Performance of Array.flat() vs spread operator to flatten multiple arrays into one

Question

Which is a better and more efficient way to flatten multiple arrays into one?

Spread operator:

let arr = [];
for (const array of arrays) {
  arr.push(…array);
}

Or with .flat():

let arr = [];
for (const array of arrays) {
  arr.push(array);
}
arr = arr.flat();

I am using a loop instead of just const arr = arrays.flat() because I want to test something very specific.

Answer 1

If you need single-level flattening, according to this JSBench I cooked up,

let arr = [].concat(...arrays);

is by far the fastest (on my Windows Chrome 93, according to comments not so on Safari), more than 3x the speed of your implementation.

Answer 2

Running the same bench as the accepted answer, that solution is faster than the others tested, but I tested a couple of others and found another solution that is around ~12% faster:

let arr = [].concat.apply([], arrays);

Benchmark testing: https://jsbench.me/adlib26t2y/2