Why splice() method changes the value of other array?

Question

I have an array say

var list = ["first", "second"];

now I assign list to some other variable say

var temp = list;

Now when I use splice on list like

list.splice(0,1);

Now when I check the value of list it shows

list = ["second"]

Also when I check the value of temp then it says

temp = ["second"]

I want to know why is that so? Why the value of temp is changed?

Because `temp` is only a **pointer** to the actual array, it's not being copied. — Shadow The GPT Wizard, Apr 17 '12 at 08:38
possible duplicate of [Why does changing an Array in JavaScript affect copies of the array?](http://stackoverflow.com/questions/6612385/why-does-changing-an-array-in-javascript-affect-copies-of-the-array) — Shadow The GPT Wizard, Apr 17 '12 at 08:39

Fabrizio Calderan · Accepted Answer · 2018-08-28T06:01:06.990

7

when you do an assignment like var temp = list you're creating a reference temp to list. So since splice changes list array in-place, you're also changing temp

Use slice instead which returns a copy of the array, like so

var temp = list.slice();

edited Aug 28 '18 at 06:01

answered Apr 17 '12 at 08:39

Fabrizio Calderan

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So how do I overcome this problem? – me_digvijay Apr 17 '12 at 08:40

score 1 · Answer 2 · answered Apr 17 '12 at 08:42

A common mistake in JS

This is a pointer, not a clone of the object:

var temp = list;

If you want to actually copy the object, there are a few ways. The simplest is just to concat it with itself:

var temp = list.concat([]);
// or
var temp = list.slice();

Note that this is somewhat dangerous, it only gets the base values out of the array. There are more advanced methods for cloning objects and creating 'perfect' array clones.

score 1 · Answer 3 · edited May 23 '17 at 10:08

1

slice

should do the trick:

var temp = list.slice(0);

About object cloning, look here How do you clone an Array of Objects in Javascript?

edited May 23 '17 at 10:08

Community

1
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answered Apr 17 '12 at 08:42

mplungjan

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score 0 · Answer 4 · answered Apr 17 '12 at 08:48

You can use a prototype method to add this functionality.

Object.prototype.clone = function() {
  var newObj = (this instanceof Array) ? [] : {};
  for (i in this) {
    if (i == 'clone') continue;
    if (this[i] && typeof this[i] == "object") {
      newObj[i] = this[i].clone();
    } else newObj[i] = this[i]
  } return newObj;
};

Which then allows you to do this:

var a = [1,2,3];

var b = a.clone();

a[1] = 5;

console.log(a); //1,5,3
console.log(b); //1,2,3

Disclaimer: shamelessly borrowed from here (the whole article is worth a read): http://my.opera.com/GreyWyvern/blog/show.dml/1725165

Why splice() method changes the value of other array?

4 Answers4