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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.

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.

• javascript
• arrays
• duplicates
• copy
• slice

Share Improve this question Follow edited Jun 26, 2021 at 5:06 AbhishekGowda28 1,0541010 silver badges1919 bronze badges asked Oct 20, 2010 at 13:43 Marco DemaioMarco Demaio 34.4k3333 gold badges130130 silver badges161161 bronze badges 3

• 5 jsben.ch/#/wQ9RU <= a benchmark for the most common ways to clone an array – EscapeNetscape Commented Oct 24, 2016 at 18:40
• See also javascript - Copy array by value - Stack Overflow -- (some answer in that question does performance comparison) – user202729 Commented Dec 6, 2021 at 10:14
• Has anyone tried benchmarking using a specific function that returns the desired array? E.g. const getInitialArray = () => {return [[1, 2], [3, 4]} – rybo111 Commented Jun 7, 2023 at 20:33

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27 Answers 27

Sorted by: Reset to default 916

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

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

what about es6 way?

arr2 = [...arr1];

Easiest way to deep clone Array or Object:

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

本文标签： Fastest way to duplicate an array in JavaScriptslice vs 39for39 loopStack Overflow

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