Discussion on: Javascript Array.push is 945x faster than Array.concat 🤯🤔

While that is true, and although it just so happened that in my use case immutability doesn't matter, but it is still worthwhile to consider using other methods to merge arrays instead of concat, while still preserving the source arrays.

If you needed to merge thousands of small arrays in a loop without modifying the source arrays for example, you should create a new result array and use push the source arrays instead to avoid the penalty of copying the first array during each loop when you use concat.

Naive code to merge 1000 arrays with concat (basically what I did, which in my case is 15,000 arrays):

for(var i = 0; i < 1000; i++){
  arr1 = arr1.concat(arr2)   // arr1 gets bigger and bigger, so this gets more expensive over time
}

Faster code to merge 1000 arrays with push w/o modification to source arrays:

let result = []
result.push(...arr1)
for(var i = 0; i < 1000; i++){
  result.push(...arr2) 
}

Also, given by the name, it's not very obvious to people that concat creates a new array instead of modifying the first array. And even though I frequently read MDN docs, I still sometimes forget that concat creates the result as new array.