How to preserve nested array value when modify a value of an cloned array?

Question

I have a default nested array called default_array, very simple :

default_array = [
        ["a", "b", "c"] 
];

And I create a obj called obj where the value of his array attribute is the copy of default_array :

obj = {

    "array" : default_array.slice(0)
};

But when I modify a element of the obj.array like this :

obj.array[0][0] = "z";

This modify also the default_array. I want this modification doesn't affect default_array. I want to preserve default_array.

Any idea ?

Try var copy = default_array.map(function(e) { return e; }); Then use copy — Tushar
– Tushar, Commented Oct 16, 2015 at 17:57
Use Pollyfill from MDN. This should be added in Question, IE8 — Tushar
– Tushar, Commented Oct 16, 2015 at 18:09

Vishnu · Accepted Answer · 2015-10-21 04:36:41Z

2

function copy(array) {
    var result = [];
    for (var i = 0, len = array.length; i < len; i++) {
        result.push(array[i].slice());
    }
    return result;
}

var default_array = [
    ["a", "b", "c"]
];
var obj = {
    "array": copy(default_array)
};

obj.array[0][0] = 'z';
console.log(obj.array);
console.log(default_array);

edited Oct 21, 2015 at 4:36

Vishnu

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answered Oct 16, 2015 at 18:09

Oleksandr T.

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1 Comment

Oleksandr T. Over a year ago

Extended version for case if in parent array not only arrays jsfiddle.net/_alexander_/o96hL8L9/1

Fabio Beltramini · Accepted Answer · 2015-10-16 18:09:39Z

0

This may be overkill for your problem, but if you are running into it more and more, consider using Immutable.js

It provides efficient immutable datatypes.... just look at https://facebook.github.io/immutable-js/ for example code that uses the ibrary.

answered Oct 16, 2015 at 18:09

Fabio Beltramini

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Comments

Andrei Kropotin · Accepted Answer · 2015-10-16 18:12:12Z

0

You can try this:

obj = {
    "array" : JSON.parse(JSON.stringify(default_array))
};

answered Oct 16, 2015 at 18:12

Andrei Kropotin

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Comments

Abhas Tandon · Accepted Answer · 2015-10-21 04:15:17Z

0

Try using map or Or you can use combination of JSON.parse & JSON.stringify since you are having a nested/multidimensional array:

default_array = [
    ["a", "b", "c"]
];
var newArray = default_array.map(function(arr) {
    return arr.slice();
});
//Or you can use combination of JSON.parse & Stringify
var newArray = JSON.parse(JSON.stringify(default_array));
obj = {
    "array": newArray.slice(0)
};

edited Oct 21, 2015 at 4:15

answered Oct 16, 2015 at 18:03

Abhas Tandon

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3 Comments

totoaussi Over a year ago

map not supported by IE8. Without .map() and without jquery is possible ?

Abhas Tandon Over a year ago

you can use combination of JSON.parse & JSON.stringify see edit

thefourtheye Over a year ago

Nit: This will not work if all the elements of default_array are not arrays.

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