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

Solution 1:

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

• loop
• slice
• Array.from()
• concat
• spread operator (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

Solution 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://web.archive.org/web/20170824010701/https://jsperf.com/cloning-arrays/3

Solution 3:

what about es6 way?

arr2 = [...arr1];