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?
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.
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.
Basically, the slice() operation clones the array and returns the reference to the new array - basically this is a deep copy then –
Household .slice() with .splice(), which gives you an empty array. Big difference. –
Cupulate var newArray = JSON.parse(JSON.stringify(orgArray)); to pass by value –
Leucopenia In Javascript, deep-copy techniques depend on the elements in an array. Let's start there.
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 () {} ];
Based on the types of elements in the array, we can use various techniques to deep copy.
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();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.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;
}
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?
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
arr1. It's very rare that that's going to be the case. Using splice obliterates arr1, so that's not a copy at all. Using JSON will fail if any of the values in the array are Functions or have prototypes (such as a Date). –
Siddon [0,"1",{2:3},function random() {return 4;}, [[5,6,7],[8,9,10],[11,12,13]]] and any other array. –
Darnel .slice() to work, since I was modifying an object through the reference... after the deep copy - works like a charm. –
Iormina arr.map(it => ({ ...it })), if you have an array of arrays: arr.map(Array.from) –
Dario arr.map(it => ({ ...it })), if you have an array of arrays (without elements that are objects): arr.map(Array.from)." –
Chanson for-in loop (as it has higher performance than .map) to deep copy elements into a new array –
Chanson x.clone()? –
Athamas 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.
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.
var arr2 = JSON.stringify(arr1); arr2 = JSON.parse(arr2); –
Squish 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 ...)
Date, or indeed, anything that has a prototype. In addition, undefineds get converted to nulls. –
Siddon 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();.
[].concat(array1) returns [array1] e.g. if its undefined you'll get [undefined]. I sometimes do var array2 = [].concat(array1 || []); –
Schear 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
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});.slice() solution is completely unintuitive. Thanks for this. –
Fanaticize 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.
.slice() still works fine even if you have objects in your array: jsfiddle.net/edelman/k525g –
Plankton 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>So how do we copy this arr?
let arr = [1,2,3,4,5];
let arrCopy = [...arr];
let arrCopy = arr.slice();
let arrCopy = [].concat(arr);
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.
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.
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
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.
I would personally prefer this way:
JSON.parse(JSON.stringify( originalObject ));
I found this method comparatively easier:
let arr = [1,2,3,4,5];
let newArr = [...arr];
console.log(newArr);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];
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];
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.
var arr2 = [arr1];). –
Flanna document.write(arr2) to console.log(arr2) so that nested array structure would show and better illustrate correct comment from @Timothy003. var arr3 = [...arr1] did the trick, though. Run code snippet to see results. (Output from document.write(arr2) was a bit misleading, hence I don't blame you). –
Yokefellow You could use ES6 with spread Opeartor, its simpler.
arr2 = [...arr1];
There are limitations..check docs Spread syntax @ mozilla
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.
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]);
}
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
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.
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.
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 falseOften 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.
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);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 referenceSo 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 cloneSo 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.
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,dUsing jQuery deep copy could be made as following:
var arr2 = $.extend(true, [], arr1);
You can also use ES6 spread operator to copy Array
var arr=[2,3,4,5];
var copyArr=[...arr];
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 );For ES6 array containing objects
cloneArray(arr) {
return arr.map(x => ({ ...x }));
}
Quick Examples:
var arr1 = ['a','b','c'];
// arr1 and arr2 are independent and primitive elements are stored in
// different places in the memory
var arr2 = arr1.slice();
arr2.push('d');
console.log(arr1); // [ 'a', 'b', 'c' ]
console.log(arr2); // [ 'a', 'b', 'c', 'd' ]var arr1 = [{ x: 'a', y: 'b'}, [1, 2], [3, 4]];
// arr1 and arr2 are independent and reference's/addresses are stored in different
// places in the memory. But those reference's/addresses points to some common place
// in the memory.
var arr2 = arr1.slice();
arr2.pop(); // OK - don't affect arr1 bcos only the address in the arr2 is
// deleted not the data pointed by that address
arr2[0].x = 'z'; // not OK - affect arr1 bcos changes made in the common area
// pointed by the addresses in both arr1 and arr2
arr2[1][0] = 9; // not OK - same above reason
console.log(arr1); // [ { x: 'z', y: 'b' }, [ 9, 2 ], [ 3, 4 ] ]
console.log(arr2); // [ { x: 'z', y: 'b' }, [ 9, 2 ] ]var arr1 = [{ x: 'a', y: 'b'}, [1, 2], [3, 4]];
arr2 = JSON.parse(JSON.stringify(arr1));
arr2.pop(); // OK - don't affect arr1
arr2[0].x = 'z'; // OK - don't affect arr1
arr2[1][0] = 9; // OK - don't affect arr1
console.log(arr1); // [ { x: 'a', y: 'b' }, [ 1, 2 ], [ 3, 4 ] ]
console.log(arr2); // [ { x: 'z', y: 'b' }, [ 9, 2 ] ]Here's a variant:
var arr1=['a', 'b', 'c'];
var arr2=eval(arr1.toSource());
arr2.push('d');
console.log('arr1: '+arr1+'\narr2: '+arr2);
/*
* arr1: a,b,c
* arr2: a,b,c,d
*/
toSource is not standard and will not work in Chrome for example. –
Teeters There's the newly introduced Array.from, but unfortunately, as of the time of this writing it's only supported on recent Firefox versions (32 and higher). It can be simply used as follows:
var arr1 = [1, 2, 3];
console.log(Array.from(arr1)); // Logs: [1, 2, 3]
Reference: Here
Or Array.prototype.map may be used with an identity function:
function identity(param)
{
return param;
}
var arr1 = [1, 2, 3],
clone = arr1.map(identity);
Reference: Here
Array.from, which is now supported on all major browsers except Internet Explorer (source). . –
Kreutzer Array.from will mutate data, doesn't provide deep copying/deep cloning. –
Consanguineous You can do that in following way :
arr2 = arr1.map(x => Object.assign({}, x));
Here is how you can do it for array of arrays of primitives of variable depth:
// If a is array:
// then call cpArr(a) for each e;
// else return a
const cpArr = a => Array.isArray(a) && a.map(e => cpArr(e)) || a;
let src = [[1,2,3], [4, ["five", "six", 7], true], 8, 9, false];
let dst = cpArr(src);
Just writting:
arr2 = arr1.concat();
You are generating new a new array with a copy of the first. Be aware that is a functional way to push an element into the array.
If your code is based on ES6 you can use spread operator as well:
arr2 = [...arr1];
structuredClone is a new way of deep cloning.
structuredClone(value)
structuredClone(value, { transfer })
transfer is an array of transferable objects in value that will be moved rather than cloned to the returned object.
You may find it's algorithm quite interesting.
None of these options worked for me as I was dealing with an array of deeply nested objects. For ES6 I found this solution helpful.
const old_array = [{name:"Nick", stats:{age:25,height:2}},{name:"June", stats:{age:20,height:2}}];
const new_array = old_array.map(e => {
if (e.name === 'June') {
e = { ...e };
e.stats = {...e.stats, age: 22};
}
return e;
});
Only the new_array will be affected by this.
After digging on it, I figured out that a clean approach can be:
const arr1 = [['item 1-1', 'item 1-2'], ['item 2-1', 'item 2-2'], ['item 3-1', 'item 3-2']];
/**
* Using Spread operator, it will create a new array with no reference to the first level.
*
* Since, the items are not primitive, they get their own references. It means that any change on them,
* it will be still reflected on the original object (aka arr1).
*/
const arr2 = [...arr1];
/**
* Using Array.prototype.map() in conjunction Array.prototype.slice() will ensure:
* - The first level is not a reference to the original array.
* - In the second level, the items are forced (via slice()) to be created as new ones, so there is not reference to the original items
*/
const arr3 = arr1.map(item => item.slice());
You need to understand the complexity of the arrays you want to work with to then apply the best solution (aka ➡️ referenced items inside a referenced array)
© 2022 - 2024 — McMap. All rights reserved.
sliceandspliceoperations and new spread operator andArray.fromhave much slower implementation. Look at perfjs.fnfo – Lamiavar arr2 = arr1.splice();to deep copy, but this technique won't work if the elements in your array contain literal structures (i.e.[]or{}) or prototype objects (i.e.function () {},new, etc). See my answer below for further solutions. – Chansonlet arr2 = [...arr1];developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… – Incandescencea = b;you actually tell the program to point in both cases to a same symbolic link in random access memory. And when a value at this symbolic link is changed it affectsaandb... So if you use a spread operatora= [...b];program will create an additional symbolic link to a different location in random access memory and you can then manipulateaandbindependently. – Inveterate