Copy array by value

Question

When copying an array in JavaScript to another array:

var arr1 = ['a','b','c'];
var arr2 = arr1;
arr2.push('d');  // Now, arr1 = ['a','b','c','d']

I realized that arr2 refers to the same array as arr1, rather than a new, independent array. How can I copy the array to get two independent arrays?

Answer 1

Use this:

let oldArray = [1, 2, 3, 4, 5];

let newArray = oldArray.slice();

console.log({newArray});

Basically, the slice() operation clones the array and returns a reference to a new array.

Also note that:

For references, strings and numbers (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.

Primitives such as strings and numbers are immutable, so changes to the string or number are impossible.

Answer 2

In Javascript, deep-copy techniques depend on the elements in an array. Let's start there.

Three types of elements

Elements can be: literal values, literal structures, or prototypes.

// Literal values (type1)
const booleanLiteral = true;
const numberLiteral = 1;
const stringLiteral = 'true';

// Literal structures (type2)
const arrayLiteral = [];
const objectLiteral = {};

// Prototypes (type3)
const booleanPrototype = new Bool(true);
const numberPrototype = new Number(1);
const stringPrototype = new String('true');
const arrayPrototype = new Array();
const objectPrototype = new Object(); // or `new function () {}

From these elements we can create three types of arrays.

// 1) Array of literal-values (boolean, number, string) 
const type1 = [ true, 1, "true" ];

// 2) Array of literal-structures (array, object)
const type2 = [ [], {} ];

// 3) Array of prototype-objects (function)
const type3 = [ function () {}, function () {} ];

Deep copy techniques depend on the three array types

Based on the types of elements in the array, we can use various techniques to deep copy.

Deep copy techniques

Benchmarks

https://www.measurethat.net/Benchmarks/Show/17502/0/deep-copy-comparison

• Array of literal-values (type1)
  The [ ...myArray ], myArray.splice(0), myArray.slice(), and myArray.concat() techniques can be used to deep copy arrays with literal values (boolean, number, and string) only; where slice() has the highest performance in Chrome, and spread ... has the highest performance in Firefox.

• Array of literal-values (type1) and literal-structures (type2)
  The JSON.parse(JSON.stringify(myArray)) technique can be used to deep copy literal values (boolean, number, string) and literal structures (array, object), but not prototype objects.

• All arrays (type1, type2, type3)

  • structuredClone();
  • The Lo-dash cloneDeep(myArray) or jQuery extend(true, [], myArray) techniques can be used to deep-copy all array-types. Where the Lodash cloneDeep() technique has the highest performance.
  • And for those who avoid third-party libraries, the custom function below will deep-copy all array-types, with lower performance than cloneDeep() and higher performance than extend(true).

function copy(aObject) {
  // Prevent undefined objects
  // if (!aObject) return aObject;

  let bObject = Array.isArray(aObject) ? [] : {};

  let value;
  for (const key in aObject) {

    // Prevent self-references to parent object
    // if (Object.is(aObject[key], aObject)) continue;
    
    value = aObject[key];

    bObject[key] = (typeof value === "object") ? copy(value) : value;
  }

  return bObject;
}

So to answer the question...

Question

var arr1 = ['a','b','c'];
var arr2 = arr1;

I realized that arr2 refers to the same array as arr1, rather than a new, independent array. How can I copy the array to get two independent arrays?

Answer

Because arr1 is an array of literal values (boolean, number, or string), you can use any deep copy technique discussed above, where slice() and spread ... have the highest performance.

arr2 = arr1.slice();
arr2 = [...arr1];
arr2 = arr1.splice(0);
arr2 = arr1.concat();
arr2 = JSON.parse(JSON.stringify(arr1));
arr2 = structuredClone(arr1);
arr2 = copy(arr1); // Custom function needed, and provided above
arr2 = _.cloneDeep(arr1); // Lo-dash.js needed
arr2 = jQuery.extend(true, [], arr1); // jQuery.js needed

Answer 3

You can use array spreads ... to copy arrays.

const itemsCopy = [...items];

Also if want to create a new array with the existing one being part of it:

var parts = ['shoulders', 'knees'];
var lyrics = ['head', ...parts, 'and', 'toes'];

Array spreads are now supported in all major browsers but if you need older support use typescript or babel and compile to ES5.

More info on spreads

Answer 4

No jQuery needed... Working Example

var arr2 = arr1.slice()

This copys the array from the starting position 0 through the end of the array.

It is important to note that it will work as expected for primitive types (string, number, etc.), and to also explain the expected behavior for reference types...

If you have an array of Reference types, say of type Object. The array will be copied, but both of the arrays will contain references to the same Object's. So in this case it would seem like the array is copied by reference even though the array is actually copied.

Answer 5

This is how I've done it after trying many approaches:

var newArray = JSON.parse(JSON.stringify(orgArray));

This will create a new deep copy not related to the first one (not a shallow copy).

Also this obviously will not clone events and functions, but the good thing you can do it in one line, and it can be used for any kind of object (arrays, strings, numbers, objects ...)

Answer 6

An alternative to slice is concat, which can be used in 2 ways. The first of these is perhaps more readable as the intended behaviour is very clear:

var array2 = [].concat(array1);

The second method is:

var array2 = array1.concat();

Cohen (in the comments) pointed out that this latter method has better performance.

The way this works is that the concat method creates a new array consisting of the elements in the object on which it is called followed by the elements of any arrays passed to it as arguments. So when no arguments are passed, it simply copies the array.

Lee Penkman, also in the comments, points out that if there's a chance array1 is undefined, you can return an empty array as follows:

var array2 = [].concat(array1 || []);

Or, for the second method:

var array2 = (array1 || []).concat();

Note that you can also do this with slice: var array2 = (array1 || []).slice();.

Answer 7

Important!

Most of answers here works for particular cases.

If you don't care about deep/nested objects and props use (ES6):

let clonedArray = [...array]

but if you want to do deep clone use this instead:

let cloneArray = JSON.parse(JSON.stringify(array))*

*functions won't be preserved (serialized) while using stringify, you will get result without them.

---

For lodash users:

let clonedArray = _.clone(array) documentation

and

let clonedArray = _.cloneDeep(array) documentation

Answer 8

I personally think Array.from is a more readable solution. By the way, just beware of its browser support.

// clone
let x = [1, 2, 3];
let y = Array.from(x);
console.log({y});

// deep clone
let clone = arr => Array.from(arr, item => Array.isArray(item) ? clone(item) : item);
x = [1, [], [[]]];
y = clone(x);
console.log({y});

Answer 9

Some of mentioned methods work well when working with simple data types like number or string, but when the array contains other objects these methods fail. When we try to pass any object from one array to another it is passed as a reference, not the object.

Add the following code in your JavaScript file:

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;
};

And simply use

var arr1 = ['val_1','val_2','val_3'];
var arr2 = arr1.clone()

It will work.

Answer 10

From ES2015,

var arr2 = [...arr1];

Answer 11

If you are in an environment of ECMAScript 6, using the Spread Operator you could do it this way:

var arr1 = ['a','b','c'];
var arr2 = [...arr1]; //copy arr1
arr2.push('d');

console.log(arr1)
console.log(arr2)

<script src="http://www.wzvang.com/snippet/ignore_this_file.js"></script>

Answer 12

Primitive values are always pass by its value (copied). Compound values however are passed by reference.

So how do we copy this arr?

let arr = [1,2,3,4,5];

Copy an Array in ES6

let arrCopy = [...arr]; 

Copy n Array in ES5

let arrCopy = arr.slice(); 
let arrCopy = [].concat(arr);

Why `let arrCopy = arr` is not passing by value?

Passing one varible to another on Compound values such as Object/Array behave difrently. Using asign operator on copand values we pass reference to an object. This is why the value of both arrays are changing when removing/adding arr elements.

Exceptions:

arrCopy[1] = 'adding new value this way will unreference';

When you assign a new value to the variable, you are changing the reference itself and it doesn’t affect the original Object/Array.

read more

Answer 13

Adding to the solution of array.slice(); be aware that if you have multidimensional array sub-arrays will be copied by references. What you can do is to loop and slice() each sub-array individually

var arr = [[1,1,1],[2,2,2],[3,3,3]];
var arr2 = arr.slice();

arr2[0][1] = 55;
console.log(arr2[0][1]);
console.log(arr[0][1]);

function arrCpy(arrSrc, arrDis){
 for(elm in arrSrc){
  arrDis.push(arrSrc[elm].slice());
}
}

var arr3=[];
arrCpy(arr,arr3);

arr3[1][1] = 77;

console.log(arr3[1][1]);
console.log(arr[1][1]);

same things goes to array of objects, they will be copied by reference, you have to copy them manually

Answer 14

let a = [1,2,3];

Now you can do any one of the following to make a copy of an array.

let b = Array.from(a); 

OR

let b = [...a];

OR

let b = new Array(...a); 

OR

let b = a.slice(); 

OR

let b = a.map(e => e);

Now, if i change a,

a.push(5); 

Then, a is [1,2,3,5] but b is still [1,2,3] as it has different reference.

But i think, in all the methods above Array.from is better and made mainly to copy an array.

Answer 15

I would personally prefer this way:

JSON.parse(JSON.stringify( originalObject ));

Answer 16

I found this method comparatively easier:

let arr = [1,2,3,4,5];
let newArr = [...arr];
console.log(newArr);

Answer 17

You must use best practice for this question when there are a lot of answers.

I recommend to you use array spreads … to copy arrays.

var arr1 = ['a','b','c'];

var arr2 = […arr1];

Answer 18

As we know in Javascript arrays and objects are by reference, but what ways we can do copy the array without changing the original array later one?

Here are few ways to do it:

Imagine we have this array in your code:

var arr = [1, 2, 3, 4, 5];

1) Looping through the array in a function and return a new array, like this:

 function newArr(arr) {
      var i=0, res = [];
      while(i<arr.length){
       res.push(arr[i]);
        i++;
       }
   return res;
 }

2) Using slice method, slice is for slicing part of the array, it will slice some part of your array without touching the original, in the slice, if don't specify the start and end of the array, it will slice the whole array and basically make a full copy of the array, so we can easily say:

var arr2 = arr.slice(); // make a copy of the original array

3) Also contact method, this is for merging two array, but we can just specify one of arrays and then this basically make a copy of the values in the new contacted array:

var arr2 = arr.concat();

4) Also stringify and parse method, it's not recommended, but can be an easy way to copy Array and Objects:

var arr2 = JSON.parse(JSON.stringify(arr));

5) Array.from method, this is not widely supported, before use check the support in different browsers:

const arr2 = Array.from(arr);

6) ECMA6 way, also not fully supported, but babelJs can help you if you want to transpile:

const arr2 = [...arr];

Answer 19

You could use ES6 with spread Opeartor, its simpler.

arr2 = [...arr1];

There are limitations..check docs Spread syntax @ mozilla

Answer 20

Dan, no need to use fancy tricks. All you need to do is make copy of arr1 by doing this.

var arr1 = ['a','b','c'];
var arr2 = [];

var arr2 = new Array(arr1);

arr2.push('d');  // Now, arr2 = [['a','b','c'],'d']

console.log('arr1:');
console.log(arr1);

console.log('arr2:');
console.log(arr2);

// Following did the trick:
var arr3 = [...arr1];
arr3.push('d');  // Now, arr3 = ['a','b','c','d'];

console.log('arr3:');
console.log(arr3);

Now arr1 and arr2 are two different array variables stored in separate stacks. Check this out on jsfiddle.

Answer 21

var arr2 = arr1.slice(0);

This way just work for simple Arrays.

If you have Complex Array like array of Objects then you must use another solutions like:

const arr2 = JSON.parse(JSON.stringify(arr1)); 

For example, we have an array of objects that each cell have another array field in its object ... in this situation if we use slice method then the array fields will copy by Ref and that's mean these fields updates will affect on orginal array same element and fields.

Answer 22

structuredClone

The modern way to copy array by value in JavaScript is to use structuredClone:

var arr1 = ['a', 'b', 'c'];
var arr2 = structuredClone(arr1); //copy arr1
arr2.push('d');

console.log(arr1); //a,b,c
console.log(arr2); //a,b,c,d

Answer 23

In my particular case I needed to ensure the array remained intact so this worked for me:

// Empty array
arr1.length = 0;
// Add items from source array to target array
for (var i = 0; i < arr2.length; i++) {
    arr1.push(arr2[i]);
}

Answer 24

Make copy of multidimensional array/object:

function deepCopy(obj) {
   if (Object.prototype.toString.call(obj) === '[object Array]') {
      var out = [], i = 0, len = obj.length;
      for ( ; i < len; i++ ) {
         out[i] = arguments.callee(obj[i]);
      }
      return out;
   }
   if (typeof obj === 'object') {
      var out = {}, i;
      for ( i in obj ) {
         out[i] = arguments.callee(obj[i]);
      }
      return out;
   }
   return obj;
}

Thanks to James Padolsey for this function.

Source: Here

Answer 25

If your array contains elements of the primitive data type such as int, char, or string etc then you can user one of those methods which returns a copy of the original array such as .slice() or .map() or spread operator(thanks to ES6).

new_array = old_array.slice()

or

new_array = old_array.map((elem) => elem)

or

const new_array = new Array(...old_array);

BUT if your array contains complex elements such as objects(or arrays) or more nested objects, then, you will have to make sure that you are making a copy of all the elements from the top level to the last level else reference of the inner objects will be used and that means changing values in object_elements in new_array will still affect the old_array. You can call this method of copying at each level as making a DEEP COPY of the old_array.

For deep copying, you can use the above-mentioned methods for primitive data types at each level depending upon the type of data or you can use this costly method(mentioned below) for making a deep copy without doing much work.

var new_array = JSON.parse(JSON.stringify(old_array));

There are a lot of other methods out there which you can use depending on your requirements. I have mentioned only some of those for giving a general idea of what happens when we try to copy an array into the other by value.

Answer 26

If you want to make a new copy of an object or array, you must explicitly copy the properties of the object or the elements of the array, for example:

var arr1 = ['a','b','c'];
var arr2 = [];

for (var i=0; i < arr1.length; i++) {
   arr2[i] = arr1[i];
}

You can search for more information on Google about immutable primitive values and mutable object references.

Answer 27

When we want to copy an array using the assignment operator ( = ) it doesn't create a copy it merely copies the pointer/reference to the array. For example:

const oldArr = [1,2,3];

const newArr = oldArr;  // now oldArr points to the same place in memory 

console.log(oldArr === newArr);  // Points to the same place in memory thus is true

const copy = [1,2,3];

console.log(copy === newArr);  // Doesn't point to the same place in memory and thus is false

Often when we transform data we want to keep our initial datastructure (e.g. Array) intact. We do this by making a exact copy of our array so this one can be transformed while the initial one stays intact.

Ways of copying an array:

const oldArr = [1,2,3];

// Uses the spread operator to spread out old values into the new array literal
const newArr1 = [...oldArr];

// Slice with no arguments returns the newly copied Array
const newArr2 = oldArr.slice();

// Map applies the callback to every element in the array and returns a new array
const newArr3 = oldArr.map((el) => el);

// Concat is used to merge arrays and returns a new array. Concat with no args copies an array
const newArr4 = oldArr.concat();

// Object.assign can be used to transfer all the properties into a new array literal
const newArr5 = Object.assign([], oldArr);

// Creating via the Array constructor using the new keyword
const newArr6 = new Array(...oldArr);

// For loop
function clone(base) {
	const newArray = [];
    for(let i= 0; i < base.length; i++) {
	    newArray[i] = base[i];
	}
	return newArray;
}

const newArr7 = clone(oldArr);

console.log(newArr1, newArr2, newArr3, newArr4, newArr5, newArr6, newArr7);

Be careful when arrays or objects are nested!:

When arrays are nested the values are copied by reference. Here is an example of how this could lead to issues:

let arr1 = [1,2,[1,2,3]]

let arr2 = [...arr1];

arr2[2][0] = 5;  // we change arr2

console.log(arr1);  // arr1 is also changed because the array inside arr1 was copied by reference

So don't use these methods when there are objects or arrays inside your array you want to copy. i.e. Use these methods on arrays of primitives only.

If you do want to deepclone a javascript array use JSON.parse in conjunction with JSON.stringify, like this:

let arr1 = [1,2,[1,2,3]]

let arr2 = JSON.parse(JSON.stringify(arr1)) ;

arr2[2][0] = 5;

console.log(arr1);  // now I'm not modified because I'm a deep clone

Performance of copying:

So which one do we choose for optimal performance. It turns out that the most verbose method, the for loop has the highest performance. Use the for loop for really CPU intensive copying (large/many arrays).

After that the .slice() method also has decent performance and is also less verbose and easier for the programmer to implement. I suggest to use .slice() for your everyday copying of arrays which aren't very CPU intensive. Also avoid using the JSON.parse(JSON.stringify(arr)) (lots of overhead) if no deep clone is required and performance is an issue.

Source performance test

Answer 28

Using jQuery deep copy could be made as following:

var arr2 = $.extend(true, [], arr1);

Answer 29

You can also use ES6 spread operator to copy Array

var arr=[2,3,4,5];
var copyArr=[...arr];

Answer 30

Here are few more way to copy:

const array = [1,2,3,4];

const arrayCopy1 = Object.values(array);
const arrayCopy2 = Object.assign([], array);
const arrayCopy3 = array.map(i => i);
const arrayCopy4 = Array.of(...array );