover it could make some trouble with Data objects which are stored inside the object.

edited Mar 28, 2016 at 22:28

answered Mar 28, 2016 at 21:38

Querenker

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2 Comments

Felix Kling Over a year ago

You might want to explain the limitations of this approach so that others are not blindly copying this.

Querenker Over a year ago

@FelixKling Thanks for your advice. I edited the post to mention some limitations I know.

Natarajan Ganapathi · Accepted Answer · 2016-03-28 21:39:21Z

0

javascript is always reference based. if you want to make a duplicate copy, please do deep object copy instead of sallow copy. In angular, angular.copy() will perform deep copy.

var newboard=angular.copy(board.slice());

answered Mar 28, 2016 at 21:39

Natarajan Ganapathi

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2 Comments

ak_ Over a year ago

There is no angular tag on this question.

Natarajan Ganapathi Over a year ago

that is for just example to understand the concept.

user162097 · Accepted Answer · 2016-03-28 21:50:08Z

0

It's because nested arrays are shallow-copied by reference. You can use a recursive function like this to deep-copy a multidimensional array such as the one you have above, making sure that each nested array is copied by value:

function copyMultidimensionalArray(array) {
    var r = array.slice(0);
    for (var i = 0, l = r.length; i < l; ++i) {
         if (Array.isArray(r[i])) {
             r[i] = copyMultidimensionalArray(r[i]);
         }
    }
    return r;
}

/* example */

var board = [[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0]],
    anotherBoard = copyMultidimensionalArray(board);

anotherBoard[0][0] = 99;

document.write([
    "board[0][0] ===",
    board[0][0],
    "&& anotherBoard[0][0] ===",
    anotherBoard[0][0]
].join(" "));

edited Mar 28, 2016 at 21:50

answered Mar 28, 2016 at 21:33

user162097

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user1626594 · Accepted Answer · 2016-03-28 22:43:46Z

0

You can also use board.map(function(cv){ return cv.slice();}) to copy the board:

var board = [[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0]];

var board2 = board.map(function(cv){ return cv.slice();});

board2[0][0] = 1;

console.log(board);
console.log(board2);

answered Mar 28, 2016 at 22:43

user1626594

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