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?

  • 2
    It looks like currently in Chrome 53 and Firefox 48 we have cool performance for slice and splice operations and new spread operator and Array.from have much slower implementation. Look at perfjs.fnfo– PencroffSep 16 '16 at 13:32
  • jsben.ch/#/wQ9RU <=this benchmark gives an overview over the different ways to copy an array– EscapeNetscapeOct 24 '16 at 18:47
  • For this array (one that contains primitive strings) you can use var 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.– tfmontagueAug 27 '17 at 22:37
  • 7
    It's 2017, so you might consider using ES6 features: let arr2=[...arr1]; developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…– HinrichSep 22 '17 at 8:30

29 Answers29

up vote2283down voteaccepted

Use this:

var newArray=oldArray.slice();

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.

  • 6
    Regarding performance the following jsPerf tests actually show that var arr2=arr1.slice() is just as fast as var arr2=arr1.concat(); JSPerf: jsperf.com/copy-array-slice-vs-concat/5 and jsperf.com/copy-simple-array . The result of jsperf.com/array-copy/5 kind of surprised me to the point I am wondering if the test code is valid.– CohenDec 19 '12 at 18:46
  • 83
    Even though this has already received a ton of upvotes, it deserves another because it properly describes references in JS, which is sort of rare, unfortunately.– Wayne BurkettJan 20 '14 at 16:29
  • 31
    @GáborImre you'd add an entire library simply for readability? Really? I'd just add a comment if I were that concerned for readability. See: var newArray=oldArray.slice(); //Clone oldArray to newArray– dudewadFeb 8 '16 at 19:47
  • 10
    @GáborImre I get that, sure. But answering a specific engineering problem by including an entire library in my opinion is not helpful, it's design bloat. I see developers do that a lot, and then you end up with a project that included an entire framework to replace having to write a single function. Just my M.O., though.– dudewadFeb 9 '16 at 19:48
  • @dudewad Generally you are right. In this case my whole JS experience suggests using this lib in almost every project. Lodash for arrays or objects, like Moment for dates, like Express for NodeJS server, like Angular or React for frontend.– Gábor ImreFeb 9 '16 at 21:02

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)var booleanLiteral=true;var numberLiteral=1;var stringLiteral='true';// Literal structures (type2)var arrayLiteral=[];var objectLiteral={};// Prototypes (type3)var booleanPrototype=new Bool(true);var numberPrototype=new Number(1);var stringPrototype=new String('true');var arrayPrototype=new Array();var objectPrototype=new Object(); # or "new function () {}"

From these elements we can create three types of arrays.

// 1) Array of literal-values (boolean, number, string) var type1=[true, 1, "true"];// 2) Array of literal-structures (array, object)var type2=[[], {}];// 3) Array of prototype-objects (function)var 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.

Javascript deep copy techniques by element types

  • Array of literal-values (type1)
    The 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 higher performance than Concat (http://jsperf.com/duplicate-array-slice-vs-concat/3).

  • 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)
    The jQuery $.extend(myArray) technique can be used to deep-copy all array-types. Libraries like Underscore and Lo-dash offer similar deep-copy functions to jQuery $.extend(), yet have lower performance. More surprisingly, $.extend() has higher performance than the JSON.parse(JSON.stringify(myArray)) technique http://jsperf.com/js-deep-copy/15.
    And for those developers that shy away from third-party libraries (like jQuery), you can use the following custom function; which has higher performance than $.extend, and deep-copies all arrays.

    function copy(o) {var output, v, key;output=Array.isArray(o) ? [] : {};for (key in o) {v=o[key];output[key]=(typeof v==="object" && v !==null) ? copy(v) : v;}return output;} 

So to answer the question...


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

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


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

// Highest performance for deep copying literal valuesarr2=arr1.slice();// Any of these techniques will deep copy literal values as well,// but with lower performance.arr2=arr1.splice(0);arr2=arr1.concat();arr2=JSON.parse(JSON.stringify(arr1));arr2=$.extend(true, [], arr1); // jQuery.js neededarr2=_.extend(arr1); // Underscore.js neededarr2=_.cloneDeep(arr1); // Lo-dash.js neededarr2=copy(arr1); // Custom-function needed - as provided above
  • 1
    Many of these approaches do not work well. Using the assignment operator means that you have to reassign the original literal value of 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).– DancrumbSep 18 '14 at 19:53
  • Using splice is a partial solution. It will fail under far more cases than JSON. Splice creates a deep-copy of strings and numbers, when it moves values - never said it returns a copy.– tfmontagueOct 3 '14 at 6:50
  • 1
    Why splice(0)? Shouldn't it be slice() ? I think it's supposed not to modify original array, which splice does. @JamesMontagne– helpseMay 14 '15 at 15:29
  • 2
    splice will create pointers to the elements in the original array (shallow copy). splice(0) will allocate new memory (deep copy) for elements in the array which are numbers or strings, and create pointers for all other element types (shallow copy). By passing a start value of zero to the splice function-method, it won't splice any elements from the original array, and therefore it doesn't modify it.– tfmontagueMay 21 '15 at 9:35
  • 1
    Actually, there is only one type of array: an array of "somethings". There is no difference between [0,"1",{2:3},function random() {return 4;}, [[5,6,7],[8,9,10],[11,12,13]]] and any other array.– wizzwizz4Apr 7 '16 at 9:12

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

    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.

    • 8
      No this would not be a deep copy.– jondavidjohnOct 14 '14 at 22:16

    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();.

    • 31
      Actually you can also do: var array2=array1.concat(); It's a lot faster regarding performance. (JSPerf: jsperf.com/copy-simple-array and jsperf.com/copy-array-slice-vs-concat/5– CohenDec 19 '12 at 18:50
    • 4
      Its worth noting that if array1 isn't an array then [].concat(array1) returns [array1] e.g. if its undefined you'll get [undefined]. I sometimes do var array2=[].concat(array1 || []);– lee penkmanAug 1 '14 at 8:27

    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 ...)

    • 3
      This is the best one. I use the same method a long time ago and think that there is no more sense in old school recursive loops– Vladimir KharlampidiMay 5 '14 at 20:28
    • 1
      Be aware that this option doesn't handle well graph-like structures: crashes in presence of cycles, and doesn't preserve shared references.– RubenJun 28 '14 at 23:12
    • 1
      This also fails for things like Date, or indeed, anything that has a prototype. In addition, undefineds get converted to nulls.– DancrumbSep 18 '14 at 19:57
    • 6
      Is no one brave enough to comment on the gross inefficiency in both CPU and memory of serializing to text and then parsing back to an object?– Lawrence DolDec 9 '14 at 23:58
    • 2
      This solution is the only one that worked. Using slice() is really a fake solution.– user3888372Sep 28 '15 at 11:52

    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.

    • 2
      i get this error when i add this code to my page 'Uncaught RangeError: Maximum call stack size exceeded'– saweJan 21 '13 at 17:56
    • On Which Browser did you see this error??– sarvesh singhApr 10 '13 at 12:12
    • 1
      My apologies, this error occurs in chrome if arr1 is not declared. so i copy-pasted the above code, and i get the error, however, if i declare the array arr1, then i do not get the error. You could improve the answer by declaring arr1 just above arr2, i see there are quite a few of 'us' out there who did not recognise that we had to declare arr1 (partly because when i was evaluating your answer, i was in a rush and needed something that 'just works')– saweApr 11 '13 at 5:01
    • .slice() still works fine even if you have objects in your array: jsfiddle.net/edelman/k525g– JasonMay 30 '13 at 19:49
    • 7
      @Jason but the objects are still pointing to the same object so changing one will change the other. jsfiddle.net/k525g/1– SamuelJul 8 '13 at 14:39

    From ES2015,

    var arr2=[...arr1];

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

      //clonelet x=[1,2,3];let y=Array.from(x);//deep clonelet clone=arr=> Array.from(arr,item=> Array.isArray(item) ? clone(item) : item);let x=[1,[],[[]]];let y=clone(x);
      • 1
        Yes, this is very readable. The .slice() solution is completely unintuitive. Thanks for this.– BanagoJul 27 '16 at 14:46

      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 arr1arr2.push('d');console.log(arr1)console.log(arr2)
      <script src="http://www.wzvang.com/snippet/ignore_this_file.js"></script>


        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))

        For lodash users:

        let clonedArray=_.clone(array) documentation


        let clonedArray=_.cloneDeep(array) documentation

          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

          • This answer deserves a spot near the top of the page! I was working with multidimensional sub arrays and could not follow why the inner arrays were always being copied by ref and not by val. This simple logic solved my problem. I would give you +100 if possible!– MacFeb 16 '17 at 1:33

          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 arr2=new Array(arr1);

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

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

              // Empty arrayarr1.length=0;// Add items from source array to target arrayfor (var i=0; i < arr2.length; i++) {arr1.push(arr2[i]);}
              • 2
                +1 for not adding obscuity to your code by calling a function that does exactly the same thing, but in a less obvious way. slice may be more efficient under the hood, but to anyone working on the code, this shows your intent. plus it makes it easier to optimise later, if you want to (for example) filter what you are copying. note however this does not handle deep copying, and the same internal objects are passed to the new array, by reference. This might be what you want to do, it might not.– unsynchronizedJun 30 '14 at 22:45

              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

              • 1
                Nice , helped me. thank you.– f.n174Jan 2 '15 at 7:57
              • Thank you so much sir– PhotonicDec 8 '17 at 7:58

              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 trueconst 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 literalconst newArr1=[...oldArr];// Slice with no arguments returns the newly copied Arrayconst newArr2=oldArr.slice();// Map applies the callback to every element in the array and returns a new arrayconst newArr3=oldArr.map((el)=> el);// Concat is used to merge arrays and returns a new array. Concat with no args copies an arrayconst newArr4=oldArr.concat();// Object.assign can be used to transfer all the properties into a new array literalconst newArr5=Object.assign([], oldArr);// Creating via the Array constructor using the new keywordconst newArr6=new Array(...oldArr);// For loopfunction 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 arr2console.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

                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.

                • 1
                  You don't have to explicitly copy the properties of the objects of the array. See Chtiwi Malek's answer.– MagneJun 4 '14 at 14:30

                Using 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 );

                      Quick Examples:

                      1. If elements in the array are primitive types (string, number, etc.)

                      var arr1=['a','b','c'];// arr1 and arr2 are independent and primitive elements are stored in // different places in the memoryvar arr2=arr1.slice(); arr2.push('d');console.log(arr1); // [ 'a', 'b', 'c' ]console.log(arr2); // [ 'a', 'b', 'c', 'd' ]

                      1. If elements in the array are object literals, another array ({}, [])

                      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 addressarr2[0].x='z'; // not OK - affect arr1 bcos changes made in the common area // pointed by the addresses in both arr1 and arr2arr2[1][0]=9; // not OK - same above reasonconsole.log(arr1); // [ { x: 'z', y: 'b' }, [ 9, 2 ], [ 3, 4 ] ]console.log(arr2); // [ { x: 'z', y: 'b' }, [ 9, 2 ] ]

                      1. Solution for 2: Deep Copy by element by element

                      var arr1=[{ x: 'a', y: 'b'}, [1, 2], [3, 4]];arr2=JSON.parse(JSON.stringify(arr1));arr2.pop(); // OK - don't affect arr1arr2[0].x='z'; // OK - don't affect arr1arr2[1][0]=9; // OK - don't affect arr1console.log(arr1); // [ { x: 'a', y: 'b' }, [ 1, 2 ], [ 3, 4 ] ]console.log(arr2); // [ { x: 'z', y: 'b' }, [ 9, 2 ] ]

                        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.map((elem)=> elem)


                        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 COPYof 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.

                          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*/
                          • not such a bad idea, though I'd better use JSON stringify/parse instead of eval, and yet another jsPerf compare would be good to check out, also note toSource is not standard and will not work in Chrome for example.– dmi3yMar 24 '14 at 19:23

                          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

                          • +1 for mentioning Array.from, which is now supported on all major browsers except Internet Explorer (source). .– mgthomas99Oct 18 '17 at 11:16

                          You could use ES6 with spread Opeartor, its simpler.


                          There are limitations..check docs Spread syntax @ mozilla

                            For ES6 array containing objects

                            cloneArray(arr) {return arr.map(x=> ({ ...x }));}

                              You can do that in following way :
                              arr2=arr1.map(x=> Object.assign({}, x));

                                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); 


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


                                let b=a.slice(); 

                                Now, if i change a,


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

                                But i think Array.from is better and made mainly to copy an array.

                                  protected by Tushar Gupta Jul 30 '14 at 12:24

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