Fastest way to duplicate an array in JavaScript - slice vs. 'for' loop

Question

In order to duplicate an array in JavaScript: Which of the following is faster to use?

Slice method

var dup_array = original_array.slice();

For loop

for(var i = 0, len = original_array.length; i < len; ++i)
   dup_array[i] = original_array[i];

---

I know both ways do only a shallow copy: if original_array contains references to objects, objects won't be cloned, but only the references will be copied, and therefore both arrays will have references to the same objects. But this is not the point of this question.

I'm asking only about speed.

Answer 1

There are at least 6 (!) ways to clone an array:

• loop
• slice
• Array.from()
• concat
• spread syntax (FASTEST)
• map A.map(function(e){return e;});

There has been a huuuge BENCHMARKS thread, providing following information:

• for blink browsers slice() is the fastest method, concat() is a bit slower, and while loop is 2.4x slower.

• for other browsers while loop is the fastest method, since those browsers don't have internal optimizations for slice and concat.

This remains true in Jul 2016.

Below are simple scripts that you can copy-paste into your browser's console and run several times to see the picture. They output milliseconds, lower is better.

while loop

n = 1000*1000;
start = + new Date();
a = Array(n); 
b = Array(n); 
i = a.length;
while(i--) b[i] = a[i];
console.log(new Date() - start);

slice

n = 1000*1000;
start = + new Date();
a = Array(n); 
b = a.slice();
console.log(new Date() - start);

Please note that these methods will clone the Array object itself, array contents however are copied by reference and are not deep cloned.

origAr == clonedArr //returns false
origAr[0] == clonedArr[0] //returns true

Answer 2

Technically slice is the fastest way. However, it is even faster if you add the 0 begin index.

myArray.slice(0);

is faster than

myArray.slice();

https://jsben.ch/F0SZ3

Answer 3

what about es6 way?

arr2 = [...arr1];

Answer 4

Easiest way to deep clone Array or Object:

var dup_array = JSON.parse(JSON.stringify(original_array))

Answer 5

🏁 Fastest Way to Clone an Array

I made this very plain utility function to test the time that it takes to clone an array. It is not 100% reliable. However, it can give you a bulk idea as to how long it takes to clone an existing array:

function clone(fn) {
  const arr = [...Array(1000000)];
  console.time('timer');
  fn(arr);
  console.timeEnd('timer');
}

And tested different approaches:

1)   5.79ms -> clone(arr => Object.values(arr));
2)   7.23ms -> clone(arr => [].concat(arr));
3)   9.13ms -> clone(arr => arr.slice());
4)  24.04ms -> clone(arr => { const a = []; for (let val of arr) { a.push(val); } return a; });
5)  30.02ms -> clone(arr => [...arr]);
6)  39.72ms -> clone(arr => JSON.parse(JSON.stringify(arr)));
7)  99.80ms -> clone(arr => arr.map(i => i));
8) 259.29ms -> clone(arr => Object.assign([], arr));
9) Maximum call stack size exceeded -> clone(arr => Array.of(...arr));

---

UPDATES

1. Tests were made back in 2018, so most likely, you'll get different results with current browsers.
2. Out of these methods, the best way to deep clone an array is by using JSON.parse(JSON.stringify(arr))
3. Do not use the above if your array might include functions as it will return null.
   Thank you @GilEpshtain for this update.

Answer 6

var cloned_array = [].concat(target_array);

Answer 7

I put together a quick demo: http://jsbin.com/agugo3/edit

My results on Internet Explorer 8 are 156, 782, and 750, which would indicate slice is much faster in this case.

Answer 8

a.map(e => e) is another alternative for this job. As of today .map() is very fast (almost as fast as .slice(0)) in Firefox, but not in Chrome.

On the other hand, if an array is multi-dimensional, since arrays are objects and objects are reference types, none of the slice or concat methods will be a cure... So one proper way of cloning an array is an invention of Array.prototype.clone() as follows.

Array.prototype.clone = function(){
  return this.map(e => Array.isArray(e) ? e.clone() : e);
};

var arr = [ 1, 2, 3, 4, [ 1, 2, [ 1, 2, 3 ], 4 , 5], 6 ],
    brr = arr.clone();
brr[4][2][1] = "two";
console.log(JSON.stringify(arr));
console.log(JSON.stringify(brr));

Answer 9

Fastest way to clone an Array of Objects will be using spread operator

var clonedArray=[...originalArray]
or
var clonedArray = originalArray.slice(0); //with 0 index it's little bit faster than normal slice()

but the objects inside that cloned array will still pointing at the old memory location. hence change to clonedArray objects will also change the orignalArray. So

var clonedArray = originalArray.map(({...ele}) => {return ele})

this will not only create new array but also the objects will be cloned to.

disclaimer if you are working with nested object in that case spread operator will work as SHALLOW CLONE. At that point better to use

var clonedArray=JSON.parse(JSON.stringify(originalArray));

Answer 10

Take a look at: link. It's not about speed, but comfort. Besides as you can see you can only use slice(0) on primitive types.

To make an independent copy of an array rather than a copy of the refence to it, you can use the array slice method.

Example:

To make an independent copy of an array rather than a copy of the refence to it, you can use the array slice method.

var oldArray = ["mip", "map", "mop"];
var newArray = oldArray.slice();

To copy or clone an object :

function cloneObject(source) {
    for (i in source) {
        if (typeof source[i] == 'source') {
            this[i] = new cloneObject(source[i]);
        }
        else{
            this[i] = source[i];
  }
    }
}

var obj1= {bla:'blabla',foo:'foofoo',etc:'etc'};
var obj2= new cloneObject(obj1);

Source: link

Answer 11

ECMAScript 2015 way with the Spread operator:

Basic examples:

var copyOfOldArray = [...oldArray]
var twoArraysBecomeOne = [...firstArray, ..seccondArray]

Try in the browser console:

var oldArray = [1, 2, 3]
var copyOfOldArray = [...oldArray]
console.log(oldArray)
console.log(copyOfOldArray)

var firstArray = [5, 6, 7]
var seccondArray = ["a", "b", "c"]
var twoArraysBecomOne = [...firstArray, ...seccondArray]
console.log(twoArraysBecomOne);

References

• 6 Great Uses of the Spread Operator
• Spread syntax

Answer 12

Benchmark time!

function log(data) {
  document.getElementById("log").textContent += data + "\n";
}

benchmark = (() => {
  time_function = function(ms, f, num) {
    var z = 0;
    var t = new Date().getTime();
    for (z = 0;
      ((new Date().getTime() - t) < ms); z++)
      f(num);
    return (z)
  }

  function clone1(arr) {
    return arr.slice(0);
  }

  function clone2(arr) {
    return [...arr]
  }

  function clone3(arr) {
    return [].concat(arr);
  }

  Array.prototype.clone = function() {
    return this.map(e => Array.isArray(e) ? e.clone() : e);
  };

  function clone4(arr) {
    return arr.clone();
  }


  function benchmark() {
    function compare(a, b) {
      if (a[1] > b[1]) {
        return -1;
      }
      if (a[1] < b[1]) {
        return 1;
      }
      return 0;
    }

    funcs = [clone1, clone2, clone3, clone4];
    results = [];
    funcs.forEach((ff) => {
      console.log("Benchmarking: " + ff.name);
      var s = time_function(2500, ff, Array(1024));
      results.push([ff, s]);
      console.log("Score: " + s);

    })
    return results.sort(compare);
  }
  return benchmark;
})()
log("Starting benchmark...\n");
res = benchmark();

console.log("Winner: " + res[0][0].name + " !!!");
count = 1;
res.forEach((r) => {
  log((count++) + ". " + r[0].name + " score: " + Math.floor(10000 * r[1] / res[0][1]) / 100 + ((count == 2) ? "% *winner*" : "% speed of winner.") + " (" + Math.round(r[1] * 100) / 100 + ")");
});
log("\nWinner code:\n");
log(res[0][0].toString());

<textarea rows="50" cols="80" style="font-size: 16; resize:none; border: none;" id="log"></textarea>

The benchmark will run for 10s since you click the button.

My results:

Chrome (V8 engine):

1. clone1 score: 100% *winner* (4110764)
2. clone3 score: 74.32% speed of winner. (3055225)
3. clone2 score: 30.75% speed of winner. (1264182)
4. clone4 score: 21.96% speed of winner. (902929)

Firefox (SpiderMonkey Engine):

1. clone1 score: 100% *winner* (8448353)
2. clone3 score: 16.44% speed of winner. (1389241)
3. clone4 score: 5.69% speed of winner. (481162)
4. clone2 score: 2.27% speed of winner. (192433)

Winner code:

function clone1(arr) {
    return arr.slice(0);
}

Winner engine:

SpiderMonkey (Mozilla/Firefox)

Answer 13

As @Dan said "This answer becomes outdated fast. Use benchmarks to check the actual situation", there is one specific answer from jsperf that has not had an answer for itself: while:

var i = a.length;
while(i--) { b[i] = a[i]; }

had 960,589 ops/sec with the runnerup a.concat() at 578,129 ops/sec, which is 60%.

This is the lastest Firefox (40) 64 bit.

---

@aleclarson created a new, more reliable benchmark.

Answer 14

It depends on the browser. If you look in the blog post Array.prototype.slice vs manual array creation, there is a rough guide to performance of each:

Results:

Answer 15

There is a much cleaner solution:

var srcArray = [1, 2, 3];
var clonedArray = srcArray.length === 1 ? [srcArray[0]] : Array.apply(this, srcArray);

The length check is required, because the Array constructor behaves differently when it is called with exactly one argument.

Answer 16

Remember .slice() won't work for two-dimensional arrays. You'll need a function like this:

function copy(array) {
  return array.map(function(arr) {
    return arr.slice();
  });
}

Answer 17

It depends on the length of the array. If the array length is <= 1,000,000, the slice and concat methods are taking approximately the same time. But when you give a wider range, the concat method wins.

For example, try this code:

var original_array = [];
for(var i = 0; i < 10000000; i ++) {
    original_array.push( Math.floor(Math.random() * 1000000 + 1));
}

function a1() {
    var dup = [];
    var start = Date.now();
    dup = original_array.slice();
    var end = Date.now();
    console.log('slice method takes ' + (end - start) + ' ms');
}

function a2() {
    var dup = [];
    var start = Date.now();
    dup = original_array.concat([]);
    var end = Date.now();
    console.log('concat method takes ' + (end - start) + ' ms');
}

function a3() {
    var dup = [];
    var start = Date.now();
    for(var i = 0; i < original_array.length; i ++) {
        dup.push(original_array[i]);
    }
    var end = Date.now();
    console.log('for loop with push method takes ' + (end - start) + ' ms');
}

function a4() {
    var dup = [];
    var start = Date.now();
    for(var i = 0; i < original_array.length; i ++) {
        dup[i] = original_array[i];
    }
    var end = Date.now();
    console.log('for loop with = method takes ' + (end - start) + ' ms');
}

function a5() {
    var dup = new Array(original_array.length)
    var start = Date.now();
    for(var i = 0; i < original_array.length; i ++) {
        dup.push(original_array[i]);
    }
    var end = Date.now();
    console.log('for loop with = method and array constructor takes ' + (end - start) + ' ms');
}

a1();
a2();
a3();
a4();
a5();

If you set the length of original_array to 1,000,000, the slice method and concat method are taking approximately the same time (3-4 ms, depending on the random numbers).

If you set the length of original_array to 10,000,000, then the slice method takes over 60 ms and the concat method takes over 20 ms.

Answer 18

In ES6, you can simply utilize the Spread syntax.

Example:

let arr = ['a', 'b', 'c'];
let arr2 = [...arr];

Please note that the spread operator generates a completely new array, so modifying one won't affect the other.

Example:

arr2.push('d') // becomes ['a', 'b', 'c', 'd']
console.log(arr) // while arr retains its values ['a', 'b', 'c']

Answer 19

I was redirected here from another question about shallow cloning Arrays, and after finding out that most of the links are either dead, outdated, or broken, figured I'd post a solution that you can just run in your own environment.

The following code should semi-accurately measure how much time it takes to clone an array using specific ways. You can run it in your Browser directly using the Developer console or Node.JS. The latest version of it can always be found here.

function benchTime(cycles, timeLimit, fnSetup, ...fnProcess) {
    function measureCycle(timeLimit, fn, args) {
        let tmp = null;
        let end, start = performance.now();
        let iterations = 0;

        // Run until we exceed the time limit for one cycle.
        do {
            tmp = fn.apply(null, args);
            end = performance.now();
            ++iterations;
        } while ((end - start) <= timeLimit);
        tmp = undefined;

        // Build a result object and return it.
        return {
            "iterations": iterations,
            "start": start,
            "end": end,
            "duration": end - start,
            "opsPerSec": (iterations / (end - start)) * 1000.0,
        };
    }

    console.log(`Measuring ${fnProcess.length} functions...`);
    let params = fnSetup();
    //console.log("Setup function returned:", params);

    // Perform this for every function passed.
    for (let fn of fnProcess) {
        let results = [];
        console.groupCollapsed(`${fn.name}: Running for ${cycles} cycles...`);
        // Perform this N times.
        for (let cycle = 0; cycle < cycles; cycle++) {
            let result = {
                "iterations": Number.NaN,
                "start": Number.NaN,
                "end": Number.NaN,
                "duration": Number.NaN,
                "opsPerSec": Number.NaN,
            };

            try {
                result = measureCycle(timeLimit, fn, params);
                results.push(result);
            } catch (ex) {
                console.error(`${fn.name}:`, ex);
                break;
            }

            console.log(`Cycle ${cycle}/${cycles}: ${result.iterations}, ${result.end - result.start}, ${result.opsPerSec} ops/s`);
        }

        // If we have more than 3 repeats, drop slowest and fastest as outliers.
        if (results.length > 3) {
            console.log("Dropping slowest and fastest result.");
            results = results.sort((a, b) => {
                return (a.end - a.start) > (b.end - b.start);
            }).slice(1);
            results = results.sort((a, b) => {
                return (a.end - a.start) < (b.end - b.start);
            }).slice(1);
        }
        console.groupEnd();

        // Merge all results for the final average.
        let iterations = 0;
        let totalTime = 0;
        let opsPerSecMin = +Infinity;
        let opsPerSecMax = -Infinity;
        let opsPerSec = 0;
        for (let result of results) {
            iterations += result.iterations;
            totalTime += result.duration;
            opsPerSec += result.opsPerSec;
            if (opsPerSecMin > result.opsPerSec) {
                opsPerSecMin = result.opsPerSec;
            }
            if (opsPerSecMax < result.opsPerSec) {
                opsPerSecMax = result.opsPerSec;
            }
        }
        let operations = opsPerSec / results.length; //iterations / totalTime;
        let operationVariance = opsPerSecMax - opsPerSecMin;
        console.log(`${fn.name}: ${(operations).toFixed(2)}±${(operationVariance).toFixed(2)} ops/s, ${iterations} iterations over ${totalTime} ms.`);
    }
    console.log("Done.");
}

function spread(arr) { return [...arr]; }
function spreadNew(arr) { return new Array(...arr); }
function arraySlice(arr) { return arr.slice(); }
function arraySlice0(arr) { return arr.slice(0); }
function arrayConcat(arr) { return [].concat(arr); }
function arrayMap(arr) { return arr.map(i => i); }
function objectValues(arr) { return Object.values(arr); }
function objectAssign(arr) { return Object.assign([], arr); }
function json(arr) { return JSON.parse(JSON.stringify(arr)); }
function loop(arr) { const a = []; for (let val of arr) { a.push(val); } return a; }

benchTime(
    10, 1000,
    () => {
        let arr = new Array(16384);
        for (let a = 0; a < arr.length; a++) { arr[a] = Math.random(); };
        return [arr];
    },
    spread,
    spreadNew,
    arraySlice,
    arraySlice0,
    arrayConcat,
    arrayMap,
    objectValues,
    objectAssign,
    json,
    loop
);

I've run this for a few sizes, but here's a copy of the 16384 element data:

Test	NodeJS v20.12.2	Firefox v127.0b2	Edge 124.0.2478.97
spread	13.180±1.022 ops/ms	7.436±1.110 ops/ms	3.321±0.239 ops/ms
spreadNew	4.727±0.532 ops/ms	1.010±0.022 ops/ms	21.045±2.982 ops/ms
arraySlice	12.912±2.127 ops/ms	11046.737±237.575 ops/ms	494.359±32.726 ops/ms
arraySlice0	13.192±0.477 ops/ms	10665.299±500.553 ops/ms	492.209±66.837 ops/ms
arrayConcat	16.590±0.656 ops/ms	7923.657±224.637 ops/ms	476.975±112.053 ops/ms
arrayMap	6.542±0.301 ops/ms	32.960±3.743 ops/ms	52.127±3.472 ops/ms
objectValues	4.339±0.111 ops/ms	10840.392±619.567 ops/ms	115.369±3.217 ops/ms
objectAssign	0.270±0.013 ops/ms	10471.860±202.291 ops/ms	83.135±3.439 ops/ms
json	0.205±0.039 ops/ms	4.014±1.679 ops/ms	7.730±0.319 ops/ms
loop	6.138±0.287 ops/ms	6.727±1.296 ops/ms	27.691±1.217 ops/ms

Overall, it appears as if the following is true:

• (arraySlice)array.slice(), (arraySlice0)array.slice(0) and (arrayConcat)[].concat(array) are equivalently fast. arraySlice appears to be minimally faster.
• In Chromium-based Browsers only, (objectAssign)Object.assign([], array) is faster with massive arrays, but brings the downside of eliminating any entries that are undefined.
• The worst ways overall to shallow-clone arrays are (spreadNew)New Array(...array), (arrayMap)array.map(i => i), (objectValues)Object.values(array), (objectAssign)Object.assign(array), (json)JSON.parse(JSON.stringify(array), (loop)let arr = new Array(); for (let e of array) { arr.push(e); }; return arr; }.
• Firefox appears to be optimizing in a way that outright looks like cheating. I've failed to identify a way to make Firefox look normal.

Disclaimer: These results are from my personal system, and I reserve the right to have made an error in my code.

Answer 20

        const arr = ['1', '2', '3'];

         // Old way
        const cloneArr = arr.slice();

        // ES6 way
        const cloneArrES6 = [...arr];

// But problem with 3rd approach is that if you are using muti-dimensional 
 // array, then only first level is copied

        const nums = [
              [1, 2], 
              [10],
         ];

        const cloneNums = [...nums];

// Let's change the first item in the first nested item in our cloned array.

        cloneNums[0][0] = '8';

        console.log(cloneNums);
           // [ [ '8', 2 ], [ 10 ], [ 300 ] ]

        // NOOooo, the original is also affected
        console.log(nums);
          // [ [ '8', 2 ], [ 10 ], [ 300 ] ]

So, in order to avoid these scenarios to happen, use

        const arr = ['1', '2', '3'];

        const cloneArr = Array.from(arr);

Answer 21

There were several ways to clone an array. Basically, Cloning was categorized in two ways:

1. Shallow copy
2. Deep copy

Shallow copies only cover the 1st level of the array and the rest are referenced. If you want a true copy of nested elements in the arrays, you’ll need a deep clone.

Example :

const arr1 = [1,2,3,4,5,6,7]           
// Normal Array (shallow copy is enough)     
const arr2 = [1,2,3,[4],[[5]],6,7]          
// Nested Array  (Deep copy required) 


Approach 1 : Using (...)Spread Operator  (Shallow copy enough)
const newArray = [...arr1] // [1,2,3,4,5,6,7]

Approach 2 : Using Array builtIn Slice method (Deep copy)  
const newArray = arr1.slice()  // [1,2,3,4,5,6,7]

Approach 3 : Using Array builtIn Concat method (Deep a copy)
const newArray = [].concat(arr1)  // [1,2,3,4,5,6,7]

Approach 4 : Using JSON.stringify/parse. (Deep a copy & fastest)
const newArray = JSON.parse(JSON.stringify(arr2));)  // [1,2,3,[4],[[5]],6,7]

Approach 5: Using own recursive function or using loadash's __.cloneDeep method. (Deep copy)

Answer 22

A simple solution:

original = [1,2,3]
cloned = original.map(x=>x)

Answer 23

Fast ways to duplicate an array in JavaScript in Order:

#1: array1copy = [...array1];

#2: array1copy = array1.slice(0);

#3: array1copy = array1.slice();

If your array objects contain some JSON-non-serializable content (functions, Number.POSITIVE_INFINITY, etc.) better to use

array1copy = JSON.parse(JSON.stringify(array1))

Answer 24

You can follow this code. Immutable way array clone. This is the perfect way to array cloning

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

const newArray = [...array]
newArray.push(6)
console.log(array)
console.log(newArray)

Answer 25

If you want a REAL cloned object/array in JS with cloned references of all attributes and sub-objects:

export function clone(arr) {
    return JSON.parse(JSON.stringify(arr))
}

ALL other operations do not create clones, because they just change the base address of the root element, not of the included objects.

Except you traverse recursive through the object-tree.

For a simple copy, these are OK. For storage address relevant operations I suggest (and in most all other cases, because this is fast!) to type convert into string and back in a complete new object.

Answer 26

May 2024

Chrome

Firefox

never finishes this benchmark, certainly an issue with the benchmarking tool.

https://jsben.ch/lO6C5

Also, structuredClone is now available which is very slow (3%)

Answer 27

If you are taking about slice it is used to copy elements from an array and create a clone with same no. of elements or less no. of elements.

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

function slc() {
  var sliced = arr.slice(0, 5);
// arr.slice(position to start copying master array , no. of items in new array)
  console.log(sliced);
}
slc(arr);