Best practices constructing an empty array

What is the performance cost of using arrays with unspecified capacity?

There is certainly a cost when you start populating the slice. If you know how big the slice should grow, you can allocate capacity of the underlying array from the very begging as opposed to reallocating every time the underlying array fills up.

Here is a simple example with timing:

package main

import "fmt"

func main() {

    limit := 500 * 1000 * 1000
    mySlice := make([]int, 0, limit) //vs mySlice := make([]int, 0)

    for i := 0; i < limit; i++ {
        mySlice = append(mySlice, i)
    }

    fmt.Println(len(mySlice))
}

On my machine:

time go run my_file.go

With preallocation:

real    0m2.129s
user    0m2.073s
sys     0m1.357s

Without preallocation

real    0m7.673s
user    0m9.095s
sys     0m3.462s

Is there any difference between the 3 ways to make an empty array?

if empty array means len(array)==0, the answer is no, but actually only myArr3==nil is true.

What is the default capacity of an array when unspecified?

the default capacity will be same with the len you specify.

What is the performance cost of using arrays with unspecified capacity?

none

First, it's a slice not an array. Arrays and slices in Go are very different, arrays have a fixed size that is part of the type. I had trouble with this at first too :)

1. Not really. Any if the three is correct, and any difference should be too small to worry about. In my own code I generally use whatever is easiest in a particular case.
2. 0
3. Nothing, until you need to add an item, then whatever it costs to allocate the storage needed.