Declaring variables in Go
There are three ways to declare variables in Go: var, :=, and const.
The var keyword is the standard method for declaring a variable. It can be
used anywhere in your program, with a value or without:
var s1 string = "Hello"
func main() {
var s2 = "World"
var s3 string
s3 += "!"
}
Variable declarations without a value require a type (s3 above), but types are
otherwise optional.
Declare a variable with with var when you need a variable at the top-level
of your program or want to initialize it with no value.
The short-declaration operator, :=, works like the var keyword for declaring
variables with three differences: a) it can only be used within the scope of a
function, b) it cannot initialize an empty variable, and c) it does not accept a
type. Of the var examples above, only one could be replaced with a :=:
// s1 := "Hello" // Outside of function scope
func main() {
s2 := "World"
// s3 := // No empty variables
}
These may sounds like significant limitations, but most of your program will be
written inside a function’s scope and rarely do variables need to be declared
without a value. So the benefit of using := comes from letting Go infer the
type of your variable so that you can focus on the code.
Declare a variable with := when you are working within a function with
initial values.
The const keyword tells the compiler that the variable is a constant — it
will not be changed or reassigned. Like var it can be used anywhere in your
program but can only be used with some types: boolean, float64, int, and
string.
const PI float64 = 3.14159
If you try to use const with other types like a slice, then you will get a
compiler error:
(value of type []string) is not constant compiler(InvalidConstInit)
Declare a variable with const when you know that it will not change
throughout your program.
I am new to learning and writing in Go, but in practice I almost exclusively use
the declaration operator, :=, for declaring variables. But it’s useful to know
the reasons when and why you should reach for var or const.