What is "_," (underscore comma) in a Go declaration?

The Go compiler won't allow you to create variables that you never use.

for i, value := range x {
   total += value
}

The above code will return an error message "i declared and not used".

Since we don't use i inside of our loop we need to change it to this:

for _, value := range x {
   total += value
}

The blank identifier may be used whenever syntax requires a variable name but program logic does not, for instance to discard an unwanted loop index when we require only the element value.

Excerpt From:

The Go Programming Language (Addison-Wesley Professional Computing Series)

Brian W. Kernighan

This material may be protected by copyright.

It avoids having to declare all the variables for the returns values.
It is called the blank identifier.

As in:

_, y, _ := coord(p)  // coord() returns three values; only interested in y coordinate

That way, you don't have to declare a variable you won't use: Go would not allow it. Instead, use '_' to ignore said variable.

(the other '_' use case is for import)

Since it discards the return value, it is helpful when you want to check only one of the returned values, as in "How to test key existence in a map?" shown in "Effective Go, map":

_, present := timeZone[tz]

To test for presence in the map without worrying about the actual value, you can use the blank identifier, a simple underscore (_).
The blank identifier can be assigned or declared with any value of any type, with the value discarded harmlessly.
For testing presence in a map, use the blank identifier in place of the usual variable for the value.

As Jsor adds in the comments:

"generally accepted standard" is to call the membership test variables "ok" (same for checking if a channel read was valid or not)

That allows you to combine it with test:

if _, err := os.Stat(path); os.IsNotExist(err) {
    fmt.Printf("%s does not exist\n", path)
}

You would find it also in loop:

If you only need the second item in the range (the value), use the blank identifier, an underscore, to discard the first:

sum := 0
for _, value := range array {
    sum += value
}